Mortality Measurement: Development of the 3M™ All Patient Refined Diag

Overview of APR DRG Subclass Assignment

The process of determining the subclasses for an All Patient Refined Diagnosis Related Groups (APR DRGs) begins by first assigning a severity of illness level and a risk of mortality level to each secondary diagnosis. The term "level" is used when referring to the categorization of a secondary diagnosis. The term "subclass" is used when referring to one of the subdivisions of an APR DRG. For secondary diagnoses, there are four distinct severity of illness levels and four distinct risk of mortality levels. The four levels are numbered sequentially from 1 to 4 indicating, respectively, minor, moderate, major or extreme severity of illness or risk of mortality. Each secondary diagnosis is assigned to one of the four severity of illness levels and one of the four risk of mortality levels. The severity of illness level and risk of mortality level associated with a patient's secondary diagnoses is just one factor in the determination of a patient's overall severity of illness subclass and risk of mortality subclass.

The assignment of a patient to a severity of illness or risk of mortality subclass takes into consideration not only the level of the secondary diagnoses but also the interaction among secondary diagnoses, age, principal diagnosis, and the presence of certain OR procedures and non-OR procedures. The subdivision of each of the 314 APR DRGs into the four subclasses, combined with the two error APR DRGs (955, 956), which are not subdivided, results in 1,258 APR DRGs.

The process of determining the severity of illness or risk of mortality subclass of a patient consists of three phases. In Phase I, the level of each secondary diagnosis is determined. Once the level of each individual secondary diagnosis is established, then Phase II determines a base subclass for the patient based on all of the patient's secondary diagnoses. In Phase III, the final subclass for the patient is determined by incorporating the impact of principal diagnosis, age, OR procedure, non-OR procedures, multiple OR procedures, and combinations of categories of secondary diagnoses. A detailed description of the determination of the severity of illness subclass and the risk of mortality subclass will be presented separately.

The three-phase process of determining the severity of illness subclass is summarized in Figure 2-1. There are six steps to Phase I, three steps to Phase II, and nine steps to Phase III for a total of 18 steps.

Figure 2-1. Three-phase process for determining patient severity of illness subclass

[D] Select for Text Description.

Phase I—Determining the severity of illness level of each secondary diagnosis

1. Eliminate secondary diagnoses associated with the principal diagnosis

If a secondary diagnosis is closely related to the principal diagnosis and does not add any distinguishing information, the secondary diagnosis is excluded from the determination of the severity of illness subclass. For example, a secondary diagnosis of urinary retention is excluded from the determination of the severity of illness subclass if the principal diagnosis is benign prostate hypertrophy because the urinary retention is caused by the benign prostate hypertrophy and will usually be present for patients hospitalized for benign prostate hypertrophy. For version 20.0 APR DRGs, the secondary diagnosis and principal diagnosis exclusion list was comprehensively reviewed and extensively modified. For version 25.0, the list was only updated.

2. Assign each secondary diagnosis to its standard severity of illness level

Each secondary diagnosis is assigned to one of the four distinct severity of illness levels. Examples of the different severity of illness levels are contained in Table 2-1.

Table 2-1. Examples of severity of illness levels

The severity of illness level for diabetes progresses from minor for uncomplicated diabetes to extreme for diabetes with hyperosmolar coma. Similarly, the severity of illness level for respiratory diagnoses progresses from minor for bronchitis to extreme for respiratory failure.

For version 20.0 APR DRGs, the standard severity of illness level was comprehensively reviewed for all secondary diagnoses codes. There were a number of revisions introduced—the majority of which were to lower the standard severity of illness level. In situations where there was a great deal of variability within an ICD-9-CM diagnosis code, the approach was to lower the standard severity of illness level and then in later steps of Phase I, consider whether modifications to the standard severity of illness level are indicated based upon specific age ranges, APR DRGs, or non-OR procedures. For example, the secondary diagnosis code 51882 Other pulmonary insufficiency NEC includes a very specific and severe condition such as adult respiratory distress syndrome, but is sufficiently broad to include other much less severe forms of pulmonary insufficiency. Beginning with version 20.0, and continuing with 25.0, the secondary diagnosis lowers from extreme to moderate, but then in a later Phase I step adjusts the severity of illness level up to major if the patient receives mechanical ventilation <96 hours, and up to extreme if the patient receives mechanical ventilation 96+ hours. The mechanical ventilation is an indicator of more severe pulmonary insufficiency and is often needed for patients with adult respiratory distress syndrome.

For version 25.0 APR DRGs there are a total of 13,677 ICD-9-CM diagnoses codes. These codes are assigned to the following severity of illness levels: 8,793 minor, 3,080 moderate, 949 major, 855 extreme.

The relatively large number of diagnoses moved to the minor severity of illness level was in part due to the decision to assign to the minor severity of illness level most secondary diagnoses related to pregnancy that were coded with an unspecified episode of pregnancy care (e.g., ICD-9-CM code 65100 Twin pregnancy without an indication of whether the encounter was for antepartum care, post partum care, or delivery). The only exceptions were diabetes mellitus, venous complications in pregnancy, and obstetrical pyemic and septic embolism, which were assigned to a higher severity of illness level. Another reason is that the APR DRG system has assigned to the minor severity of illness level most diagnoses that are described as complications of treatment. While complications of treatment are sometimes unavoidable and not due to poor quality of care, the APR DRG system has been very conservative in allowing these diagnoses to contribute to the patient's severity of illness level (the same is true for risk of mortality). Most of the ICD-9-CM complications of treatment diagnosis codes in the 990 series and the obstetrical complications of the administration of anesthesia were changed to minor severity of illness level in the version 15.0 APR DRGs. In addition, there are some other complications of treatment diagnosis codes that were changed to minor severity of illness level in version 20.0 APR DRGs (e.g., tracheostomy, gastrostomy, colostomy complications, and iatrogenic pneumothorax).

There are some secondary diagnoses that can have different meanings or implications in different circumstances and these received special attention in version 20.0 APR DRG through the various Phase I steps. To illustrate, there are circumstances where secondary diagnosis code 3481 Anoxic brain damage may be part of the patient's acute presenting condition (e.g., major trauma, poisoning, major neurological, respiratory, cardiac or infectious condition) and an indicator of high severity of illness. There are other instances where anoxic brain damage is not ordinarily expected and may represent the use of code 3481 for long standing anoxic brain damage (from a prior event), or possibly an unexpected complication of treatment. To take into account these different circumstances, version 20.0 APR DRGs lowered the standard severity of illness level for anoxic brain damage from extreme to minor, but then, in a later Phase I step, adjusts the severity level back up to extreme for selected APR DRGs where it is reasonable to expect that the anoxic brain damage may be part of the patient's presenting condition. (This was handled the same way for risk of mortality.)

Another set of secondary diagnoses that received special attention is the secondary diagnoses of cardiac arrest, ventricular fibrillation and ventricular flutter. In version 15.0 APR DRGs, these diagnoses were all assigned a severity of illness level of extreme (likewise for risk of mortality.) These secondary diagnoses unquestionably represent very extreme acute diagnoses. At the same time, there is a unique aspect to these diagnoses in that they can potentially be coded for most patients who die and whose admitting condition is not cardiac or cardiac related. If this was to occur, the subclass assignment logic, especially for risk of mortality, could become somewhat circular. To avoid this possibility, the standard severity of illness level (and standard risk of mortality level) in version 20.0 APR DRGs was changed from extreme to minor, and then for a small subset of APR DRGs adjusted back up to extreme. The subset includes APR DRGs for major neurological, respiratory, cardiovascular, and infectious conditions, and poisonings. For these APR DRGs, the patients are at a clear risk of having a cardiac arrest, ventricular fibrillation, or ventricular flutter and so these secondary diagnoses contribute to the severity of illness (and risk of mortality) assignment. This is different from other APR DRGs where the patient is not at an apparent risk of a cardiac arrest, ventricular fibrillation, or ventricular flutter. Patients in these other APR DRGs could still have a cardiac arrest, ventricular fibrillation, or ventricular flutter as part of the course of their hospitalization, but since their principal diagnosis is not cardiac or cardiac related, there is the concern for potential overcoding of these secondary diagnoses for patients who die. Versions 20.0 and 24.0 APR DRGs do not let these occurrences contribute to the patient's severity of illness level or risk of mortality level.

The process of determining the severity of illness subclass for a patient begins by assigning each secondary diagnosis its standard severity of illness level. The next step is to modify the standard severity of illness level based on other patient attributes. The patient attributes which can modify the standard severity of illness level of a secondary diagnosis are the age of the patient, the APR DRG and principal diagnosis, the APR DRG, and the presence of certain non-operating room procedures. These potential modifiers are evaluated and applied sequentially through Phase I.

3. Modify the standard severity of illness level of a secondary diagnosis based on age

The age of the patient will modify the standard severity of illness level assignment for some secondary diagnoses. For pediatric patients there are some secondary diagnoses that are modified to a higher level throughout all childhood years. For example, hypertension is modified from minor to major and really represents a different disease in children than adults. There are other secondary diagnoses that are modified only for certain childhood ages, most often early childhood. For example, many congenital anomalies and syndromes have their most difficult presentation in the neonatal time period and the first year of life, and are modified to a higher level for these ages. For example, hypoplastic left heart syndrome and combined immune deficiency are both modified from major to extreme for children less than one year of age. There are also some secondary diagnoses that are modified to a lower level for pediatric patients. For example, thrush is modified from moderate to minor for children less than one year of age.

In general, for elderly patients, for select secondary diagnoses, the severity of illness level is increased. For example, the secondary diagnoses of hypovolemia (dehydration) and chronic bronchitis are modified from minor to moderate and asthma with status asthmaticus is modified from moderate to major for patients age >69 years.

4. Modify the standard severity of illness level of a secondary diagnosis based on the APR DRG and principal diagnosis

The standard severity of illness level for some secondary diagnoses may be modified depending on the APR DRG and principal diagnosis of the patient. In version 24.0, this logic is applied only to APR DRG 190 Acute Myocardial Infarct. In general, secondary diagnoses that are closely related to the principal diagnosis are excluded from the determination of the severity of illness subclass. However, for a patient admitted for an acute anterior wall myocardial infarction, an acute anterolateral myocardial infarction represents an extension of the acute anterior wall myocardial infarction. Therefore, the acute anterolateral myocardial infarction is not excluded and is assigned a severity of illness level of moderate.

5. Modify the standard severity of illness level of a secondary diagnosis based on the APR DRG

The standard severity of illness level for many secondary diagnoses may be modified depending on the APR DRG to which the patient is assigned. Altogether, there are 3,787 modifications of the standard severity of illness level of a secondary diagnosis depending upon the APR DRG. The APR DRG specific modifications to the severity of illness level of individual secondary diagnoses reflects the disease-specific nature of the determination of severity of illness.

Some examples of APR DRG modifications are shown in table 2-2. Chronic renal failure significantly increases the severity of illness level for patients with diabetes and, thus, is increased to a major severity of illness for the APR DRG for diabetes. Cardiomegaly is not only common for congestive heart failure patients, but it is also an integral part of the disease and is reduced to a minor severity of illness level for the APR DRG for congestive heart failure. Uncomplicated diabetes is a minor secondary diagnosis, but for a vaginal delivery, represents a more difficult delivery and is therefore increased to a moderate severity of illness level.

Table 2-2. Examples of modification of standard Severity of Illness level based on APR DRG

In general, for surgical APR DRGs, secondary diagnoses that constituted or were associated with the reason for performing the procedure had their standard severity of illness level decreased. In general, for medical APR DRGs, secondary diagnoses that were closely related to the reason for the admission had their standard severity of illness level decreased. In essence, the standard severity of illness level of every secondary diagnosis was reviewed with every APR DRG and modified when appropriate. For version 20.0 APR DRGs, there were a substantial number of additions and modifications made on this basis.

6. Modify the standard severity of illness level of a secondary diagnosis based on non-OR procedures

Some secondary diagnoses can vary significantly in terms of their severity and clinical impact on patients. The presence of certain non-OR procedures can indicate a more extensive disease process. This type of modification is applied to only nine sets of non-OR procedure codes and to only a limited number of secondary diagnoses. The most important of these are the procedure codes for mechanical ventilation. Mechanical ventilation <96 hours is used to increase the standard severity level of a secondary diagnosis by an increment of one up to major; e.g., asthma with status asthmaticus would increase from level moderate to major if the patient had mechanical ventilation <96 hours. Mechanical ventilation 96+ hours is used to increase the standard severity level of illness of a secondary diagnosis by an increment of two up to extreme; e.g., other pulmonary insufficiency not elsewhere classified (which includes adult respiratory distress syndrome) increases the standard severity of illness level from moderate to extreme and a diagnosis such as pneumonia NOS which is already a level of major increases to extreme if the patient had mechanical ventilation 96+ hours. In each of these instances, the need for mechanical ventilation is indicative of a patient with more severe pulmonary illness, especially those who require ventilation for 96+ hours.

Among the other non-OR procedures that are used as part of this step, renal dialysis is used to increase the severity level of nephritis by an increment of one up to a maximum of major; total parenteral nutrition (TPN) is used to increase regional enteritis and ulcerative colitis by an increment of one up to major; and temporary pacemaker is used to increase heart block diagnoses such as trifascicular block by an increment of one up to major. Overall, non-OR procedures as part of this step in the APR DRG severity of illness logic are used more sparingly starting with version 20.0.

Phase II—Determine the base severity of illness subclass for the patient

Once each secondary diagnosis has been assigned its standard severity of illness level and the standard severity of illness level of each secondary diagnosis has been modified based on age, APR DRG and principal diagnosis, APR DRG, and presence of certain non-OR procedures, the Phase II base severity of illness subclass for the patient can be determined. The process of determining the base patient severity of illness subclass of the patient begins with the elimination of certain secondary diagnoses that are closely related to other secondary diagnoses. The elimination of these diagnoses prevents the double counting of clinically similar diagnoses in the determination of the severity of illness subclass of the patient. Once redundant diagnoses have been eliminated, the base severity of illness subclass is determined based on all of the remaining secondary diagnoses. There are three steps to Phase II.

7. Eliminate certain secondary diagnoses from the determination of the severity of iIlness subclass of the patient

Closely related secondary diagnoses are grouped together with clinically similar diagnoses. If more than one secondary diagnosis from the same secondary diagnosis group is present, then only the secondary diagnosis with the highest severity of illness level is preserved. All other secondary diagnoses in the group have their severity level reduced to minor, virtually eliminating them from contributing to the patient's base subclass determination. There are 289 secondary diagnosis groups defined for this step. For example, the secondary diagnoses of cerebral embolism with infarct and precerebral occlusion are in the same secondary diagnosis group, Cerebrovascular Diagnoses. Since the cerebral embolism with infarct is an extreme severity of illness level, and the precerebral occlusion is a moderate severity of illness level, the cerebral embolism with infarct will be preserved and the severity of illness level of the precerebral occlusion will be reduced to one when they are both present as secondary diagnoses.

8. Combine all secondary diagnoses to determine the base severity of illness subclass of the patient

Once secondary diagnoses that are related to other secondary diagnoses have had their severity levels reduced to minor, the base patient severity of illness subclass is set equal to the maximum severity of illness level across all of the remaining secondary diagnoses. For example, if there are five remaining secondary diagnoses and one is a major severity of illness level and four are a moderate severity of illness level then the base patient subclass is major.

9. Reduce the base severity of illness subclass of patients with a major or extreme subclass unless the patient has multiple secondary diagnoses at a high severity level

In order to be assigned to the major or extreme severity of illness subclass, a patient must have multiple secondary diagnoses at a high severity of illness level. High severity of illness patients are usually characterized by the presence of multiple high severity of illness secondary diagnoses. Patients with a base severity of illness subclass of extreme must have two or more secondary diagnoses that are an extreme severity of illness level, or one secondary diagnoses at an extreme severity of illness level plus at least two other secondary diagnoses at a major severity of illness level—otherwise the base severity of illness subclass is reduced to major. Patients with a base severity of illness subclass of major must have two or more secondary diagnoses that are a major severity of illness level, or one secondary diagnosis at a major severity of illness level plus at least two other secondary diagnoses at a moderate severity of illness level—otherwise the base severity of illness subclass is reduced to moderate. Thus, a secondary diagnosis of AMI is not sufficient to assign a patient to an extreme severity of illness subclass. In addition to the AMI, there must be at least one additional extreme severity of illness secondary diagnosis (e.g., acute renal failure) or two or more additional major severity of illness secondary diagnoses (e.g., congestive heart failure and diabetic ketoacidosis).

Phase III—Determine the final severity of illness subclass of the patient

Once the base patient severity of illness subclass is computed, the patient severity of illness subclass may be increased or decreased based on specific values of the following patient attributes:

• Combinations of APR DRG and principal diagnosis.
• Combinations of APR DRG and age, or APR DRG and principal diagnosis and age.
• Combinations of APR DRG and non-OR procedures.
• Combinations of APR DRG and OR procedures.
• Combinations of APR DRG and pairs of OR procedures.
• Combination of APR DRG for ECMO and presence/absence of certain OR procedures.
• Combinations of APR DRG and principal diagnoses and non-OR procedures.
• Combinations of categories of secondary diagnoses.

Phase III examines these eight patient attributes, seven of which are APR DRG specific, and then as its ninth step, computes the patient's final severity of illness subclass assignment.

In Phase I, age and non-OR procedures were used to modify the standard severity of illness level of a secondary diagnosis. However, age and non-OR procedures can also have an impact that is specific to the patient's APR DRG or to a specific principal diagnosis within the APR DRG. Thus, the impact of age and non-OR procedures is reassessed in Phase III as part of the determination of the severity of illness subclass of the patient. Based on the patient attributes listed above, a series of modifications to the base patient severity of illness subclass are made during Phase III. The final patient severity of illness subclass is computed based on the Phase II base patient severity of illness subclass and the modifications to the base severity of illness subclass made in Phase III.

10. Modify severity of illness subclass for the patient based on combinations of APR DRG and principal diagnosis

This step is used extensively in Phase III to modify a patient's severity of illness subclass. The ICD-9-CM coding system will sometimes include in a single diagnosis code both the underlying disease and an associated manifestation of the disease. For example, if the principal diagnosis code is 25020 Diabetes with hyperosmolar coma, the patient is assigned to the APR DRG for diabetes. Ordinarily, if the patient had no secondary diagnoses then the severity of illness subclass would be minor. Since the principal diagnosis includes not only the underlying diagnosis but also a major manifestation, the diabetic patient with hyperosmolar coma should be assigned to a higher patient severity of illness subclass. In order to accommodate this idiosyncrasy of ICD-9-CM, if the principal diagnosis is an ICD-9-CM diagnosis code that represents multiple diagnoses, or a diagnosis as well as a high severity manifestation, the severity of illness subclass of the patient is increased by a specified increment up to a specified maximum subclass. For example, if diabetes with hyperosmolar coma is the principal diagnosis, the severity of illness subclass of the patient is increased by one up to a maximum subclass of major. Other examples of principal diagnoses that include an important manifestation include: head trauma with prolonged or deep coma, intractable epilepsy, ruptured aortic aneurism, acute stomach ulcer with perforation and obstruction, acute appendicitis with peritonitis, and open fracture of the femur shaft.

Within specific APR DRGs there are also some principal diagnoses that are indicative of higher severity of illness relative to the other principal diagnoses in the APR DRG. For example, the severity of illness subclass of patients in APR DRG 221 Major Small & Large Bowel Procedures with a principal diagnosis of acute vascular insufficiency of the intestine is increased by one up to a maximum subclass of moderate. Relative to the other principal diagnoses associated with the procedures in APR DRG 221 (e.g., diverticulosis of colon, bowel malignancies), acute vascular insufficiency of the intestine represents a more severely ill patient. A medical example is hemophilia factor VIII that is increased by two up to major in APR DRG 661 Coagulation Disorders.

Conversely, within specific APR DRGs some principal diagnoses are indicative of lower severity of illness relative to the other principal diagnoses in the APR DRG. For example, within APR DRG 404 Thyroid, Parathyroid & Thyroglossal Procedures, patients with a principal diagnosis of nontoxic uninodular goiter will have their severity of illness subclass decreased by one if their severity of illness subclass up to this point in the process were major or moderate. Relative to the other principal diagnoses associated with the procedures in APR DRG 404 (e.g., malignant neoplasm of thyroid), nontoxic uninodular goiter represents a less severely ill patient. A medical example is first degree burns, which is decreased from moderate to minor in APR DRG 844 Partial Thickness Burns as these patients are less severely ill than second degree burn patients.

11. Modify severity of illness subclass for the patient based combinations of APR DRG and age, or APR DRG, principal diagnosis and age

For some principal diagnoses in specific APR DRGs, the patient's age essentially represents a complicating factor. For specific principal diagnoses and age combinations in certain APR DRGs, the severity of illness subclass of the patient is increased by a specified increment up to a specified maximum subclass. For example, for pediatric patients in APR DRG 344 Osteomyelitis, Septic Arthritis & Other Musculoskeletal Infections with bone infection as a principal diagnosis, the severity of illness subclass is increased by one up to a maximum of a moderate subclass. The increase in the severity of illness subclass indicates that bone infection in a pediatric patient represents a more severely ill patient. Elderly patients with certain principal diagnoses have their severity of illness subclass increased by one to a maximum subclass of moderate. For example, patients age >69 years with certain septicemia principal diagnoses in APR DRG 720 Septicemia and patients age >79 years with chronic/unspecified ulcer with hemorrhage without obstruction in APR DRG 241 Peptic Ulcer & Gastritis have their severity of illness subclass increased by one to a maximum of moderate.

For some APR DRGs the patient's severity of illness subclass is modified for all patients in an age range, not just for those certain principal diagnoses. This modification has been applied to just elderly patients and in just two MDC 10 (Endocrine, Nutritional & Metabolic Diseases and Disorders) APR DRGs and five MDC 19 (Mental Diseases & Disorders) APR DRGs. For example, patients age >79 years in APR DRG 421 Malnutrition, Failure to Thrive and Other Nutritional Disorders and APR DRG 422 Hypovolemia & Related Electrolyte Disorders will have their severity of illness subclass increased by an increment of one up to a maximum subclass of moderate.

12. Modify the severity of illness subclass for the patient based upon combinations of APR DRG and non-OR procedures

For some APR DRGs the presence of certain non-OR procedures represents a complicating factor. The most important of these are the codes for mechanical ventilation. For a number of neurological, respiratory, certain cardiovascular, neonatal, burn, and trauma patients, the need for mechanical ventilation indicates a more severely ill patient and the patient's severity of illness subclass is increased most often by an increment of one to a maximum subclass of major. In the same APR DRGs, mechanical ventilation 96+ hours is often used to increase the patient's severity of illness subclass by an increment of two up to a maximum subclass of extreme. The exact amount of the increment will vary according to the APR DRG category. For example, in the instance of neonates the increment varies depending upon birthweight and medical or surgical APR DRG. In the cardiovascular APR DRGs, balloon pulsation device is used to increase the severity subclass by an increment of one to a maximum of major for most surgical categories and by an increment of two to extreme for most medical APR DRGs.

13. Modify the severity of illness subclass for the patient based on combinations of APR DRG and OR procedure

This step is used extensively in Phase III to modify a patient's severity of illness subclass. Within specific APR DRGs, some OR procedures are indicative of higher severity of illness relative to the other OR procedures in the APR DRG. For example, the severity of illness subclass of patients in APR DRG 362 Mastectomy Procedures with an OR procedure of bilateral extended radical mastectomy is increased by one up to a maximum of a moderate subclass. Relative to the other OR procedures in APR DRG 362 (e.g., unilateral simple mastectomy), a bilateral extended radical mastectomy represents a patient that is more severely ill.

Conversely, within specific APR DRGs, some OR procedures are indicative of lower severity of illness relative to the other OR procedures in the APR DRG. For example, the severity of illness subclass of patients in APR DRGs 162 and 163 (Cardiac Valve Procedure With and Without Cardiac Catheterization) with an OR procedure of open heart valvuloplasty, is less complex than patients receiving cardiac valve replacements, and have their severity of illness subclass decreased by one for patients with a severity of illness subclass up to this point in the process that is moderate.

14. Modify the severity of illness subclass for the patient based on combinations of APR DRG and pairs of OR procedures

Within specific APR DRGs some pairs of OR procedures are indicative of higher severity of illness relative to the other patients in the APR DRG. Areas where multiple procedures are a significant determinant of severity of illness include: peripheral bypass surgery plus lower limb amputation or skin graft, cranial procedures plus face bone or jaw procedures, multiple spinal fusion procedures (anterior and posterior), and multiple procedures related to trauma such as multiple limb procedures, limb procedure plus back procedure, and limb procedure plus skin or fascia graft. For example, if a patient in APR DRG 308 Hip & Femur Procedure for Trauma receives both a femur procedure (upper leg) and one of a specified set of tibia/fibula procedures (lower leg) or shoulder/arm procedures, the severity of illness subclass will be increased by one up to a maximum subclass of extreme. Relative to other femur procedure patients, those who also have a procedure for trauma to other extremities have a higher severity of illness.

15. Modify the severity of illness subclass for the patient based upon combination of APR DRG for ECMO and presence/absence of certain OR procedures

This step is specific to the logic of how one APR DRG is defined, APR DRG 583 Neonate With ECMO (Extracorporeal Membrane Oxygenation). All of the patients who receive ECMO are severely ill but there are two subsets of ECMO patients, those who receive ECMO along with a major OR procedure for a congenital diaphragmatic hernia or heart condition and those who receive ECMO because conventional therapy has been unsuccessful at treating pulmonary hypertension and respiratory failure. To distinguish, those neonatal patients who do not have one of the major neonatal surgeries have their severity subclass decreased by one.

16. Modify the severity of illness subclass for the patient based upon combinations of APR DRG, principal diagnosis and non-OR procedure

Specific principal diagnoses within an APR DRG in combination with certain non-OR procedures will increase the severity of illness subclass by a specified increment up to a specified maximum severity of illness subclass. This step applies to a limited number of patients, mostly cancer patients receiving chemotherapy or radiation therapy. For example, patients with a principal diagnosis of malignancy in APR DRG 343 (Musculoskeletal Malignancy and Pathological Fracture Due To Musculoskeletal Malignancy) are increased by one level up to a maximum subclass of major if radiation therapy or chemotherapy is performed.

17. Establish a minimum severity of illness subclass for the patient based on the presence of specific combinations of categories of secondary diagnoses

The presence of certain combinations of secondary diagnoses has great clinical significance. The interaction of specific combinations of secondary diagnoses makes treatment more difficult and typically indicates a more extensive disease process. Therefore, a minimum patient severity of illness subclass greater than minor is established if certain combinations of secondary diagnoses are present. The presence of multiple interacting diagnoses is characteristic of high severity of illness patients. A subset of secondary diagnoses interact with each other causing patient severity of illness to be increased. All of the ICD-9-CM diagnosis codes were classified into either one of the 83 core secondary diagnosis categories applicable to all patients except MDC 15 (Newborns & Other Neonates with Conditions Originating in the Perinatal Period) or to one of the 21 secondary diagnosis categories applicable to a subset of MDC 15. The 83 core secondary diagnosis categories are shown in Table 2-3.

Each of these categories represents a disease process and is further subdivided by severity of illness level. The full numbering of the categories includes the two digits shown in table 2-3 plus a third digit for the severity of illness level of the secondary diagnoses in the category. To illustrate, secondary diagnosis category 15 Cerebrovascular Diagnoses includes diagnoses that span all four severity levels so the full numbering and titling is: 151 Cerebrovascular Diagnoses (1), e.g., cerebral atherosclerosis; 152 Cerebrovascular Diagnoses (2), e.g., occlusion and stenosis of pre-cerebral artery without cerebral infarction; 153 Cerebrovascular Diagnoses (3), e.g., occlusion and stenosis of pre-cerebral artery with cerebral infarction; and 154 Cerebrovascular Diagnoses (4), e.g., cerebral thrombosis with cerebral infarction. Not all secondary diagnosis categories contain four severity levels.

Some describe a disease process that has only three severity levels (e.g., Ear, Nose & Throat; Eye) or only two severity levels (e.g., Asthma; Hypertension). Others describe a more singular disease process that has only one severity level (e.g., Coronary Bypass Graft Status, Acute Myocardial Infarct, Hypovolemia). Altogether, the secondary diagnosis categories together with severity level breakouts contain 240 categories.

The secondary diagnosis categories for MDC 15 are shown in Table 2-4. These are intended for use with just two groups of MDC 15 patients: APR DRG 626 Neonate BWT 2000—2499 Grams, Normal Newborn Or Neonate With Other Problem and APR DRG 640 Neonate BWT >2500 Grams, Normal Newborn Or Neonate With Other Problem. The secondary diagnoses on this list are nearly all diagnoses with a severity of illness level of minor, so no further differentiation based on severity level is necessary. It is their purpose to distinguish newborns with multiple minor or other problems from those who are normal newborns or have a single minor problem. This is an important distinction because there is a very large case volume of these newborn patients.

As summarized in Table 2-5 there are nine different types of combinations of secondary diagnosis categories that will result in a minimum severity of illness subclass for a patient. For combination types 1 through 5, four significant secondary diagnoses are required in order to increase the severity of illness subclass of a patient. Two of the four secondary diagnoses must constitute one of the secondary diagnosis category combinations and must not have had their standard severity of illness level decreased as part of the Phase I severity level modifications. The addition of the third and fourth secondary diagnoses increases the likelihood that the specific combination of secondary diagnosis categories represents a more extensive and severe disease process.

Combination types 11, 13, and 15 only require a total of three significant secondary diagnoses, the two that make up the secondary diagnosis category combination and one additional secondary diagnosis. This reflects that the secondary diagnosis category combination is sufficiently significant that only one additional secondary diagnosis is required. Combination types 11, 13, and 15 are new starting with version 20.0 of the APR DRG system. Previous versions contained only types 1 through 6.

A type 1 combination consists of two secondary diagnosis categories that contain major severity of illness level diagnoses, plus any third and fourth secondary diagnosis that is at least a major severity of illness level. When a type 1 combination occurs, the minimum patient severity of illness subclass is extreme. An example of a type 1 combination is a major bacterial infection (category 9) with a major hematological/immunological diagnosis (category 44). If a diagnosis from both these categories is present plus at least two other secondary diagnoses that are at least a major severity of illness level, then the minimum patient severity of illness subclass will be extreme.

A type 2 combination is the same as a type one combination except that the two categories consist of a major severity of illness category and a moderate severity of illness category. An example of a type 2 combination is a major bacterial infection (category 9) and brain malignancy (category 11).

A type 3 combination consists of two categories that contain moderate severity of illness level diagnoses plus any third and fourth secondary diagnosis that is at least a moderate level. When a type 3 combination occurs, the minimum patient severity of illness subclass is major. An example of a type 3 combination is a moderate alcohol and drug abuse diagnosis (category 5) and a moderate electrolyte disorder except hypovolemia (category 34).

A type 4 combination consists of a moderate severity of illness category and a minor severity of illness category plus any third and fourth diagnosis that is at least a moderate severity of illness level. When a type 4 combination occurs, the minimum patient severity of illness subclass is major. An example of a type 4 combination is a moderate hematological/immunological diagnosis (category 44) and hypovolemia (category 51).

A type 5 combination consists of two categories that contain minor severity of illness level diagnoses plus two additional minor severity of illness level diagnoses. When a type 5 combination occurs the minimum patient severity of illness subclass is moderate. An example of a type 5 combination would be diabetes without mention of complication (category 30) and minor bacterial infection (category 9).

Combination type 6 is a special combination type for APR DRGs 626 and 640 to distinguish neonates with multiple "other problems," i.e., problems that are generally viewed as minor severity of illness but distinguish a neonate from being a normal newborn. An example is a neonate with transient tachypnea (category 920) and newborn feeding problem (category 911). These diagnoses have a minor severity of illness level, but are each increased to moderate for APR DRGs 626 and 640 per an earlier Phase I step, and together, as part of this step, result in the patient's severity subclass being increased to major for APR DRGs 626 and 640.

Combination types 11, 13, and 15 are new to version 20.0 and pertain mostly to multiple trauma patients and a handful of other patients such as transplant status patients. A type 11 combination consists of two secondary diagnosis categories that contain major severity of illness diagnoses, plus any third secondary diagnosis that is at least a major severity of illness. An example is a major severity of illness transplant status diagnosis (category 84) and a major TB, fungal or parasitic infection (category 74).

A type 13 combination consists of two secondary diagnosis categories that contain moderate severity of illness level diagnoses, plus any third secondary diagnosis that is at least a moderate severity of illness level. An example is a moderate cardiothoracic trauma diagnosis (category 18) and a moderate head and neck trauma with coma diagnosis (category 24).

A type 15 combination consists of two secondary diagnosis categories that contain minor severity of illness level diagnoses, plus any third secondary diagnosis that is at least a minor severity of illness level. An example is a minor severity of illness level head and neck trauma without coma diagnosis (category 43) and a minor severity of illness level pulmonary diagnosis (category 75).

18. Compute the final patient severity of illness subclass

The final patient severity of illness subclass is computed based on the Phase II base patient severity of illness subclass and the Phase III modified patient severity of illness subclasses. If all the Phase III modified severity subclasses are greater than or equal to the Phase II base severity of illness subclass, then the final severity of illness subclass is computed as the maximum of the Phase II and III severity subclasses. If all of the modified Phase III severity of illness subclasses are less than or equal to the Phase II base severity of illness subclass, then the final severity of illness subclass is computed as the Phase II base severity of illness subclass minus one.

If the Phase III modified severity of illness subclasses include modified severity of illness subclasses that are both greater and less than the Phase II based severity of illness subclass, then the modified Phase III subclass relating to procedures and combinations of secondary diagnoses will take priority in determining the final severity of illness subclass. The combination of the APR DRG and the final patient severity of illness subclass constitute the complete APR DRG description of the severity of illness of the patient.

Summary of APR DRG severity of illness subclass assignment logic

The following is a summary of the steps involved in computing the APR DRG severity of illness subclass of a patient.

Phase I—Determine the severity of illness level of each secondary diagnosis

1. Eliminate secondary diagnoses that are associated with the principal diagnosis.
2. Assign each secondary diagnosis its standard severity of illness level.
3. Modify the standard severity of illness level of each secondary diagnosis based on the age of the patient.
4. Modify the standard severity of illness level of each secondary diagnosis based on the principal diagnosis and the APR DRG to which the patient is assigned (applica�ble only to APR DRG 190 Acute Myocardial Infarct).
5. Modify the standard severity of illness level of each secondary diagnosis based on the APR DRG to which the patient is assigned.
6. Modify the standard severity of illness level of each secondary diagnosis based on the presence of certain non-OR procedures.

Phase II—Determine the base severity of illness subclass of the patient

7. Eliminate all secondary diagnoses that are in the same secondary diagnosis group except the secondary diagnosis with the highest severity of illness level.
8. Compute the base patient severity of illness subclass as the maximum of all the secondary diagnosis severity of illness levels.
9. If the base patient severity of illness subclass from Step 8 is major or extreme, then reduce the base patient severity of illness subclass to the next lower severity of ill�ness subclass unless there are multiple secondary diagnoses at a high severity of illness level.

Phase III—Determine the final severity of illness subclass of the patient

10. Modify the patient severity of illness subclass based on the APR DRG and principal diagnosis.
11. Modify the patient severity of illness subclass based on the APR DRG and age of the patient.
12. Modify the patient severity of illness subclass based on a combination of the APR DRG and the presence of certain non-OR procedures.

1. 13. Modify the patient severity of illness subclass based on the APR DRG and OR procedure.

13. Modify the patient severity of illness subclass based on combinations of APR DRGs and pairs of OR procedures.
14. Modify the patient severity of illness subclass based on the APR DRG 583 Neonate with ECMO and the presence/absence of certain OR procedures.
15. Modify the patient severity of illness subclass based on the combination of APR DRG and principal diagnosis and the presence of certain non-OR procedures.
16. Establish a minimum severity of illness subclass for the patient based on the pres�ence of specific combinations of categories of secondary diagnoses.
17. Compute the final patient severity of illness subclass based on the Phase II base patient severity of illness subclass from Step 9 and the modifications of the patient severity of illness subclasses from Steps 10-17.

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