Appendix: Outcomes Table Methodology

As described in the earlier report,^29-30 we performed this computation with 2 populations at high risk for preterm delivery, based on the placebo groups' baseline risk for delivery before 37 weeks. The general high-risk population has a baseline risk of less than 30% for delivery before 37 weeks, whereas the more selected high-risk population has a baseline risk of greater than 30%. In the outcomes table, the case for the general high-risk population incorporates the mean and 95% CIs from 2 high-risk studies in which approximately 23% of the bacterial vaginosis–positive women in the placebo group delivered before 37 weeks^50,57 for the listed outcomes. The second scenario, for the more selected high-risk population, incorporates the pooled results of the other 3 high-risk studies,^58-60 in which the percentage of bacterial vaginosis–positive women in the placebo groups who delivered before 37 weeks is greater than 30% (Table 2). The effect sizes of treatment on bacterial vaginosis–positive pregnant women are therefore based on this review.

We assumed the prevalence of unsuspected bacterial vaginosis to be 25%, both screening test sensitivity and specificity to be 95%, and adherence to treatment to be 80%. The prevalence of bacterial vaginosis in asymptomatic pregnant women has ranged from 9% to 23% in several large prospective studies.^11-13,21-33 Because the outcomes table presents estimates based on high-risk women, it is reasonable to assume that the prevalence of bacterial vaginosis is somewhat higher for this group. We see this as a realistic estimate of prevalence in this population, although it is not directly derived from the literature. For sensitivity and specificity, we assumed a high-quality screening test. We performed a sensitivity analysis to assess the influence of the alternative assumptions of lower sensitivity and specificity on the calculated benefits and harms as well.

Appendix Figure 1 provides an example of the calculations performed for the outcomes table, using the outcome of delivery before 34 weeks in the more selected high-risk population.

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