Antenatal screening for Group B Streptococcus: A diagnostic cohort study

Table 3 Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), positive and negative likelihood ratios with 95% confidence intervals for screening at 32 weeks or 36 weeks, using low vaginal (LVS) and/or perianal (PAS) swabs for 600 women and for using a risk factor strategy*.

	LVS		PAS		Either		LVS		PAS		Either
	32 Weeks (n = 600)						36 Weeks (n = 600)						(n= 699)
	Estimate + 95% CI		Estimate + 95% CI		Estimate + 95% CI		Estimate + 95% CI		Estimate + 95% CI		Estimate + 95% CI		Estimate + 95% CI
Sensitivity	63	54–71	70	61–77	72	63–79	73	65–81	76	67–83	81	73–87	19	13–26
Specificity	94	92–96	94	91–96	93	90–95	95	93–97	94	91–96	93	90–95	83	79–86
PPV*	77	68–84	76	68–84	75	66–82	82	74–88	78	70–85	77	69–84	25	18–34
NPV	90	87–92	91	89–94	92	89–94	92	90–95	93	90–95	94	92–96	77	73–80
Likelihood ratio (+ve)	11.3	7.6–16.7	11.2	7.7–16.2	10.1	7.2–14.3	15.5	10.2-23.6	12.1	8.4-17.5	11.7	8.3-16.6	1.10	0.76-1.59
Likelihood ratio (-ve)	0.39	0.31–0.49	0.32	0.25–0.42	0.30	0.23–0.40	0.28	0.21–0.37	0.26	0.19–0.35	0.21	0.15–0.29	0.98	0.90–1.06
Diagnostic Odds Ratio	28.7	16.4–50.6	34.5	19.8–60.4	33.4	19.4–57.9	55.4	30.1–103	46.5	26.1–83.1	56.7	31.4–103	1.12	0.69–1.79

* Risk factor strategy included women with GBS bacteruria at booking, birth at <37 weeks' gestation, prelabour rupture of membranes or temperature during labour of greater than or equal to 38 degrees Centrigrade.

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