Table 2.

Sensitivity, specificity, positive and negative predictive values of World Health Organization (WHO) immunological criteria for virological failure of antiretroviral therapy

Definition of virological failure No. of CD4 count and viral load measurements meeting criteria*	TP	FN	FP	TN	Sensitivity, % (95% CI)	Specificity, % (95% CI)	Positive predictive value, % (95% CI)	Negative predictive value, % (95% CI)
≥10 000 copies/ml
Based on two measurements meeting criteria	6	29	57	1917	17.1 (6.6–33.6)	97.1 (96.3–97.8)	9.5 (3.6–19.6)	98.5 (97.9–99.0)
Based on one measurement meeting criteria	76	82	245	1606	48.1 (40.1–56.2)	86.8 (85.1–88.3)	23.7 (19.1–28.7)	95.1 (94.0–96.1)
≥500 copies/ml
Based on two measurements meeting criteria	12	83	51	1863	12.6 (6.7–21.0)	97.3 (96.5–98.0)	19.0 (10.2–30.9)	95.7 (94.7–96.6)
Based on one measurement meeting criteria	92	195	229	1493	32.1 (26.7–37.8)	86.7 (85.0–88.3)	28.7 (23.8–33.9)	88.4 (86.8–89.9)

Percentages with binomial exact confidence intervals are shown. TP, true positive; FN, false negative; FP, false positive; TN, true negative.

^*Criteria for virological failure as shown in table, criteria for immunological failure as defined by WHO: decline in the CD4 cell count to the baseline value or below, a decline of at least 50% from the highest count on treatment or a persistent CD4 cell count below 100 cells/μl after six months of ART (World Health Organization 2006).