Diagnostic accuracy of fourth-generation ARCHITECT HIV Ag/Ab Combo assay and utility of signal-to-cutoff ratio to predict false-positive HIV tests in pregnancy

Abstract Background False positive HIV screening tests in pregnancy may lead to unnecessary interventions in labor. In 2014, the Centers for Disease Control and Prevention released a new algorithm for HIV diagnosis using a 4 th generation screening test, which detects antibodies to HIV as well as p24 antigen and has a shorter window period compared with prior generations. A reactive screen requires a differentiation assay, and supplemental qualitative RNA testing is necessary for nonreactive differentiation assay. One screening test, the ARCHITECT Ag/Ab Combo assay, is described to have 100% sensitivity and >99% specificity in non-pregnant populations; however, its clinical performance in pregnancy has not been well described. Objective To determine the performance of the ARCHITECT assay among pregnant women at a large county hospital and to assess if the relative signal-to-cutoff ratio can be used to differentiate between false positive versus confirmed HIV infections in women with nonreactive differentiation assay. Study Design This is a retrospective review of 4 th generation HIV testing in pregnant women at Parkland Hospital between June 1, 2015 and January 31, 2017. We identified gravidas screened using the ARCHITECT Ag/Ab Combo assay (index test), with reflex to differentiation assay. Women with reactive ARCHITECT and nonreactive differentiation assay were evaluated with a qualitative RNA assay (reference standard). We calculated sensitivity, specificity, predictive value, and false positive rate of the ARCHITECT screening assay in our population, and described characteristics of women with false positive HIV testing versus confirmed infection. Among women with nonreactive differentiation assay, we compared interventions among women with and without qualitative RNA assay result available at delivery, and examined relative signal-to-cutoff ratios of the ARCHITECT assay in women with false positive versus confirmed HIV infection. Results A total of 21,163 pregnant women were screened using the ARCHITECT assay, and 190 tested positive. Of these, 33/190 (17%) women had false positive HIV screening tests (28 deliveries available for analysis), and 157/190 (83%) had confirmed HIV-1 infection (140 available for analysis). Diagnostic accuracy of the ARCHITECT HIV Ag/Ab Combo assay in our prenatal population (with 95%CI) was as follows: sensitivity 100%(97.7%-100%), specificity 99.8%(99.8%-99.9%), positive likelihood ratio 636(453-895), negative likelihood ratio 0.0(NA), positive predictive value 83%(77%-88%), and false positive rate 0.16%(0.11%-0.22%), with prevalence 7/1,000. Women with false positive HIV testing were younger and more likely of Hispanic ethnicity. Qualitative RNA assay (reference standard) was performed prenatally in 24(86%) and quantitative viral load in 22(92%). Interventions occurred more frequently in women without qualitative RNA assay result available at delivery, including intrapartum zidovudine (75% vs 4%, p=0.002), breastfeeding delay (75% vs 8%, p=0.001) and neonatal zidovudine initiation (75% vs 4%, p=0.002). ARCHITECT signal-to-cutoff ratio was significantly lower for women with false positive HIV tests compared to those with established HIV infection (1.89 [1.27, 2.73] vs 533.65 [391.12, 737.22], respectively, p Conclusion While the performance of the 4 th generation ARCHITECT HIV Ag/Ab Combo assay among pregnant women is comparable to that reported in non-pregnant populations, clinical implications of using a screening test with positive predictive value of 83% in pregnancy are significant. When qualitative RNA assay result is unavailable, absence of risk factors in combination with ARCHITECT HIV Ag/Ab assay S/Co ratio