Low specificity of determine HIV1/2 RDT using whole blood in south west Tanzania

Abstract

Objective: To evaluate the diagnostic performance of two rapid detection tests (RDTs) for HIV 1/2 in plasma and in whole blood samples.

Methods: More than 15,000 study subjects above the age of two years participated in two rounds of a cohort study to determine the prevalence of HIV. HIV testing was performed using the Determine HIV 1/2 test (Abbott) in the first (2006/2007) and the HIV 1/2 STAT-PAK Dipstick Assay (Chembio) in the second round (2007/2008) of the survey. Positive results were classified into faint and strong bands depending on the visual appearance of the test strip and confirmed by ELISA and Western blot.

Results: The sensitivity and specificity of the Determine RDT were 100% (95% confidence interval= 86.8 to 100%) and 96.8% (95.9 to 97.6%) in whole blood and 100% (99.7 to 100%) and 97.9% (97.6 to 98.1%) in plasma respectively. Specificity was highly dependent on the tested sample type: when using whole blood, 67.1% of positive results were false positive, as opposed to 17.4% in plasma. Test strips with only faint positive bands were more often false positive than strips showing strong bands and were more common in whole blood than in plasma. Evaluation of the STAT-PAK RDT in plasma during the second year resulted in a sensitivity of 99.7% (99.1 to 99.9%) and a specificity of 99.3% (99.1 to 99.4%) with 6.9% of the positive results being false.

Conclusions: Our study shows that the Determine HIV 1/2 strip test with its high sensitivity is an excellent tool to screen for HIV infection, but that--at least in our setting--it can not be recommended as a confirmatory test in VCT campaigns where whole blood is used.

Figure 1. HIV-Testing Algorithm and Exclusion criteria Survey 1.

In Survey 1 we tested 1,889 whole blood specimen and 14,448 plasma specimen with the Determine HIV1/2 RDT. The confirmation algorithm included two different ELISAs and one Western Blot. If both ELISAs were in agreement their result was used as the reference standard result. Samples with discordant ELISA results were retested by Western Blot and the Western Blot result used. Samples with indeterminate Western Blot results were excluded from analysis. Negative RDT results were not directly confirmed, but regarded as true negative if the result of the following survey was also negative. Whole blood results where confirmation by ELISA testing was impossible due to lack of plasma, were regarded as true positive if the result in the next survey was confirmed positive and as false positive if a negative test result in the next survey was confirmed using the above reference algorithm. Results where the true HIV status could not be verified according to the reference algorithm were excluded from this analysis.

Figure 2. HIV-Testing Algorithm and Exclusion criteria Survey 2.

In Survey 2 we tested only plasma samples from 15,957 participants using the STAT-PAK-HIV1/2 RDT. The confirmation algorithm was essentially the same as in Survey 1.It included two different ELISAs and one Western Blot for RDT positive samples. Participants, who’s positive RDT result had been confirmed in Survey 1 already were not re-tested. 319 negative RDT results were directly confirmed by Behring ELISA, 11563 were regarded as true negative because the result of the following survey was also negative.