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Pooled NAAT Following Various HIV Antibody Tests

Pooled NAAT can improve detection of HIV infection, even when highly sensitive third-generation immunoassays are used for initial antibody testing.

As HIV antibody tests become increasingly sensitive, they detect earlier stages of HIV infection and thus more infections overall. However, these tests still miss the newest infections and cannot distinguish acute from chronic infection — limitations that are troubling, given the public health importance of rapidly identifying and managing people with acute HIV infection (AHI). One strategy for overcoming these limitations is to perform pooled nucleic-acid amplification testing (NAAT) on antibody-negative specimens. Previous studies have shown that this strategy can substantially improve detection of AHI (JW AIDS Clin Care Jan 11 2010). However, in most of these studies, the initial antibody testing was done with less-sensitive, earlier versions of EIAs. In the present study, investigators hypothesized that the yield of pooled NAAT for HIV diagnosis would be lower with a third-generation EIA than with first- or second-generation EIAs, because the third-generation EIA detects a higher proportion of early infections.

The study took place in Los Angeles, New York, and Florida, mostly in clinics that served high-risk populations. The Los Angeles clinics used a relatively insensitive first-generation EIA that is no longer available in the U.S., the New York clinics used a second-generation EIA, and the Florida clinics used a third-generation EIA. If a sample tested negative on first- or second-generation EIA but positive on NAAT, it was retested using the third-generation EIA, so that investigators could calculate the percentage of HIV infections missed by the less-sensitive assays.

During the 2-year study period, 99,111 people underwent HIV testing in the three clinic populations. Fifty-five cases of AHI were identified through pooled NAAT (positive predictive value, 98.0%; specificity, 99.9%). Half the AHI cases that had tested negative for HIV infection on first- or second-generation EIAs had positive third-generation EIA results and would likely have been identified during initial screening if the more-sensitive third-generation assay had been used. Overall, pooled NAAT increased HIV detection by 8.2% when a first-generation EIA was used — but only 2.2% when a third-generation EIA was used. Nonetheless, the yield of pooled NAAT following third-generation EIA was 11.9% in the clinic with the highest prevalence of HIV infection.

Comment: As early detection of HIV infection improves with more-sensitive EIAs, the benefits of subsequent pooled NAAT decline. However, these study results suggest that adding pooled NAAT to a third-generation EIA remains beneficial in populations with a high incidence of HIV infection. Improved EIAs enhance overall HIV diagnoses but diminish the number of people who are diagnosed with AHI overall, an outcome with public health implications, given the high risk of HIV transmission during AHI.

— Anna K. Person, MD, and Charles B. Hicks, MD

Dr. Person is a Fellow in the Division of Infectious Diseases at Duke University Medical Center.

Published in Journal Watch HIV/AIDS Clinical Care January 25, 2010

Citation(s):

Patel P et al. Detecting acute human immunodeficiency virus infection using 3 different screening immunoassays and nucleic acid amplification testing for human immunodeficiency virus RNA, 2006-2008. Arch Intern Med 2010 Jan 11; 170:66.

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