Failure of A Novel, Rapid Antigen and Antibody Combination Test to Detect Antigen-Positive HIV Infection in African Adults with Early HIV Infection

BACKGROUND: Acute HIV infection (prior to antibody seroconversion) represents a high-risk window for HIV transmission. Development of a test to detect acute infection at the point-of-care is urgent. METHODS: Volunteers enrolled in a prospective study of HIV incidence in four African cities, Kigali in Rwanda and Ndola, Kitwe and Lusaka in Zambia, were tested regularly for HIV by rapid antibody test and p24 antigen ELISA. Five subgroups of samples were also tested by the Determine Ag/Ab Combo test 1) Antigen positive, antibody negative (acute infection); 2) Antigen positive, antibody positive; 3) Antigen negative, antibody positive; 4) Antigen negative, antibody negative; and 5) Antigen false positive, antibody negative (HIV uninfected). A sixth group included serial dilutions from a p24 antigen-positive control sample. Combo test results were reported as antigen positive, antibody positive, or both. RESULTS: Of 34 group 1 samples with VL between 5x105 and >1.5x107 copies/mL (median 3.5x106), 1 (2.9%) was detected by the Combo antigen component, 7 (20.6%) others were positive by the Combo antibody component. No group 2 samples were antigen positive by the Combo test (0/18). Sensitivity of the Combo antigen test was therefore 1.9% (1/52, 95% CI 0.0, 9.9). One false positive Combo antibody result (1/30, 3.3%) was observed in group 4. No false-positive Combo antigen results were observed. The Combo antigen test was positive in group 6 at concentrations of 80 pg/mL, faintly positive at 40 and 20 pg/mL, and negative thereafter. The p24 ELISA antigen test remained positive at 5 pg/mL. CONCLUSIONS: Although the antibody component of the Combo test detected antibodies to HIV earlier than the comparison antibody tests used, less than 2% of the cases of antigen-positive HIV infection were detected by the Combo antigen component. The development of a rapid point-of-care test to diagnose acute HIV infection remains an urgent goal

Comparison of the p24 ELISA (Vironostika) and Combo test (antigen component) against serially diluted p24 antigen-positive controls.

*<p>Average OD (Optical Density) of two runs. An OD ≥0.10 is considered a positive result.</p

Performance of the Determine Ag/Ab Combo test.

<p>Group 1: Antigen positive, antibody negative acute HIV infection (Ag+Ab−).</p><p>Group 2: Antigen positive, antibody positive early HIV infection (Ag+Ab+).</p><p>Group 3: Antigen negative, antibody positive chronic HIV infection (Ag−Ab+).</p><p>Group 4: Antigen negative, antibody negative volunteers without HIV infection (Ag−Ab−).</p><p>Group 5: Antigen false positive, antibody negative volunteers without HIV infection (Ag−Ab−).</p><p>Ag: p24 antigen, Ab: HIV antibody, weak pos: the respective assay indicator was fainter than the control indicator. This is considered “positive” in our analyses, as per the package insert.</p

ELISA p24 Antigen Concentration and Viral Load Summary Statistics.

*<p>Upper limit of detection for the VL assay  = 15,000,000, any observation that exceeded 15,000,000 recorded as 15,000,001.</p><p>Group 1: Antigen positive, antibody negative i.e. acute HIV infection (Ag+ Ab−).</p><p>Group 2: Antigen positive, antibody positive i.e. early HIV infection (Ag+ Ab+).</p