False negativity by an anti-HIV assay kit (IMx 8B32) and evaluation of its replacement (IMx 8C98).

False negativity in a commercial anti-HIV kit (IMx HIV-1/HIV-2 3rd Generation Plus (code 8B32) was investigated, and the kit that superseded it (IMx HIV-1/HIV-2 III Plus, code 8C98) was evaluated. In a comparison on 574 freshly collected anti-HIV-1-positive specimens, 97.2% were more reactive in 8C98 than in 8B32; 35.5% were more than twice as reactive and 8.5% were more than four times as reactive. In 8B32, the signal from 55 specimens selected because of weak reactivity was enhanced 1.5 to 8.8 times by preliminary heating at 56 degrees C for 30 min. The reactivity of the 55 heated sera was then similar to that of the same specimens tested without heat treatment in the 8C98 assay. Reactivity in 8B32 was also increased in 66 of 76 (at least twofold in 20) randomly chosen anti-HIV-positive serum specimens by the addition of EDTA (10 mM final concentration). One of these specimens was false negative (signal:cutoff (S:CO) ratio 0.76) in 8B32, though its reactivity was restored by addition of EDTA (S:CO ratio 9.54). These findings indicate that the inhibitory effect that originally led to false negative findings in 8B32 was probably due to complement activity, and that the same activity was present in the freshly collected specimens used here to evaluate the replacement IMx anti-HIV assay (8C98). The specimen panel employed to evaluate 8C98 included 1,892 anti-HIV-positive and 779 anti-HIV-negative specimens. There were no false negative reactions. The lowest S:CO ratio observed was 6.2 and only 17 (0.2%) anti-HIV-positive specimens gave ratios less than 10. Nine unreproducible false positive reactions arose, all possibly attributable to specimen carryover by the IMx instrument. The performance of 8C98 was also compared with that of 10 other current anti-HIV kits using 21 sets of seroconversion specimens (127 specimens in total), and five performance assessment panels (92 specimens in total) comprised mostly of single bleeds from recent seroconverters. IMx 8C98 was the second most sensitive assay. We found no evidence that the 8C98 kit was prone to the effect that had given rise to false negative results in its predecessor (8B32).