Utility of quantitative enzyme immunoassay reactivity for predicting human immunodeficiency virus seropositivity in low- and high-prevalence populations.

To assess the utility of quantitative enzyme immunoassay (EIA) reactivity for predicting human immunodeficiency virus seropositivity, we evaluated 22,823 serum samples from homo- and bisexual men, heterosexual intravenous drug users, and other heterosexuals with initial screening by EIA, retesting of reactive samples in duplicate, and confirmatory Western blot (immunoblot) testing. Quantitative EIA reactivity was determined by a mean of the optical density ratio of the three assays performed for each reactive specimen. A total of 1,773 samples (7.8%) were repeatedly reactive, and 1,747 (7.7%) were confirmed Western blot positive. All 26 EIA-reactive-Western blot-negative samples had low-level EIA reactivity (ratio < 2.2), while most (86%) of the Western blot-positive samples had high-level reactivity (ratio, > 3.0). The positive predictive value for samples with moderate-to-high-level EIA reactivity (ratio, > 2.2) was 100% for all risk groups. These results support the value of quantitative EIA reactivity in predicting human immunodeficiency virus seropositivity and suggest that confirmatory testing of specimens with high-level reactivity is not necessary in all situations.