Evaluation of an expanded two-ELISA approach for confirmation of reactive serum samples in an HIV-screening programme for pregnant women

Serum specimens were collected from 31,232 pregnant women in Amsterdam between 1988 and 1995 in a screening programme for human deficiency virus (HIV) infection. The sera of 56 (0.179%) women tested were confirmed as positive for HIV. A total of 67 sera reacted positive or borderline by the screening enzyme-linked immunosorbent assay (ELISA) and indeterminate or negative by HIV-1 Western blot; 42 of these specimens were available for evaluation of the strategy for diagnosis of HIV infection. A two-ELISA approach with the second ELISA based on a principle different from that of the screening ELISA, expanded with the use of a membrane immunoassay based on two synthetic peptides specific for HIV-1 gp41 and HIV-2 gp36 envelope proteins, was compared with the Western blot analysis. Indeterminate results were resolved with a nucleic acid sequence-based amplification assay (NASBA) for HIV-1 RNA and a strip immunoassay (SIA) for the simultaneous detection of antibodies to HIV-1 or HIV-2 and HIV-1 p24 antigen. Eleven samples were weakly or borderline positive by the screening test and gave indeterminate results by Western blot. The expanded two-ELISA approach designated these sera as HIV-negative, and confirmed negative by NASBA and the SIA. Twenty-one samples showed borderline or positive results on the screening test and negative results by Western blot. Again, these sera were characterised as HIV-negative by the expanded two-ELISA procedure, and this characterisation was confirmed by both NASBA and the SIA. Five HIV-2-positive serum samples were recognised by the expanded two-ELISA approach and the SIA; these sera were negative by NASBA. Finally, another five serum samples were weakly or borderline positive by both ELISAs and positive by the membrane immunoassay; of these five, two sera generated positive patterns and the other three indeterminate patterns on Western blots, and four were positive by the NASBA assay. Follow-up serum specimens from these five women were negative and the reactivity of the initial specimens was thus likely to have been the result of cross-contamination. Our results demonstrate the effectiveness of a simple confirmation approach of two HIV ELISAs expanded with a membrane spot assay to discriminate between infection with HIV-1 or HIV-2. The data also indicate the importance of retesting individuals with indeterminate or positive confirmational results to exclude the possibility of contamination as the cause of reactivity of the original specimen.