False negative results in a competitive immunoassay for anti-HIV-antibodies due to the presence of rheumatoid factors.

One serum from a blood donor, which had both HIV-antibodies and rheumatoid factor, gave positive results with EIA anti-HIV screening tests and Western blot, but negative results with the Abbott's ENVACOR assay. Moreover, this serum and 5 out of 6 sera containing only rheumatoid factors were able to interfere with ENV and CORE reactions. Rheumatoid factors bind to the beads used in ENVACOR test and are also able to react with the conjugate. Most of the rheumatoid factors bind to the ENV conjugate, though with a wide range of activity. Obvious binding of rheumatoid factors to the CORE conjugate could not be demonstrated. Binding of rheumatoid factors both to the bead and to the conjugate leads to high optical density values and therefore results in the negative range according to the normal interpretation of the ENVACOR competition procedure.