Difficulty in determining the ABO blood group led to the discovery of an anti-I cold auto-agglutinin in the serum of a blood donor. The peculiarity of this antibody is that its activity is enhanced in the presence of a preservative found in manufactured anti-sera: sodium azide. At 4°C, the auto-antibody agglutinates RBCs even without NaN(3) but the drug increases its effect in the cold. At 22°C, the drug is necessary for the adsorption of the antibody on red cells.

One serum from a blood donor, which had both HIY-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.