Method for avoiding false-positive results occurring in immunoglobulin M enzyme-linked immunosorbent assays due to presence of both rheumatoid factor and antinuclear antibodies.

In a double-sandwich immunoglobulin M (IgM) enzyme-linked immunosorbent assay recently developed for the detection of IgM antibodies to Toxoplasma, the presence of either rheumatoid factor or antinuclear antibodies did not cause false-positive results. We recently noted, however, that in certain sera containing both rheumatoid factor and antinuclear antibodies, false-positive results do occur. In experiments to define the nature of the cross-reaction, these false-positive results were not found to be a consequence of interactions of the sera with Toxoplasma antigens, but rather were due to interactions of rheumatoid factor-antinuclear antibodies with the Fc portion of IgG antibodies used for the enzyme conjugate. This was avoided when the F(ab')2 fragment of IgG was used for the conjugate. The use of such F(ab')2 conjugates did not affect the sensitivity and, thereby, the usefulness of the double-sandwich IgM enzyme-linked immunosorbent assay for the diagnosis of acute acquired or congenital Toxoplasma infections. We concluded that F(ab')2 fragments of IgG antibodies can be used as enzyme conjugates to avoid false-positive results in sera positive for both rheumatoid factor and antinuclear antibodies in either the conventional test or in our double-sandwich IgM enzyme-linked immunosorbent assay.