Difficulties associated with serological diagnosis of Toxoplasma gondii infections.

Physicians often rely on serology to help determine whether a patient has had a recent infection with Toxoplasma gondii and as an aid in estimating the possible teratogenic effect on the fetus. For this reason the diagnostic laboratory should take every precaution to avoid misleading results. The best serological analysis is based on a rise in IgG titer with two appropriately spaced serum samples. Also, the presence of a high IgM titer in one serum sample is generally considered to be good evidence that infection has occurred recently. The indirect fluorescent antibody (IFA) test has been the most widely used test for detection of IgG or IgM. Recently enzyme-linked immunosorbent assays (ELISA) have also been developed for this purpose. In this study we reaffirm that false IgM positive results can occur with these tests because of the presence of rheumatoid factor in serum, and false negative results can also occur because of competitive inhibition by specific IgG. We show that a preabsorption of serum with a Staphylococcus/Streptococcus preparation (Staffinoc, MA Bioproducts, Walkersville, MD) removes IgG and IgA and eliminates many of the false reactions. We have also found that elevated levels of specific IgM can persist for at least several years in some women. This suggests that the presence of IgM alone is not always an indication of recent infection.