Detection of circulating antigens in immunocompromised patients during reactivation of chronic toxoplasmosis.

One hundred and fifteen sera from nineteen patients undergoing bone marrow transplant and four recipients of kidney transplant who showed serological or clinical reactivation of chronic toxoplasmosis were tested for Toxoplasma gondii circulating antigen (TCA) by enzyme-linked immunosorbent assay using antitoxoplasma IgG antibody coupled to alkaline phosphatase. Seven bone marrow transplant patients and one kidney transplant recipient were TCA positive either before or at the time of antitoxoplasma IgG antibody increase. TCA continued to be detected in one patient with neurological toxoplasmosis until his death. In the other patients, TCA disappeared when IgG antibodies rose, probably due to the formation of immunocomplexes consisting of TCA and immunoglobulins. In the TCA seronegative patients, the presence of circulating immunocomplexes or TCA kinetics of short duration may explain these results. Patients receiving immunosuppressive therapy should be tested for TCA.