A novel simple immunoassay for rapid detection of human IgG antibodies to Toxoplasma gondii.

The carbon immunoassay (CIA) is a direct serological test relying on a specific reaction between the carbon particles of India ink and rabbit immunoglobulin G. On addition of the India ink reagent, particulate antigens may be visualized in the light microscope after rabbit antibodies have reacted with them. Serum, India ink reagent and antigen may be mixed together in a single step and the result read within 5 min. The applicability of CIA can be expanded to include human sera by pretreatment of India ink with staphylococcal protein A. Sera from 50 suspected toxoplasmosis patients were examined by immunofluorescence (IF), the dye test of Sabin-Feldman (SF) and CIA. There was good correlation between results by the CIA and IF. Sera with low SF titres were usually negative by IF and negative or weakly positive by CIA. With sera positive by all 3 tests, CIA and IF gave similar titres, but SF titres were generally higher. CIA provides a simple inexpensive and rapid laboratory test for diagnosis of toxoplasmosis.