RESUMEN
Interpretation of IgM anti-Toxoplasma can be problematic given the phenomena of "natural" IgM. We analyzed 1,503 sera obtained during prenatal care program, and we established natural and false-positive results by doing follow-up. In 101 samples the concordance between enzyme linked immunosorbent assay (ELISA) and two semi-automatized systems: electro-chemiluminescence immunoassay (ELECSYS) and vitek immunodiagnostic assay system (VIDAS) was calculated. In 1,503 serum, 71 (4,7%) had ELISA IgG negative and ELISA IgM positive results and in 44 of these had a second sample 4 weeks later. In second samples, 27 (61,3 %) were IgM and IgG negative (false positive result in the first sample) and 13 (29,5%) were ELISA IgM positive and IgG negative (natural IgM). ELISA assay had a poor concordance with enzyme-linked fluorescent immunoassay as well as ELECSYS tests, contrarily, enzyme-linked fluorescent immunoassay and ELECSYS had optimal concordance, with 100 of 101 sera obtaining the same result by both tests. We recommended to use automatized assays to measure IgM anti-Toxoplasma.