Revision of the positive predictive value of IgM anti-Toxoplasma antibodies as an index of recent infection

The severity of congenital Toxoplasma gondii infection underlines the need for a precise diagnosis of acute infection during pregnancy. The search for specific IgM has been widely used for this purpose, but their possible early disappearance or persistence over time limits their meaning. In order to estimate the positive predictive value of anti-Toxoplasma IgM testing, we made an epidemiological analysis of the presence of anti-Toxoplasma IgG and IgM using ELISA in 4786 subjects attending the Hospital of Legnano in 2004-2005: 1360 seen for a clinical check-up and 3426 pregnant women for serological screening. In relation to IgG avidity, the positive predictive value of IgM was 45.98% (95% CI: 35.51-56.45) as a whole: this increased to 83.87% (95% CI: 70.92-96.82) in the patients with a highly positive test for IgM, but decreased to 9.52% (95% CI: 0.00-22.07) in pregnant women with a weakly positive test for IgM. Our results indicate that a highly positive IgM value in patients can be a good index of recent infection, but its poor predictive value in pregnant women underlines the need for additional tests with a follow-up if necessary.