Commercial laboratory IgM testing for Toxoplasma gondii in pregnancy: a 20-year experience

This study was performed to review the clinical utility of commercial laboratory Toxoplasmosis-specific IgM testing during pregnancy and outcomes of the gestation at our institution.

A retrospective review of all women referred for suspected acute Toxoplasma gondii infection during pregnancy from 1984 through 2004 was performed. Women were diagnosed with suspected acute toxoplasmosis based on commercial laboratory serologic antibody testing. All women had blood sent to a recognized reference laboratory for antibody testing within 2 weeks of the commercial laboratory results. The study protocol was approved by the Institutional Review Board. Chi-square analysis were used with a significance of P < .05.

A total of 130 women were evaluated during the study period with 116 IgM positive results from the commercial laboratories. The commercial laboratory antibodies were as follows: IgM positive with IgG negative (n = 20), IgM positive with IgG positive (n = 96), and IgM negative with IgG positive (n = 14). There was a significant reduction in the IgM positive results when comparing commercial laboratory (n = 116) with the reference laboratory results (n = 28; p < .001). Acute toxoplasmosis infection was diagnosed in 7 (5%) of the women. All cases of acute toxoplasmosis infection had a positive commercial laboratory IgM result. The false positive rate for the commercial laboratory IgM was 88.6% and the diagnostic indices were sensitivity 100%, specificity 11.4%, positive predictive value 6% and negative predictive value 100%.

Commercial laboratory Toxoplasmosis-specific IgM is associated with a high false positive rate. The commercial and reference laboratory IgM results identified all cases of acute toxoplasmosis infection. Commercial laboratories reflexively obtaining reference laboratory confirmation of positive results could reduce costs associated with testing, referrals, retesting, and invasive procedures.