The value of CMV IgG avidity and immunoblot for timing the onset of primary CMV infection in pregnancy

Background: Primary CMV infections in pregnancy are usually asymptomatic and only detected by serology. Estimating the onset of infection is a major diagnostic goal, since primary infections around conception and in early gestation hold a higher risk for congenital disease than those in later pregnancy. Objectives: To assess the ability of serological supplementary CMV assays to date the onset of primary infection. Study design: From our routine diagnosis we identified 61 pregnant women (n = 188 serum samples) with precisely determined onset of CMV primary infection either by IgG seroconversion (n = 24) or by significant IgG antibody rise (n=37). One hundred and forty-seven sera were investigated using the VIDAS® CMV IgG avidity EIA (BioMerieux) and 83 sera using the recomBlot CMV IgG with avidity (Mikrogen). Results: Both assays proofed to be reliable in terms of timing the onset of CMV primary infection. An avidity index (AI) in the VIDAS avidity EIA of <40% indicated primary infection within the last 20 weeks (positive predictive value 93.4%; 99/106), whereas an intermediate AI excluded primary infection within the last 12 weeks (negative predictive value 88.2%; 15/17). The recomBlot showed high reliability (PPV 96.9%; 31/33) for timing the onset of infection within the last 14 weeks. Avidity testing by blot however could not be interpreted in 11 of 47 sera (23.4%). Conclusion: For timing the onset of infection (before or in early pregnancy) CMV avidity testing is most helpful if carried out within the first trimester up to the beginning of second trimester.