Background: Human cytomegalovirus (CMV) is the most common cause of viral intrauterine infection. Primary CMV infection in early pregnancy bears a high risk of fetal damage. Accurate measurement of CMV-specific IgG avidity may help to improve the serodiagnosis of CMV-infected women by determining the time of infection and fetal outcome.
Objectives: To study the performance of the CMV avidity assay with the fully automated Vidas analyzer (bioMérieux) as a function of the concentration of CMV-specific IgG present in the serum sample.
Study design: Eighty-two serum samples were investigated from 3 clinical scenarios: 18 individuals with sera negative for CMV-specific IgG and IgM (control group), 20 pregnant women (44 samples) containing CMV-specific IgG- and IgM-antibodies suggesting acute or recent primary infection and 20 patients with evidence of past infection (CMV-IgG positive and CMV-IgM negative).
Results: In the group with presumed acute or recent primary infection 12 of 44 sera had CMV-specific IgG values above 100 arbitrary units (AU, bioMérieux)/ml and in these cases an increase in AI was measurable upon dilution of the serum sample. In two cases, AI's were shifted towards or above the cut-off value of AI>or=0.8, indicative of past infection. Dilution of sera which were CMV-specific IgM positive and had specific IgG concentrations of <or=100 AU/ml produced only minor changes in AI. In individuals with serology indicative of past infection, dilution effects similar to the group of acute or recently infected individuals were observed and if CMV-specific IgG was above 100 AU/ml an increase in AI to above or equal to the cut-off could only be calculated after serum dilution.
Conclusions: The results obtained from undiluted patients' sera with high CMV-IgG concentrations indicate that falsely low avidity indices are obtained if these sera are not diluted to below an empirically determined CMV-specific IgG concentration. In addition, the cut-off value for this commercial CMV-IgG avidity assay should be revised.