The relationship between cytomegalovirus IgM index value and low IgG avidity

<b>Background:</b> Serological markers like anti-CMV IgG, IgM, and IgG avidity are used to diagnose cytomegalovirus (CMV) infection. This study assessed the seroprevalence of CMV antibodies and the predictive value of the CMV IgM index for low IgG avidity.<br /> <b>Materials and methods:</b> A total of 5,666 serum samples collected between January 2017 and June 2021 were analyzed for CMV IgG and IgM using chemiluminescence immunoassay. IgG avidity was tested by enzyme-linked immunosorbent assay in 40 IgG-positive and borderline/positive IgM samples.<br /> <b>Results:</b> Anti-CMV IgM seroprevalence was 4.3%. Low IgG avidity was detected in 2 of 40 cases (5.0%). Receiver operating characteristic (ROC) analysis identified 2.26 as the optimal IgM index cut-off, yielding 100% sensitivity, 86.8% specificity (area under the curve = 0.895, p = 0.06).<br /> <b>Conclusions:</b> High CMV IgM index values may indicate low IgG avidity but are not sufficient alone. Results should always be interpreted alongside avidity testing. Threshold use without confirmation is not recommended.