Background Cases of congenital rubella syndrome are reported annually in Japan. Rubella-specific IgM antibody titers can show false positivity or remain positive for a long period. Furthermore, the seroprevalence of rubella hemagglutination inhibition (HI) and IgM antibodies is unknown, and the relevance of measuring rubella-specific IgM antibodies in asymptomatic individuals is unclear. Hence, this study aimed to assess the validity of rubella-specific IgM antibody titers as a diagnostic

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... , by retrospectively assessing the prevalence of rubella HI and IgM antibodies among pregnant Japanese women. The study also aimed to estimate the incidence of rubella among rubella IgM antibody-positive pregnant women. Methods This single-center, retrospective study measured rubella HI and IgM antibody titers of pregnant Japanese women during prenatal visits from January 2007 to December 2020 at Yokohama City University Medical Center Perinatal Center for Maternity and Neonates. Rubella-specific IgM antibody titers were measured by enzyme immunoassay. Values > 1.2 and ≤ 1.2 were defined as positive and negative, respectively. Results A total of 14965 pregnant women were included in the study, of whom 186 (1.2%) were IgM-positive. One patient (0.5%) was clinically diagnosed with rubella. Of the IgM-positive patients, 136 (73.1%) had rubella HI antibody titers of < 1:256. The patient with clinically diagnosed rubella had a fever and skin rash and showed a high titer value for both rubella HI (1:2048) and rubella-specific IgM. The patient decided to terminate the pregnancy without a repeat blood examination. The correlation coefficient between rubella-specific IgM and rubella HI antibody titers was 0.2527 (p < 0.0001), indicating a weak positive correlation. Conclusion This study showed that almost all asymptomatic rubella-specific IgM-positive patients measured in the context of high rubella HI antibody titers were not acutely infected regardless of whether the rubella-specific IgM antibody titer was high or low.