Appropriateness of Immunoglobulin M Testing for Measles, Mumps, and Rubella

Testing for immunity to measles, mumps, and rubella should include only immunoglobulin G (IgG); immunoglobulin M (IgM) testing is appropriate only if acute illness is suspected. The appropriateness of measles, mumps, and rubella IgM testing was evaluated in a national administrative dataset. Laboratory testing for measles, mumps, and rubella during 2019–2022 was analyzed in 2024 using HealthVerity administrative claims and laboratory data. IgG, IgM, and reverse-transcriptase polymerase chain reaction (RT-PCR) testing are described by year, demographics, and region. IgM testing was examined for appropriateness, defined as an IgM test combined with diagnostic codes indicative of acute illness. During 2019–2022, IgM testing represented a small proportion of serologic testing (measles: 3.3%, mumps: 2.4%, rubella: 2.1%) but appeared to be appropriately performed in only 15.4% of cases for measles, 32.8% of cases for mumps, and 10.2% of cases for rubella. IgM testing was more commonly performed for female patients, with the largest discrepancy seen for rubella (90.5% female vs 9.5% male). IgM for measles and mumps was more often performed appropriately for persons aged 0–19 years (37.6% and 60.1%) compared with persons aged 20–49 years (11.8% and 22.0%) and 50+ years (16.5% and 33.8%). The majority of IgM testing for measles, mumps, and rubella during this period appeared inappropriate. Clinicians and health systems could ensure that IgG testing alone is performed when evaluating for immunity through modifications to electronic medical records and commercial laboratories could ensure that providers are able to test for IgG alone when evaluating immunity.