Immunoglobulin M for Acute Infection: True or False?

Immunoglobulin M (IgM) tests have clear clinical utility but also suffer disproportionately from false-positive results, which in turn can lead to misdiagnoses, inappropriate therapy, and premature closure of a diagnostic workup. Despite numerous reports in the literature, many clinicians and laboratorians remain unaware of this issue. In this brief review, a series of virology case examples is presented. However, a false-positive IgM can occur with any pathogen. Thus, when an accurate diagnosis is essential for therapy, prognosis, infection control, or public health, when the patient is sick enough to be hospitalized, or when the clinical or epidemiologic findings do not fit, IgM detection should not be accepted as a stand-alone test. Rather, whenever possible, the diagnosis should be confirmed by other means, including testing of serial samples and the application of additional test methods.