From the Guidelines
Measles IgM testing should be performed as soon as possible after the first clinical encounter with a person who has suspected or probable measles, using a sensitive and specific assay such as the direct-capture IgM EIA method 1. When interpreting serologic data, it is crucial to consider the timing of specimen collection in relation to rash onset and the characteristics of the antibody assay used. Measles IgM may be detectable at the time of rash onset, peaks approximately 10 days after rash onset, and is usually undetectable 30-60 days after rash onset 1. Some key points to consider when testing for measles IgM include:
- Measles IgM is detectable for at least 1 month after rash onset 1
- If measles IgM is not detected in a serum specimen obtained in the first 72 hours after rash onset, another specimen should be obtained at least 72 hours after rash onset and tested for measles IgM antibody 1
- Persons with febrile rash illnesses who are seronegative for measles should be tested for rubella 1 It is essential to note that correct interpretation of serologic data depends on the timing of specimen collection and the characteristics of the antibody assay used, and false negatives can occur if testing is done too early 1.
From the Research
IgM Response to Measles
- The IgM response to measles virus infection is a key indicator of primary infection, with studies showing that 97% of nonimmune children develop IgM antibodies after primary measles vaccination 2.
- IgM antibodies are also detectable in individuals with a prior history of vaccination who develop clinical measles virus infection, with 96% of cases having detectable IgM antibodies 2.
- The IgM response is typically absent on revaccination of those previously immunized, suggesting that IgM antibodies are a reliable marker of primary infection 2.
Diagnostic Performance of IgM Assays
- Commercially available IgM assays have been shown to have significant agreement and high diagnostic accuracy in primary infection, with areas under the curve (AUC) ranging from 0.875 to 0.931 3.
- However, the diagnostic performance of IgM assays can be affected by factors such as vaccine failure and reinfection, with studies showing that IgM tests may have lower sensitivity in these cases 3, 4.
- The performance characteristics of IgM assays, including sensitivity and specificity, can vary between different commercial kits, ranging from 50 to 83% and 86.9 to 98%, respectively 4.
IgM Antibody Response
- IgM antibody levels have been shown to correlate well with the response to measles virus proteins, including the fusion protein (F), haemagglutinin (H), and nucleoprotein (N) 5.
- IgM levels decline rapidly after infection, with studies showing that they are lost 3-6 months after infection 5.
- The IgG subclass response to measles virus infection is characterized by a predominant IgG(1) and IgG(3) response during the acute phase, with IgG(3) being lost in the convalescent phase 5.