Which laboratory test most likely indicates measles exposure in a community?

From the Guidelines

The most likely laboratory test to indicate community spread of measles is the measles IgM antibody test, which should be performed on a serum specimen collected during the first clinical encounter with a person who has suspected or probable measles. This test is recommended by the CDC as the primary method for diagnosing measles, especially during outbreaks 1. The measles IgM antibody test detects the presence of measles-specific immunoglobulin M antibodies, which are typically detectable at the time of rash onset, peak approximately 10 days after rash onset, and are usually undetectable 30-60 days after rash onset 1.

Key Points to Consider

• The timing of specimen collection is crucial for accurate interpretation of serologic data, and measles IgM may not be detectable with some less sensitive assays until at least 72 hours 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.
• The CDC recommends that persons with febrile rash illnesses who are seronegative for measles should be tested for rubella 1.

Outbreak Control Strategies

• The local or state health department should be contacted immediately when suspected cases of measles occur in a community, and all reports of suspected measles cases should be investigated promptly 1.
• Prompt vaccination of susceptible persons at risk for exposure may help prevent dissemination of measles, and control activities should not be delayed pending the return of laboratory results from persons with suspected or probable cases 1.

From the Research

Laboratory Tests for Measles Diagnosis

The Centers for Disease Control and Prevention (CDC) recommends the use of laboratory tests to confirm measles diagnosis. The following are some of the laboratory tests that can indicate measles in a community:

• IgM serology tests: These tests detect the presence of IgM antibodies against the measles virus in the blood. According to 2, the sensitivity of IgM serology tests ranges from 82.8% to 88.6%, and specificity ranges from 86.6% to 99.6%.
• IgM capture assays: These tests are considered more specific than IgM serology tests and are used for confirmatory testing. The CDC IgM capture assay has a sensitivity of 87.0% and specificity of 94.8% 2.
• Reverse transcription polymerase chain reaction (RT-PCR): This test detects the genetic material of the measles virus in the blood or other bodily fluids. According to 3, RT-PCR can play an important role in laboratory confirmation of acute measles infections.
• Point-of-care tests (POCTs): These tests detect measles-specific IgM antibodies in serum or oral fluid specimens. According to 4, a POCT has a sensitivity of 90.8% and specificity of 93.6% in serum specimens, and a sensitivity of 90.0% and specificity of 96.2% in oral fluid specimens.

Factors Affecting Test Performance

The performance of laboratory tests for measles diagnosis can be affected by several factors, including:

• Timing of specimen collection: The best seropositivity rates are observed with samples collected 6 to 14 days after the onset of symptoms 2.
• Type of test used: Different tests have varying levels of sensitivity and specificity 2, 5.
• Presence of reinfection: IgM tests may have lower diagnostic accuracy in cases of reinfection 5.

Surveillance and Diagnosis

Laboratory tests play a crucial role in measles surveillance and diagnosis. According to 6, rapid diagnostic tests (RDTs) have the potential to transform measles surveillance by allowing real-time case confirmation outside of central/regional laboratories. RDTs can also enable the detection of measles virus genes and facilitate timely public health response.