What laboratory tests are recommended to diagnose measles?

Laboratory Diagnosis of Measles

Order serum measles-specific IgM antibody testing immediately during the first clinical encounter with any patient suspected of having measles, using a sensitive and specific assay such as the direct-capture IgM EIA method. 1, 2, 3

Primary Diagnostic Test

• Measles IgM antibody is the first-line laboratory test recommended by the CDC and American Academy of Pediatrics for confirming acute measles infection 2, 3
• Collect blood for serology at the first clinical encounter with suspected measles, regardless of timing relative to rash onset 1
• The direct-capture IgM EIA method is preferred over indirect assays due to superior sensitivity and specificity 1, 2, 3

Critical Timing Considerations

• Optimal detection window: IgM testing achieves 92-100% seropositivity when collected 6-14 days after symptom onset 2, 4
• IgM may be detectable at rash onset, peaks at approximately 10 days, and remains detectable for 30-60 days 1, 3
• If the initial IgM test is negative and collected within 72 hours of rash onset, obtain a second specimen at least 72 hours after rash onset, as IgM may not be detectable with some assays in the first 3 days 1, 2, 3

Additional Laboratory Tests

Acute and Convalescent Serology (Alternative Method)

• Obtain paired serum specimens demonstrating a significant rise in measles antibody titer if IgM testing is unavailable or inconclusive 1, 3
• Collect acute-phase specimen within 1-3 days after rash onset and convalescent-phase specimen 2-4 weeks later 1, 3
• This method has been largely supplanted by single-specimen IgM testing 1

Viral Isolation and Molecular Testing

• Collect urine or nasopharyngeal specimens for measles virus isolation and genetic characterization as close to rash onset as possible 1, 3
• Contact your local or state health department immediately for guidance on proper collection and shipping procedures 1
• Molecular characterization is critical for epidemiologic tracking but cannot be used for acute diagnosis due to processing time 1

Important Pitfalls and Caveats

False-Negative Results

• Early specimen collection (within 72 hours of rash) is the most common cause of false-negative IgM results 1, 2
• Always repeat testing if clinical suspicion remains high despite negative initial results 1, 2

False-Positive Results

• Parvovirus infection (fifth disease) can cause false-positive measles IgM results with some commercial ELISA assays 1, 2
• Other viral infections and rheumatoid factor positivity may also cause false positives 2
• Consider confirmatory testing with direct-capture IgM EIA when IgM is positive in a patient without identified exposure or epidemiologic linkage to a confirmed case 1

Test Performance Variability

• Commercial indirect IgM assays show sensitivities ranging from 82.8% to 88.6% and specificities from 86.6% to 99.6% 4
• IgM capture assays demonstrate 92.2% sensitivity and 86.6% specificity, with improved detection rates (80%) in acute-phase samples compared to indirect assays (57-77%) 4

Public Health Requirements

• Report suspected cases immediately to local or state health departments—do not wait for laboratory confirmation 1, 2, 3
• Laboratory confirmation is essential for facilitating appropriate public health response and contact tracing 2, 3
• Control activities and outbreak investigation should proceed without delay pending laboratory results 1, 3
• Test persons with febrile rash illness who are seronegative for measles for rubella 1