What labs are recommended for a measles workup?

Laboratory Testing for Measles Diagnosis

For measles diagnosis, blood for serologic testing should be collected during the first clinical encounter with suspected cases, with measles IgM antibody testing as the primary diagnostic test, supplemented by specimen collection for virus isolation from urine or nasopharyngeal samples. 1

Primary Laboratory Criteria for Measles Diagnosis

The Advisory Committee on Immunization Practices (ACIP) recommends the following laboratory tests for measles diagnosis:

1. Serologic testing:

   • Measles IgM antibody testing (primary method)
   • Significant rise in measles antibody level by standard serologic assay
   • Specimen collection timing is critical

2. Virus isolation:

   • Collection of clinical specimens (urine or nasopharyngeal mucus)
   • Important for molecular characterization and epidemiologic investigation

Detailed Testing Algorithm

Serologic Testing Protocol

1. Initial specimen collection:

   • Collect serum during first clinical encounter with suspected measles case
   • Test for measles IgM antibody using sensitive and specific assay (direct-capture IgM EIA method preferred) 1

2. Timing considerations:

   • Measles IgM may be detectable at rash onset
   • Peaks approximately 10 days after rash onset
   • Usually undetectable 30-60 days after rash onset 1

3. Follow-up testing:

   • If initial IgM test is negative and specimen was collected within 72 hours of rash onset, obtain second specimen at least 72 hours after rash onset
   • IgM remains detectable for at least 1 month after rash onset 1

4. Alternative serologic approach:

   • Paired acute and convalescent sera showing significant rise in antibody titer
   • Acute specimen: collect within 1-3 days after rash onset
   • Convalescent specimen: collect 2-4 weeks later
   • Less commonly used now that IgM testing is widely available 1

Virus Isolation and Molecular Characterization

1. Specimen collection:

   • Collect urine or nasopharyngeal specimens as close to rash onset as possible
   • Delay reduces chance of isolating measles virus
   • Contact local/state health department for specific collection and shipping instructions 1

2. Purpose:

   • Molecular characterization for epidemiologic investigation
   • Documentation of measles elimination efforts
   • Not used for immediate diagnosis (requires considerable time) 1

Important Considerations and Pitfalls

1. False positive results:

   • More common as measles becomes rare
   • Can occur in patients with parvovirus infection (fifth disease)
   • Confirmatory testing with sensitive and specific assay recommended when:
     • IgM is detected in patient with no identified source of infection
     • No epidemiologic linkage to another confirmed case 1

2. False negative results:

   • May occur if specimen collected too early (within first 72 hours of rash)
   • Patients with negative measles serology should be tested for rubella 1

3. Test performance:

   • Sensitivity of commercial IgM tests ranges from 82.8% to 88.6% 2
   • Specificity ranges from 86.6% to 99.6% 2
   • Best seropositivity rates (92-100%) observed with samples collected 6-14 days after symptom onset 2

4. Alternative specimen types:

   • Saliva testing shows good agreement with serum results (sensitivity 95.5%, specificity 66.7%) 3
   • May be useful as non-invasive alternative, especially in children 3

Public Health Considerations

1. Immediate reporting:

   • Report suspected measles cases immediately to local/state health department
   • Do not delay control activities pending laboratory results 1

2. Case definition:

   • Laboratory-confirmed case: meets laboratory criteria
   • Clinical case: generalized rash ≥3 days, fever ≥101°F, and cough/coryza/conjunctivitis
   • A laboratory-confirmed case need not meet clinical case definition 1

By following this testing algorithm, clinicians can ensure accurate and timely diagnosis of measles, which is critical for appropriate patient management and public health response to prevent further transmission.