Ongoing measles activity in other countries will result in sporadic cases of measles in the United States. Evaluation and interpretation of measles diagnostic results can be complex, particularly in measles elimination settings. Detection of specific IgM antibodies in a serum sample collected within the first few days of rash onset can provide presumptive evidence of a current or recent measles virus infection. However, because no assay is 100% specific, serologic testing of non-measles cases using any assay will occasionally produce false positive IgM results. Serologic tests can also result in false-negative results when serum specimens are collected too early with respect to rash onset.
In countries such as the United States where endemic circulation of measles has been eliminated, most suspected cases are not measles, and rash and fever illnesses are more likely due to a number of other rash–causing illnesses such as parvovirus B19, enteroviruses, or human herpesvirus–6 (roseola). The presence of rheumatoid factor (RF) can also result in a false positive IgM. Indirect EIA’s appear to be more affected by RF than IgM capture assays. Additionally, patients with throat or ear infections occasionally develop a rash following administration of antibiotics. Subsequent serologic specimens sent for measles IgM testing may result in a false positive test.
Blood for serologic testing of adults is collected by venipuncture. Collect 7–10 ml of blood in a red-top or serum-separator tube (SST). The preferred volume for IgM and IgG testing at CDC is 0.5–1 ml of serum to allow for re–testing; however, testing can be done with as little as 0.1 ml (100 µl) if necessary. Do not freeze tubes containing whole blood. Centrifuge blood collection tubes (10 min at 2200 – 2500 rpm) to separate serum from clot. Gel separation tubes should be centrifuged no later than 2 hours after collection. Aseptically transfer serum to a sterile tube that has an externally threaded cap with an o–ring seal. Store specimens at 4°C and ship on wet ice packs. Hemolyzed and lipemic serum and plasma are noted and tested, usually without apparent interferences.
Blood for serologic testing of infants or small children can be collected by finger/heel stick. Capillary tubes can be utilized for infants. Capillary tubes require the submitter to have access to a hematocrit centrifuge. Clinical laboratories should have 50 or 100 µl capillary tubes that are typically used for a variety of tests such as hematocrits or total lipids on newborns. At least 3 of the 50 µl hematocrit capillary tubes should be collected and spun in a hematocrit centrifuge.
What tests are available for laboratory confirmation of measles?
A capture IgM EIA (non-quantitative) that incorporates a recombinant measles nucleocapsid protein as the antigen is used to detect measles IgM. A commercial, indirect EIA (non-quantitative) assay is used for detection of IgG. Viral detection methods include standard methods of culturing virus in appropriate cell lines and techniques, such as real time RT-PCR to detect measles viral RNA. RT-PCR is available at many state public health laboratories and though the APHL/CDC Vaccine Preventable Disease Reference CentersExternal.
Who can send samples to CDC?
To determine where to send specimens for testing please contact your health department. Specimens can be shipped to the state laboratory, the VPD-RC, or CDC. See the Specimen Collection, Storage and Shipment page.
Does CDC provide testing for any suspected case of measles?
During outbreaks of measles, the CDC measles laboratory can assist with testing.
To expedite confirmation of measles from highly suspicious cases (recent travel, classic measles signs and symptoms), particularly in a setting with many potential exposures, an aliquot of serum, and a nasopharyngeal swab can be sent to CDC while testing is underway at the SPHL or local clinical laboratory.
Requests for measles serologic testing that are strictly for rule–out purposes (low index of suspicion, does not meet clinical case definition, no travel or contact with cases), should be directed to either SPHLs or clinical laboratories. Prior communication with CDC’s Viral Vaccine-Preventable Diseases Branch (VVPDB) regarding suspected cases of measles that require assistance is preferred.
What information is needed with the specimen and when will the result be available?
A completed 50.34 submission form must be submitted with each specimen. It is important to include the date of measles vaccinations, if applicable, and the dates of rash onset and blood collection. A patient identification number and/or name as well as the patient date of birth should be entered on the form.
Serum specimens for measles serologic testing (IgG, IgM) are delivered by courier to the STAT (Specimen Triage and Tracking) laboratory at CDC. Samples are then delivered to the VVPDB measles lab (Unit #81). The turnaround–time for testing samples for IgM and IgG is 7 working days after the delivery of samples to the measles reference lab by the STAT lab.
When situations require immediate notification of a result, please inform the laboratory (Paul Rota 404-639-4181 or Carole Hickman 404-639-3339) upon submission of the sample. Provide a telephone number, email address, and fax number.
IgG avidity testing and a plaque reduction neutralization (PRN) assay are available when case classification requires additional methods to confirm measles infection, usually when the result obtained for IgM is suspected of being a false negative or a false positive (Sowers et al). These assays may be offered when confirmation by RT-PCR was not successful or a molecular sample was not available. Prior approval from the VVPDB laboratory (Carole Hickman 404-639-3339) should be obtained. Avidity testing and the PRN assay require specialized reagents and their use is limited to unusual cases, typically in an outbreak setting, when cases with mild or a non-classic presentation of measles are detected.
A single serum can be tested for IgG avidity; however, samples must have detectable IgG. Both an acute and a convalescent phase serum are recommended for PRN testing.
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- Hickman CJ, Sowers SB, Mercader S, McGrew M, Williams NJ, Beeler JA et al. Laboratory characterization of measles virus infection in previously vaccinated and unvaccinated individuals. J Infect Dis. 2011;204(Supplement 1):S549-558.
- Rota JR, Hickman CJ, Sowers SB, Rota PA, Mercader S, and Bellini WJ . Two case studies of modified measles in vaccinated physicians exposed to primary measles cases: High risk of infection but low risk of transmission. J Infect Dis. 2011;204(Supplement 1):S559-563.
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