Serologic testing for measles in low prevalence setting
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 specimens 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 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 enzyme immunoassays (EIAs) 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.
Laboratory evidence of immunity is based on testing to detect measles IgG. People who have negative or equivocal results for measles IgG should be vaccinated or revaccinated. In some cases, vaccination is not possible, and testing with a second line diagnostic assay may be necessary to determine the patient’s immune status. CDC has evaluated the performance of several of the commercial EIAs used to detect measles IgG (Latner et al, 2020). Some of the commercial, manually-processed EIAs are slightly more sensitive than high-throughput automated IgG test platforms that are typically used by large commercial laboratories. Contact CDC or the state public health laboratory for information on second line assays.
Instructions for blood collection (serum):
Blood for serologic testing of measles at CDC is collected as described in the Infectious Disease Laboratories Test Directory entry for each test, listed below.
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.
Submission of specimens to CDC for measles serologic testing: Q & A
What tests are available for laboratory confirmation of measles?
A capture IgM EIA (non-quantitative) that incorporates a recombinant measles virus 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 through the APHL/CDC Vaccine Preventable Disease Reference Centers.
Who can send specimens to CDC?
To determine where to send specimens for testing, please contact your state or local 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 either a nasopharyngeal or throat 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 CDC Specimen Submission Form (50.34) must be submitted with each specimen. Provide the following information on the CDC Specimen Submission Form (50.34): clinical signs and symptoms, date of onset, date of specimen collection, date(s) of MMR vaccination (if known), and travel history. Provide any preliminary results available. 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 STATT (Specimen Triage and Tracking Team) laboratory at CDC. Specimens are then delivered to the VVPDB measles lab (Unit #81). The turnaround–time for testing specimens for IgM and IgG is 7 working days after the delivery of specimens to the measles reference lab by the STATT laboratory.
When situations require immediate notification of a result, please inform the laboratory (Dr. Stephen Crooke 404-718-4003 or Dr. Paul Rota 404-639-4181) upon submission of the specimen. Provide a telephone number, email address, and fax number.
Specialized serologic testing available at CDC: avidity and PRN assays
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 specimen was not available. Prior approval from the VVPDB laboratory (Dr. Stephen Crooke 404-718-4003) 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, specimens must have detectable IgG. The turnaround–time for testing specimens for IgG avidity is 30 working days after the delivery of the specimens to the measles reference lab by the STATT laboratory. Both an acute and a convalescent phase serum are recommended for PRN testing. There is no established turnaround–time for measles PRN testing.
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