Serologic Testing for Measles in Low Prevalence Setting
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.
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, enterviruses, or human herpesvirus–6 (roseola). In addition, testing for measles is frequently requested for people with ear infections or sore throats who were given antibiotics which resulted in a rash. The presence of rheumatoid factor can also result in a false positive IgM. However, ongoing measles activity in many other countries will result in sporadic cases of measles in the United States.
Instructions for Blood Collection (Serum)
Blood for serologic testing is collected by venipuncture or by finger/heel stick. Use tubes without additives—either a plain, red–top tube or a serum separator tube. 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 ul) if necessary. Generally, 5 ml of blood (yield about 1.5 ml of serum) can easily be collected from adults. Do not freeze the tube before serum has been removed. Centrifuge the tube 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. Fresh, sterile serum can be shipped overnight on a wet ice pack. Hemolyzed and lipemic serum and plasma are noted and tested, usually without apparent interferences.
Capillary tubes can be utilized for infants. Capillary tubes require the submitter to have access to the appropriate centrifuge for these capillary tubes. Clinical laboratories should have 50 or 100 ul 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 ul hematocrit capillary tubes should be collected and spun in a hematocrit centrifuge.
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Submission of Samples to CDC for Measles Serologic Testing: Q & A
What tests are used at CDC?
The reference laboratory at CDC uses assays developed at CDC for measles serologic testing of IgM and IgG. The assays utilize a recombinant measles nucleoprotein (NP) antigen in the EIA. The CDC measles IgM is a capture IgM format EIA; the CDC measles IgG test is an indirect EIA.
Who can send samples to CDC?
In general, the State Public Health Laboratory (SPHL) refers serum to the CDC for measles testing, usually for confirmatory testing. The mission of the CDC national laboratory is to provide support for measles surveillance as needed. Certain circumstances sometimes require that initial testing is done at CDC. Shipment of serum samples can be arranged by local public health district offices or private clinics and hospitals to CDC to expedite testing. However, the cases should be referred for testing following notification of the CDC measles laboratory and the appropriate state health department and/or infectious disease control official.
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 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 Measles Mumps, Rubella, and Herpesvirus Laboratory Branch (MMRHLB) regarding suspected cases of measles that require assistance is preferred.
What information is needed with the serum sample and when will the result be available?
A completed submission form must be submitted with each serum sample. It is important to include the date of the last measles vaccination, 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, formerly DASH) laboratory at CDC. Samples are then delivered to the MMRHLB measles lab (Unit #81). The turnaround–time for testing samples for IgM and IgG is 3 working days after the delivery of samples to the measles reference lab by the STAT lab.
Reports are sent back to the submitter (usually the SPHL) by mail. The address of the SPHL should appear in the upper right of the CDC 50.34 form. When situations require immediate notification of a result, please inform the laboratory coordinator (firstname.lastname@example.org) upon submission of the sample. Provide a telephone number, email address, or fax number.
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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. These assays may be offered when confirmation by RT-PCR was not successful or a sample was not available. Prior approval from the MMRHLB chief 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 generally must have detectable IgG. Both an acute and a convalescent phase serum are recommended for PRN testing.