Lab Testing for Measles at the MDH Public Health Laboratory
Measles is a highly infectious disease. Although it is no longer endemic in the U.S., an imported case can spread rapidly. Suspected measles cases should be immediately reported to MDH to facilitate case confirmation and ensure a rapid public health response. Case confirmation is based on clinical presentation, epidemiologic factors, and laboratory test results.
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Measles may be tested by one or more of the following methods in order of preference:
- Detection of the virus by reverse transcription-polymerase chain reaction (RT-PCR),
- A positive viral culture for measles,
- A positive measles-specific IgM antibody*,
- A significant rise in IgG antibody between acute and convalescent paired sera (not commonly done).
*False positive or negative results can occur and further interpretation or testing may be needed.
Measles PCR is a highly sensitive and timely laboratory test. A positive PCR result is considered confirmatory. Contact an MDH epidemiologist at 651-201-5414 or toll-free at 877-676-5414 to facilitate testing for situations in which rapid case confirmation is critical.
PCR test specimens
Clinical specimens for PCR should be taken as soon as measles is suspected. Viral RNA is more likely to be detected when the specimens are collected within 3 days of rash onset; however it is acceptable to collect specimens for PCR up to 9 days past rash onset. Viral isolation requires the same specimens and may be done if PCR is negative.
Measles virus can best be isolated from –in order of preference: throat, urine, and nasal specimens. Ideally, all three specimens should be collected. NOTE: Flocked swabs are preferred for specimen collection. Viral transport media (VTM) containing ~1% protein (e.g., bovine serum albumin or gelatin) best supports measles virus; however, sterile saline is acceptable for PCR testing.
- Throat swab
Vigorously swab tonsillar areas and posterior nasopharynx with a viral culturette. Use tongue blade to depress tongue to prevent contamination of swab with saliva. Place swab into VTM.
- Urine specimen
Collect 10-40 ml of urine in a STERILE 50 ml centrifuge tube or a urine specimen container. First-morning voided specimens are ideal, but any urine collection is adequate. Have patient void directly into container, collecting from the first part of the urine stream if possible.
- Nasal or nasopharyngeal swab
Use sterile swabs to swab the nasal passage or the nasopharynx with either a viral culture swab or culturette. Do not use special (e.g., anaerobic) media. Place swab into VTM.
Measles serology may give false positive or false negative results. Contact an MDH epidemiologist to discuss serology testing and interpretation.
Serologic test specimens
Draw 1-2 ml blood in a red top tube; spin down serum if possible. NOTE: 50-100 µl of capillary blood (approximately 3 capillary tubes) may be collected in situations where venipuncture is not preferred, such as for young children.
- Draw blood for IgM antibody testing as soon as possible. Occasionally, false-negative measles IgM results occur when blood specimens are collected within 72 hours after rash onset. A second blood sample, collected 72 hours after rash onset, should be tested in this situation.
- A positive serologic test result for measles IgM antibody indicates recent infection or recent vaccination.
- Other diseases, such as parvovirus infection, infectious mononucleosis, or rheumatologic disease can cause false positive measles IgM antibody results.
- Obtain acute and convalescent serum specimens for measles specific IgG antibody to confirm a measles diagnosis. Acute and convalescent specimens should be tested as paired sera.
- Draw blood for acute IgG as early as possible when measles infection is suspected. For convenience, the blood drawn for measles IgM antibody testing may be used for the acute IgG.
- Draw blood for convalescent IgG approximately 2 weeks after blood was drawn for the acute IgG.
Possible interpretations of a single positive measles IgG antibody result include:
- Current infection to which the individual is developing immunity.
- Immunity due to either a past infection or vaccination.
- Presence of maternal antibody (in infants < 15 months).
- The earlier that blood is drawn for measles IgG antibody following the onset of symptoms, the more likely a positive result is due to past infection or vaccination.
Note: In accordance with recommendations from the Centers for Disease Control and Prevention, the MDH lab routinely tests for both measles and rubella when specimens are submitted for either disease.
- Label tubes/containers with patient name, date of birth, and date collected
- Include an MDH Virology Specimen Submission Form for each specimen (submission forms are available at Infectious Disease Laboratory - Forms)
- Send to MDH Public Health Laboratory via overnight shipping.
- Specimens should be kept and transported at refrigeration temperature (2-8° C).
- Before shipping, always consult with an MDH epidemiologist at 651-201-5414 or toll-free at 877-676-5414.
- Send specimens to:
MDH Public Health Laboratory
Attn: Biological Accessioning
601 Robert St. N.
St. Paul, MN 55155-2531