4. Lab Testing for Measles Takes Time
Lab testing to rule out measles is unlikely to be timely enough to help with measles control. Lab testing is important for confirmation, but the history and clinical features can make a reasonable clinical diagnosis or rule out measles well before lab test results are received. Leukopenia or lymphopenia is typically seen in measles but is not diagnostic. Serology and viral detection are the available tools for confirmation.
Serology is more widely available. IgM antibodies begin to appear 1-2 days after rash onset, although the IgM response can be delayed. About 20% of patients with measles will be IgM negative during the first 74 hours after rash onset; repeat testing should be performed if suspicion for measles remains. IgM assays are not 100% specific, and low-level positives around the cutoff level may represent false positives. Serial testing can resolve this. Thus, depending on lab turnaround, the diagnosis may not be confirmed by serology until 3-6 days after rash onset.
Measles virus can be detected by polymerase chain reaction (PCR), which provides the most definitive confirmation. The throat and nasopharynx are the preferred sites to sample for PCR, and sampling more than one site may increase detection. However, PCR for measles virus is not currently available through commercial labs in the United States. Measles PCR testing can be arranged through local or state health departments and is performed in conjunction with CDC. The health department will provide guidance on the need for testing and can help with managing quarantines and contact tracing.
Patients with measles are highly contagious. The spread is by true aerosols that can remain suspended in the air and infectious for up to two hours. If they need to go to a lab to have blood drawn, you should notify the lab and infection control to help prevent exposing other patients.
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Any views expressed above are the author's own and do not necessarily reflect the views of WebMD or Medscape.
Cite this: Never Seen Measles? 5 Things to Know - Medscape - May 02, 2019.