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Measles tests: What PCPs need to know
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Although the United States appears to have avoided losing its nearly 20-year status as a measle-free nation, there have been at least 1,243 cases of measles from 31 states reported to the CDC thus far in 2019 — more than any year since 1992.
There are multiple tests available that doctors can use to ascertain if a patient has measles, but physicians may not be familiar with all the testing options or the benefits and disadvantages of each test.
Healio Primary Care asked Anna Sick-Samuels, MD, MPH, instructor of pediatric infectious diseases at Johns Hopkins University School of Medicine, and Sheldon Kaplan, MD, an infectious diseases expert with Texas Children’s Hospital, to discuss the measles tests that are currently available for physicians and the pros and cons of each.
Testing options
- There are four tests that can determine a patient’s measles status, Sick-Samuels told Healio Primary Care. These are:
- a reverse transcriptase-polymerase chain reaction (RT-PCR) test, which can detect measles RNA in samples from blood, the throat, nasopharynx, or urine;
- an immunoglobulin M (IgM) antibody test, which is limited to blood samples;
- an immunoglobulin G (IgG) antibody titers test, where a fourfold rise in the IgG titer would be supportive of a recent infection; and
- a viral cell culture.
Pros, cons of tests
The factor that links the tests’ strengths and weaknesses is the time it takes to get results, both doctors suggested.
Sick-Samuels noted that RT-PCR can detect the measles virus as soon as the patient has symptoms and for about 3 days after the rash develops. Kaplan pointed out that the specimen must be collected as quickly as possible after the onset of measles.
The IgM can be completed at a commercial lab, Kaplan, who is also a professor of pediatrics at Baylor College of Medicine, added.
The IgM test does have a caveat, Sick-Samuels said in the interview.
“IgM may be detectable within 3 to 4 four days after initial symptoms develop and stays positive for about 1 month. If tested early in the course of the illness, about 20% of IgM may be negative. Also, people immunized with measles vaccine may not have a detectable IgM response, so a negative test may not rule out infection,” she said.
Conversely, IgG develops later after measles immunization or infection. Physicians can obtain IgG levels during the acute infection period and then again 10 days later.
Sick-Samuels added that the last option, viral cell culture, is not recommended because cell isolation can take 14 days and “this timeframe is not helpful for clinical diagnosis or case confirmation.”
Role of tests in overall protection plan
Kaplan explained some of the public health benefits of measles tests.
“These tests confirm the diagnosis of measles, which contributes to the recognition of outbreaks,” he said in the interview.
Sick-Samuels agreed that the benefits of measles tests extend beyond the patient with the illness.
“Testing is important for the public to help reduce spread of the virus in the community. It is also helpful for health care setting employees and public health officials to know if other potentially exposed people need to be notified to take additional precautions to protect their health,” she said. – by Janel Miller
References:
CDC. Measles cases and outbreaks. https://www.cdc.gov/measles/cases-outbreaks.html. Accessed Sept. 20, 2019.
Paules CI, et al. N Eng J Med. 2019;doi:10.1056/NEJMp1905099.
WHO. Measles. https://www.who.int/news-room/fact-sheets/detail/measles. Accessed Sept. 20, 2019.
Disclosure: Sick-Samuels and Kaplan report no relevant financial disclosures.