CDC seeks to clarify TB test questions
CDC seeks to clarify TB test questions
Low-risk HCWs may have false positives
A hospital employee tests positive on a tuberculin skin test. Should you retest with a blood test to confirm that? An employee tests positive on a TB blood test but works in a low-risk area and has had no known exposures. Should you recommend treatment for latent TB infection?
Such questions have been vexing for employee health professionals. Updated guidelines from the Centers for Disease Control and Prevention provide some answers, but until more research clarifies the interpretation of blood TB tests, clinical judgment still plays an important role.
"You have to look at the context of testing how much risk they had and you have to look at the magnitude of the response [to the test]," says Gerald Mazurek, MD, captain in the U.S. Public Health Service and medical officer and epidemiologist in CDC's Division of TB Elimination.
The introduction of blood tests for TB interferon gamma release assays (IGRAs) in 2001 offered a promising new way to identify tuberculosis infection. The blood tests measure interferon gamma release in response to antigens that represent M. tuberculosis.
Three IGRAs are currently available: QuantiFERON-TB Gold and QuantiFERON-TB Gold In Tube, which measure the interferon gamma concentration, and T-Spot TB, which measures the number of interferon gamma-producing cells.
The blood tests do have some advantages over the tuberculin skin test. They require only one visit to have a blood sample drawn. They don't react to BCG vaccination and are recommended for use in people who have had the BCG vaccine.
But some uncertainties remain about interpreting the blood tests, acknowledges Mazurek. Here are a few key considerations when using either the skin test or blood test for TB screening of health care workers:
Do not test people at low risk of exposure. The likelihood of false positive results either with the TST or IGRAs increases among people who are at low risk. In its 2005 updated Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Setting, CDC recommends annual TB screening for health care workers at medium risk and baseline screening upon hire for those at low risk.
"Even if a test is 99% specific, when the risk [of infection] is less than 1%, you'll have more false positives than true positives," says Mazurek. "In any of the health care settings where they screen people and they're only following people who have been negative in the past, their risk is less than 1%. You have a significant number of false positives with both tests."
If someone at low risk has a positive test result, "it's reasonable to repeat it," he says. "If you make the judgment that it was a false positive, it's reasonable not to exclude them from the serial testing."
More research is needed to clarify the cut-off points for IGRAs. The T-Spot can produce a borderline or indeterminate result and the QuantiFERON tests can produce indeterminate results. And while the skin test has different interpretation criteria based on risk, the blood tests have a single cut-off point.
"You can have very little change and be a converter," says Mazurek. It's reasonable to use clinical judgment, including considering the person's risk of exposure, if their result is near the cut-off, he says. You also may want to consider the expertise of the laboratory performing the tests, he says. Errors in handling the blood specimens, for example, can affect the results.
The tests can produce false-negative results. You should not rely on any TB test alone to rule out tuberculosis in someone with symptoms, Mazurek says."If I have someone with symptoms of tuberculosis, I don't care how many skin tests or QuantiFERONs or T-Spots I have that are negative. The sensitivity isn't 100%," he says.
A hospital employee tests positive on a tuberculin skin test. Should you retest with a blood test to confirm that? An employee tests positive on a TB blood test but works in a low-risk area and has had no known exposures. Should you recommend treatment for latent TB infection?Subscribe Now for Access
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