New TB test can be used for baseline HCW testing
New TB test can be used for baseline HCW testing
Role less clear for routine surveillance
A recently approved tuberculosis test can be used to diagnosis latent TB infection in health care workers without generating false positives due to "boosting" effects of the traditional TB skin test, the Centers for Disease Control and Prevention (CDC) reports.
The new test (QuantiFERON-TB or QFT; manufactured by Cellestis Limited, Carnegie, Victoria, Australia) measures the release of interferon-gamma in whole blood in response to stimulation by purified protein derivative (PPD). Tuberculin skin testing (TST) has been used for years as an aid in diagnosing latent tuberculosis infection. TST includes measurement of the delayed type hypersensitivity response 48 to 72 hours after intradermal injection of PPD.
The new diagnostic test requires phlebotomy, but it can be accomplished in a single visit because the worker does not have to come back to have the results read. The QFT assesses responses to multiple antigens simultaneously and does not boost anamnestic immune responses. Compared with TST, QFT results are less subject to reader bias and error, the CDC notes.1
"Lot’s of people prefer it because it might also give you some advantage to distinguish people who are BCG-positive [due to previous Calmette-Guérin TB vaccination], and you cannot get that out of the skin test," says Margarita E. Villarino, MD, medical epidemiologist in the CDC division of TB elimination. "It’s not very expensive, and you don’t have the person back to have the test read. It doesn’t boost. You wouldn’t get a false-positive skin test if you test after using quantiferon. The opposite is not recommended. It is not recommended that you do a QFT after you test with a skin test because the skin test does boost the quantiferon test response," she says.
TST and QFT do not measure the same components of the immunologic response and are not interchangeable. Assessment of the accuracy of the tests is further limited by lack of a standard for confirming latent TB infection.
"In the studies that were done, it had about the same sensitivity and specificity of the skin tests," Villarino says. "It measures different parts of the immune response, so they are not completely comparable. That said, it is considered to be screening test that is as good, specific, and sensitive as the skin test."
The new test is indicated for initial employment of health care workers, but its role for periodic surveillance is less clearly defined. The key variable is the risk of TB in the work environment the worker is going to be exposed to. Routine TST testing in populations with little or no TB has been discouraged because it primarily yields false positives. "We don’t really know what the recommendation should be for serial testing of somebody who is at low risk of TB exposure," she says.
"If they are going to a high-exposure setting, then they could be more equivalent to serial testing [recommended for] high-risk groups. Then the facility might choose to use the [QFT test]. We don’t know exactly how it will function in a surveillance system for periodic surveillance of health care workers. But it is indicated for the initial evaluation — their pre-employment screening — before they get exposed to a situation of high-risk TB," she says.
Interim CDC recommendations for QFT remind that the highest priority of targeted tuberculin testing programs is identifying people at increased risk for TB who will benefit from treatment for latent TB infection. Following that principle, targeted tuberculin testing should be conducted among groups at risk for recent infection with M. tuberculosis and those who, regardless of duration of infection, are at increased risk for progression to active TB. QFT can be considered for screening for latent TB infection (LTBI) as follows:
- initial and serial testing of people with an increased risk for LTBI (e.g., recent immigrants, injection-drug users, and residents and employees of prisons and jails);
- initial and serial testing of persons who are, by history, at low risk for LTBI but whose future activity might place them at increased risk for exposure, and others eligible for LTBI surveillance programs (e.g., health care workers and military personnel);
- testing of people for whom LTBI screening is performed but who are not considered to have an increased probability of infection (e.g., entrance requirements for certain schools and workplaces).
Confirmation of QFT results with TST is possible because performance of QFT does not affect subsequent QFT or TST results. The probability of LTBI is greatest when both the QFT and TST are positive. Considerations for confirmation are as follows:
- When the probability of LTBI is low, confirmation of a positive QFT result with TST is recommended before initiation of LTBI treatment. LTBI therapy is not recommended for people at low risk who are QFT-negative or who are QFT-positive but TST-negative.
- TST can also be used to confirm a positive QFT for people at increased risk for LTBI. However, the need for LTBI treatment when QFT is positive and the subsequent TST is negative should be based on clinical judgment and perceived risk.
Negative QFT results do not require confirmation, but results can be confirmed with either a repeat QFT or TST if the accuracy of the initial test is in question.
Reference
1. Centers for Disease Control and Prevention. Guidelines for using the QuantiFERON®-TB test for diagnosing latent Mycobacterium tuberculosis infection. MMWR 2002; 51: Dispatch.
A recently approved tuberculosis test can be used to diagnosis latent TB infection in health care workers without generating false positives due to boosting effects of the traditional TB skin test, the Centers for Disease Control and Prevention (CDC) reports.
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