Journal Review: ICPs must weigh pros, cons of new TB test
ICPs must weigh pros, cons of new TB test
One challenge is getting blood to lab in 12 hours
Villarino ME, Mazurek G. Tuberculosis Contacts, Concerns, and Controls: What Matters for Healthcare Workers? Editorial.Infect Control Hosp Epidemiol 2006; 27:433-435.
Conceding that the jury still is out on the issue, the Centers for Disease Control and Prevention recommends that infection control professionals screening health care workers for exposures to tuberculosis should consult with local, state, and regional TB control programs if they are considering switching from the traditional tuberculin skin test (TST) to the new QuantiFERON-TB Gold test (QFT-G).
"Infection control practitioners in health care facilities are at an important juncture in deciding the future of employee screening practices for tuberculosis," the CDC authors of this editorial state. "The 2005 CDC guidelines support the use of QFT-G for screening of health care workers and others undergoing serial evaluation for M. tuberculosis infection, stating that QFT-G can usually be used in place of (not in addition to) TST. Other experts consider that the evidence for using only QFT-G for surveillance is still too incomplete to warrant its full implementation and short-term and long-term costs, except as part of collecting post-marketing data."
TST to detect infection
Until recently, the only practical method for detecting asymptomatic infection due to M. tuberculosis was the TST. The QFT-G is an ex vivo assay that measures the release of interferon in whole blood in response to stimulation by antigens that are more specific to M. tuberculosis than is tuberculin purified protein derivative (PPD). Because the QFT-G is an ex vivo assay, this test does not cause boosting when it is repeated. In health care settings where serial testing is conducted, initial two-step testing with QFT-G is not recommended. For TST-based serial testing for workers, initial two-step testing is necessary to establish a baseline infection status, to avoid interpreting boosting as a new infection, and to prevent subsequent unnecessary treatment for latent tuberculosis infection (LTBI).
The CDC recommended that QFT-G may be used in all circumstances in which the TST is currently used, including sequential testing surveillance programs for infection control (e.g., those involving surveillance of health care workers). In such programs, a single negative test result is sufficient evidence that the health care worker is probably not infected with M. tuberculosis. A person with a positive test result need not be retested for surveillance. When using QFT-G for serial testing, a change from a negative result to a positive result should be considered a case of newly diagnosed infection.
"In many settings, the biggest challenge in performing a QFT-G is getting the blood specimen to a qualified laboratory within 12 hours after it is obtained, so that incubation can be performed while the blood cells are viable," the authors of the editorial state. "The guidelines do not recommend that all HCW testing be done with the QFT-G."
The QFT-G has been shown to have sensitivity for detection of culture-confirmed tuberculosis that is comparable to that of the TST, and the specificity of the QFT-G for detection of M. tuberculosis infection is greater than that of the TST. The test requires only one visit to a health care facility, and the result can be available within one day. In addition, many health care workers in the United States were born in countries where the incidence of tuberculosis is high and where bacille Calmette-Guérin (BCG) vaccination is routine. Because a QFT-G result is not influenced by BCG vaccination, ICPs may have increased confidence that the test detects new infection. However, errors in collecting or transporting blood specimens or in running and interpreting the assay can decrease the accuracy of QFT-G.
"In this editorial, our aim is to demonstrate what we believe might be the most important guideline of all: To find the best balance as to what infection control professionals are ready to implement for the surveillance of tuberculosis among health care workers at this point, when the most highly supported evidence-based recommendations are not yet available," the authors state.
"One-size-fits-all" guidelines do not fit all situations: The new guidelines are best used as a guide, rather than a directive for setting infection control policy. Confirming or excluding the diagnosis of tuberculosis disease and assessing the probability of LTBI among health care workers still require a combination of epidemiologic, historical, physical, and diagnostic findings that should be considered when interpreting TST results and QFT-G results.