Screening employees for tuberculosis via tuberculin skin test is a common practice in healthcare settings nation-wide. Some organizations have switched to interferon-γ release assays (IGRA) in this population, and the experience of one in dealing with serial IGRA testing is the subject of this issue of Strategies.
Even though the U.S. has a low incidence of tuberculosis (TB), healthcare workers are at risk of developing TB infection (TBI) due to repeated exposure to patients with either increased risk or active disease. For this reason, annual screening of healthcare workers for latent TBI or active TB is a routine practice in healthcare settings nation-wide. However, the tuberculin skin test (TST), the mainstay of TB testing for decades, has low specificity due to its cross-reactivity with bacille Calmette-Guerin (BCG) vaccines and nontuberculous mycobacteria. TST also has the potential for a booster effect after serial testing.
In 2001, the U.S. Food and Drug Administration approved the first IGRA as an aid in diagnosing TBI, followed by two others later in the decade. The Centers for Disease Control and Prevention in 2005 published guidelines for using IGRAs, suggesting that IGRA could replace TST, even in serial testing. The Agency’s 2010 update included preferences and special considerations in the use of IGRA versus TST. Significantly, this update also called for more research in 17 areas, underscoring that while IGRAs have higher sensitivity and specificity in detecting TBI, incomplete information remains about their performance.
In 2007, the Cleveland Clinic started using IGRA instead of TST for screening all new employees, with the intention of eventually using the former as the primary screening method for its entire workforce. Investigators at Cleveland Clinic recently reported on the health system’s experience with serial testing of healthcare workers, and their findings are the subject of this issue of Strategies (Chest 2012;142:55–62).
Co-author Belinda Yen-Lieberman, PhD, explained that Cleveland Clinic moved to use IGRA testing in this population due both to logistics and the demographics of its workforce. “We have 18 clinics, and some people would have to drive 40 minutes to have a skin test, then two days later drive back just to have the skin test read. So we had a lot of no-shows, plus the TST is a first-generation test,” she said. “We also have many young doctors and other providers from outside the U.S., and many of them had positive TST because they either had BCG vaccines or prior exposures. This required follow-up chest x-rays, which showed these results to be false positives.” Yen-Lieberman is director of clinical virology, serology, and cellular immunology at Cleveland Clinic, which is considered a medium-risk facility for transmission of TB, according to the authors.
Yen-Lieberman and her colleagues performed a 3-year chart review of serial TB testing among Cleveland Clinic employees to evaluate the performance of IGRA in this population. During the study period, 7,374 IGRAs were performed, of which 486 (6.6%) were positive at baseline, 305 (4.1%) were indeterminate, and 6,583 (89.3%) were negative. Out of the 1,867 with serial IGRA testing, 52 (2.8%) were identified as converters, meaning after an initial negative result, a second IGRA was positive. Nearly three-quarters of these had IGRA values higher than the manufacturer’s recommended cutoff of 0.35 IU/mL but less than 1 IU/mL, with a serial test median value of 0.63 IU/mL. None of these individuals had active TB or were part of an outbreak investigation; however, 40% had received BCG vaccination in the past. In addition, 14 employees were asked to repeat IGRA testing because of an initial low positive or suspected false-positive result. Of these, eight reverted, meaning after an initial positive result, subsequent tests had values below the manufacturer’s cutoff and were considered negative. Initial IGRA values for these eight ranged from 0.35–10 IU/mL, with a median of 0.42 IU/mL.
Based upon these findings and other reports in the literature, Cleveland Clinic implemented a new occupational health policy for employees whose IGRA values convert from negative to positive. They will have repeated IGRA tests, chest x-rays, and evaluations by infectious diseases staff. If they have no identified risk factors for latent TBI, Cleveland Clinic will consider treatment only in those with IGRA values >1 IU/mL, as opposed to the manufacturer’s cutoff of 0.35 IU/mL.
Aside from their study findings, another factor in Cleveland Clinic’s decision to use a different cutoff is the inherent variability of IGRAs. “This test is subject to biological variability. If the result comes back 0.36 IU/mL, it’s positive. And if the result is 0.34 IU/mL, it’s negative. But we believe it should be in a range because the test is measuring interferon gamma that’s been released by white blood cells in response to stimulation with TB antigens that have been coated inside the specimen tube. On certain days a person’s white cells might be up, and on certain days they’re down,” said Yen-Lieberman.
Matthew Bankowski, PhD, who was not involved in the study, agreed that IGRAs have inherent variability, but still questioned Cleveland Clinic’s use of a new cutoff. “This is a bioassay, so it’s taking live cells—lymphocytes—and if you’re not nice to them in the lab, they’re going to die off,” he said. “You might have started with 100 percent viability and by the time you’re done processing the sample, depending on who’s processing it, it might be down to 50 percent. So you might be seeing a lot less reactivity than you’d like to see in that tube. The problem is we don’t have a gold standard to compare this test to.” Bankowski is vice president and technical director at Queen’s Medical Center’s Diagnostic Laboratory Services and assistant professor of pathology at the University of Hawaii’s John A. Burns School of Medicine in Honolulu.
Bearing in mind IGRAs’ variability and the lack of a gold standard, Bankowski cautioned about the choice of a new cutoff. “The companies will put their interpretations of the tests out there, and if you’re going to deviate from them, you better have a good reason, and the data to support it. In this case, the authors are partially basing their change on another paper that’s from a different population. However, I think it would have been wise to say they were going to gather more information and if the data panned out they would go in this direction based on their population.” According to Yen-Lieberman, the Cleveland Clinic has follow-up studies underway to evaluate the impact of this new approach.
Bankowski suggested that another strategy to address indeterminate, low-positive, and converting results would be to save the original specimen and retest it along with a new specimen to detect the change value. “I support having a repeat on IGRA testing to see where it stands compared to the first one. But I’d hold the first test in the freezer to see if we have reproducibility on the value from that specimen compared with the second one. If the result hasn’t changed and the value is less than 1 IU/mL, then you’d be done.”
Putting aside any discussion about the choice of cutoffs, Yen-Lieberman and Bankowski agreed that proper phlebotomy and specimen handling is essential to minimizing indeterminate and false-positive IGRA results. “The collection for this test is counter-intuitive to any other blood collection. You have three tubes, and when you collect the
blood you actually hemolyze the cells because you want to get rid of the red blood cells and keep the white cells,” explained Bankowski. “If you don’t do it correctly and keep that antigen in each one of those tubes well-mixed, you can affect the end result. And you get indeterminates from that.”
When Cleveland Clinic first implemented IGRA testing among employees, Yen-Lieberman was concerned about the level of indeterminate results, which was about 11.3%. In response, she and her colleagues made a training video as part of an extensive training process, promoted a senior phlebotomist to oversee the process, and implemented other quality control measures. With these steps in place, Cleveland Clinic’s indeterminate rate now is about 0.63% and Yen-Lieberman closely monitors reports to quickly address any negative variances.