Poor TST readings lead to false-positives
Use experienced readers, risk assessment
In the world of tuberculosis screening, sometimes an unfortunate series of events leads down the path toward inappropriate treatment. Consider this real-life scenario: the antigen had changed; the tuberculin skin test (TST) reader was inexperienced; and the employees in this case, firefighters were in a low-risk community in Mississippi.
Nine firefighters tested positive for TB, although they had no known exposure and no source of TB infection could be identified. An investigation by the Centers for Disease Control and Prevention led to conclusions that are pertinent to all occupational TB screening programs, including those that are hospital-based: Target your at-risk employees, train your TST readers and don't switch antigens. Or, better yet, use the more specific blood assay test.1
"In any occupational testing, be it health care workers or firefighters or policemen, the decision to test or not should be based on an actual TB assessment of risk," says John D. Gibbins, DVM, MPH, lieutenant commander with the U.S. Public Health Service and an Epidemic Intelligence Service (EIS) officer with CDC's Health Hazard Evaluations and Technical Assistance branch. "Because of the inherent problems with the TST, they shouldn't be administered any more than necessary and only to groups that are at an increased risk."
Unfortunately, problems with the TST are all too common, says Lee B. Reichman, MD, MPH, FACP, FCCP, professor of medicine, preventive medicine and community health at the New Jersey Medical School and executive director of the Global Tuberculosis Institute in Newark.
That was demonstrated in a study by researchers at the Medical College of Virginia in Richmond. They tested the TST reading capabilities of 107 clinicians, including pediatricians, academic pediatricians, nurses and a nurse practitioner at a general hospital and a university hospital. The patient had been treated for pulmonary tuberculosis, but the readers were not told of his history. Ninety-three percent of the clinicians incorrectly read the TST as negative with an induration of less than 15 mm. A third (33%) read the induration as less than 10 mm.2
That level of error is nothing short of "scandalous," says Reichman. "If this was X-ray reading or EKG reading, there would be a congressional hearing," he says. "But with TB, no one gives a damn."
Incorrect reading of TSTs may equate to undiagnosed TB infection or could lead to health care workers who are unnecessarily placed on isoniazid treatment, he says.
The problem in Mississippi began in 2006 when the fire department implemented a TST program to comply with the National Fire Protection Associa-tion guidelines. The 101 firefighters received their two-step TST at the local hospital.
It is notoriously difficult to make sure employees return promptly to have their skin test read, and that was true for the firefighters. Of the nine who eventually were identified as having a positive skin test, six did not return for their first reading until nine to 21 days after they were placed instead of the recommended 48 to 72 hours.
Meanwhile, the hospital purchasing department had switched from Tubersol to Aplisol without informing clinicians, and the firefighters were the first to receive the new antigen. There also were problems with the way the test was read along the vertical axis of the forearm, instead of the horizontal axis. The nurse reading the TSTs was inexperienced; these were among the first she had read, says Gibbins.
Either TST antigen is considered acceptable, but the CDC recommends against switching antigens because of possible problems with interpretation, says Gibbins. "You see issues like this happening time and again," he says. "A lot of times, they do seem to be associated with the change from one tuberculin to another. Erythema can be read as a positive [TST]."
In this case, nine firefighters were diagnosed with latent tuberculosis infection. Five began isoniazid therapy and the others either refused the treatment or stopped soon after starting it. There was no known TB exposure, and as news spread of the positive TB tests, the community became concerned about the possible spread of TB.
The fire department asked for a Health Hazard Evaluation by the National Institute for Occupa-tional Safety and Health (NIOSH). The CDC investigators initially suspected the positives were caused by nontuberculous mycobacteria that is prevalent in the Southeast. However, that mycobacteria should cause an induration of less than 15 mm, says Gibbins.
When the nine firefighters were retested with the QuantiFERON-TB Gold blood test, none were positive. They ceased taking the isoniazid. CDC also underscored its recommendations related to tuberculin skin tests. (See article, above right.)
Here are some of the lessons learned:
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CDC: Take steps to avoid TST errors In response to tuberculin skin test (TST) reading errors of firefighters in Mississippi, the Centers for Disease Control and Prevention provided these recommendations for those who administer the tests:
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- Conduct a risk assessment and limit employees who are screened. "Because of the inherent problems with TST, they shouldn't be administered any more than necessary and only to groups that are at an increased risk," says Gibbins.
- No self-reading. Under no circumstances should physicians and nurses be allowed to read their own TSTs, says Reichman. "Self-reading of any kind is totally unacceptable and should be thrown out," he says. "There are all sorts of stories of misdiagnosis."
- Consider using the blood test. The QuantiFERON test is more specific than the TST and it is a one-step test, notes Reichman. In a German study, 261 health care workers who had been exposed to tuberculosis were tested with both the TST and QuantiFERON. Twenty-four percent were positive with the TST, but just 9.6% were positive with the blood test. A previous TST or vaccination with BCG could explain 98% of the discordant positive TSTs, the authors said. Meanwhile, only 60% of the positive QuantiFERON tests also were detected by the TST.3
- Investigate any unusual TB screening results. Whether you use the TST or the blood test, you should look into any unexpected screening results, says Gibbins. "The blood test is not a panacea. You're still going to have to interpret results in line with clinical signs and symptoms," he says. "If you do see something out of the ordinary, the first thing you have to do is investigate why, just as you do with the TST."
References
1. Centers for Disease Control and Prevention. Evaluation of results from occupational tuberculin skin tests Mississippi, 2006. MMWR 2007; 56:1,316-1,318.
2. Kendig EL, Kirkpatrick BV, Carter WH, et al. Under-reading of the tuberculin skin test reaction. Chest 1998; 113:1,175-1,177. Available at www.chestjournal.org/cgi/reprint/113/5/1175?ck=nck. Accessed on Feb. 11, 2008.
3. Nienhaus A, Schablon A, Le Bacle C, et al. Evaluation of the interferon-g release assay in healthcare workers. Int Arch Occ Environ Health 2007; 81:295-300.
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