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 (CDC) led to conclusions that are pertinent to all occupational TB screening programs: 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 workers who are unnecessarily placed on isoniazid treatment, he says.

Here are some of the lessons learned in the Mississippi incident:

  • 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 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.

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.