CDC ponders postscript on TST false-positives
CDC ponders postscript on TST false-positives
Warn before you switch; re-test with other reagent
According to published research, the two commercially available skin-test reagents (Aplisol, made by Monarch Pharmaceuticals, and Tubersol, from Aventis Pasteur) do equally well in their ability to detect latent TB infection. But the research certainly hasn’t stopped occasional reports that one of the reagents — usually Aplisol — has caused another rash of apparently false-positive reactions.
To help clinicians sort things out, federal TB experts are mulling some additional instructions on the subject, probably for publication in some future edition of Morbidity and Mortality Weekly Report. According to Elsa Villarino, MD, MPH, chief of diagnostics and therapeutics at the Division of TB Elimination of the Centers for Disease and Prevention, the new directive should go something like this:
• If you’re planning to change skin-test reagents, let people know what’s about to happen.
• If you do see a rise in positive tests, estimate the likelihood of disease, and weigh risks and benefits of possible interventions (such as X-ray, sputum test, or treatment with isoniazid.)
• Consider re-testing with the other reagent.
Lot-to-lot variations the culprit?
Even so, Villarino stands by her landmark study comparing the two reagents, which showed they give comparable results.1 The fact that Tubersol is 1% more specific than Aplisol made no difference in the way tests in the study were read, Villarino reminded an audience at a recent meeting of the Advisory Council to Eliminate Tuberculosis (ACET). Still, some TB experts at the ACET meeting argued, when lots of people in low-incidence populations are getting tested, a reagent that’s slightly more specific should have better luck separating true reactions from mycobacterial background noise.
That’s not the only possible explanation for Aplisol’s seemingly erratic performance. In a recent letter to the Journal of the American Medical Association, Henry Blumberg, MD, associate professor of medicine at Emory University in Atlanta, reported eleven conversions among health care workers at Grady Memorial Hospital (which had switched to Aplisol after years of using Tubersol). In the letter, Blumberg theorized that lot-to-lot variations in Aplisol might explain its seeming unpredictability. Perhaps Villarino’s study didn’t pick up on such discrepancies, he wrote, because Aplisol samples used in the study came from only two lots.2
Before the CDC makes any public comments, Villarino is waiting on two additional sources of information. MedWatch, a consumer-complaint log maintained by the Food and Drug Administration, has made available its reports on tuberculin skin tests dating back to January 1998. Analysis of the MedWatch material may turn up some useful data, says Villarino.
Second, the New Jersey Model TB Center recently convened a panel of skin-test experts, Villarino among them, who made presentations on the subject. The talks are being edited into a monograph, due for publication sometime this summer. That will supplant the only other extant monograph on the subject, which dates back to 1993, and may shed some light on the two-reagent controversy, notes Villarino.
It’s enough to make a person yearn for a new way to test for latent TB infection. That, of course, is just what the FDA recently approved in Quantiferon, the one-stop blood test for TB latency. Given all the curiosity about and interest in the new test, Villarino told the ACET meeting that though she’d prefer to wait on more research, the CDC will probably soon publish some stopgap recommendations on how to use the new product.
References
1. Villarino E, Burman W, Wang YC, et al. Comparable specificity of commercial tuberculin reagents in persons at low risk for tuberculosis infection. JAMA 1999; 281:169-171.
2. Blumberg HM, White N, Parrott P, et al. False positive tuberculin skin test results among health care workers. JAMA 2000; 283:2793.
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