Ten years ago, a colleague’s purified protein derivative (PPD) test was read by several physicians as being positive at 13-mm induration. Disturbed by the results, we tried testing with PPD again. The area became red but not indurated. Since then, yearly PPD tests have been nonreactive. I presume that the original response was just a local inflammatory reaction. Should we confirm every “positive” reaction with a subsequent PPD test?
—Julie Lavinder, MD, Alliance, Ohio

No. According to consensus guidelines from the American Thoracic Society and the CDC (Am J Respir Crit Care Med. 2000;161:1376-1395), the original PPD was a positive TB skin test (TST), as it was >10 mm and the patient was working in the health-care setting. In the absence of active TB, this warranted isoniazid (INH) prophylaxis for nine months (JAMA. 2005; 293:2776-2784). Note that with the initial positive result described, your patient has a 10% lifetime risk of developing active TB, and the risk does not decrease with serial TSTs because anergy may develop as the time from initial exposure increases. The subsequent tests were false negatives, and the first positive TST was likely a true positive, so INH is still indicated.
—Cedric W. Spak, MD, MPH (110-17)

Continue Reading