As a clinician working for an institution that houses hundreds of mentally disabled residents, I am often asked to check on employees’ purified protein derivative (PPD) conversions to decide if and when those employees should be allowed to return to work. (The institution’s policy is to check employees’ PPD status yearly.) I have been confirming the conversions and requesting stat chest x-rays. Asymptomatic employees whose x-rays show no evidence of active TB are cleared for return to duty. They are then referred to the health department for further evaluation and treatment while attempts to identify index cases are pursued. When x-rays are not available for a few days (usually on weekends, when staff shortage is critical), should asymptomatic employees be allowed to return to work?—Duc Nguyen, MD, Gainesville, Fla.

Yes. A positive TB skin test is a useful screening tool in at-risk populations. As such, it should be used only to guide additional diagnostics, such as chest radiograph and/or clinical exam. If clinical evaluation reveals no obvious symptoms, then the chances of there being acid-fast bacilli in a respiratory sample are exceedingly remote provided (1) the person does not have HIV and (2) he is not extremely immunocompromised in some other way (solid organ transplant recipient, bone marrow transplant recipient, or receiving other T-cell inhibiting immunomodulation).—Cedric W. Spak, MD, MPH (142-7)