Historically, other antigens as an "anergy panel" (e.g. Candida, mumps, or tetanus toxoid preparations) have been used to evaluate an immunosuppressed patient's ability to react to the PPD.

For example, if such a patient is anergic (i.e. non-reactive to Candida, mumps, or tetanus toxoid antigen) and the tuberculin skin test is non-reactive, then one cannot definitively rule-out prior TB infection; therefore, the tuberculin skin test is 'uninterpretable'.

Recently, the CDC has recommended discontinuation of anergy testing in HIV+ individuals since anergy in this population is often in flux; hence results of an anergy panel may vary over time, thereby misleading interpretation of the tuberculin skin test.

Evaluation of a Positive Tuberculin Skin Test

An individual with positive tuberculin skin test (+PPD) or a history of a positive skin test in whom a diagnostic evaluation for active TB is negative should be evaluated for preventive therapy (e.g. INH prophylaxis). The decision to initiate preventive therapy is a complex one where the physician has to balance the risk of reactivation of TB versus the possible side effects of the prophylactic agent. In general, a patient with a +PPD has a 10% lifetime risk of reactivation. The risk of reactivation is greatest within the first two years of PPD conversion. Individuals infected with HIV have a reactivation risk of 10% per year if the tuberculin skin test is positive. INH prophylaxis is very effective in preventing the reactivation of TB in PPD+ individuals over the course of a lifetime (for healthy skin test converters, the efficacy is 93-98%). If assistance is needed to decide if preventive therapy is appropriate for a patient, help can be obtained from the infectious diseases or pulmonary consult services and the Winchester Chest Clinic (for inpatients and outpatients, respectively).