Clinincal Briefs-By Louis Kuritzky, MD
Anergy Testing and PPD?
Probably the most common method of administration of tuberculin skin testing (PPD) includes application of control antigens to assess integrity of immune response, and rule out anergy as a cause of a false-negative PPD test. For instance, as many as 20% of AIDS patients with active TB are PPD negative at presentation. Indeed, most persons with miliary TB are PPD negative.
Anergy may reflect immune compromise from a variety of sources. For instance, protein malnutrition is associated with anergy, as is major trauma, histamine, and calcium channel blocker treatment.
The use of anergy testing has not been proven and might even produce misleading information. In a population of HIV-positive persons (n = 623), mumps/candida anergic individuals were just as likely to be PPD positive as nonanergic individuals. As many as 20% of persons with active TB who are PPD negative maintain antigen responsivity to other standard skin-testing antigens. In this scenario, clinicians might be falsely misdirected away from a correct diagnosis by relying on intact cutaneous reactivity to indicate likelihood of appropriate responsiveness to PPD. Slovis and colleagues conclude that the only helpful skin test in TB screening is a positive PPD, as a negative test does not exclude infection and requires further investigation in the setting of high clinical suspicion.
Slovis BS, et al. JAMA 2000;283:2003-2007.
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