In a few situations, however, the PPD test may be nondiagnostic. Individuals who have received bacille Calmette-Guerin (BCG) immunization for tuberculosis prevention would be expected to have a positive response. Some individuals with remote BCG immunization lose PPD reactivity over time, but if a second PPD is placed within a few weeks of the first, a falsely positive booster effect may be seen.13 Exposure to nontuberculosis mycobacteria can also result in a false-positive PPD reaction.
A competent immune system is required to yield a positive response to the PPD if there has been exposure to M. tuberculosis. Therefore, individuals with abnormal immune systems, such as in HIV, may lose their ability to mount a delayed-type hypersensitivity reaction required to produce a positive PPD reaction. Although the Centers for Disease Control (CDC) no longer recommend routine anergy testing among HIV-positive individuals, 16 some clinicians utilize control agents such as mumps or candida to assess at-risk patients for anergy. Controls are placed on the opposite forearm, and failure to respond with induration may indicate anergy and thus an unreliable PPD. False negatives have also been demonstrated with improper PPD administration. Nonreactivity to a PPD has been occasionally reported in patients with active culture-positive tuberculosis; therefore, a PPD test may occasionally be unreliable in acute stages of the disease.
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