Will a positive purified protein derivative (PPD) test return to normal, i.e., become nonreactive, after isoniazid treatment?
—Maj. James R. Northern Sr, DO, Mexico, Mo.

In an immunocompetent patient, the PPD is a marker of active or latent infection by Mycobacterium tuberculosis. Once the individual has been treated appropriately with isoniazid, there may be reversion to a negative skin test in approximately 10% of persons who had formerly tested positive. This occurs more commonly in younger patients. Repeating a PPD after someone has been treated is not contraindicated. However, as reversion to negative does not happen with great frequency, it should not be used as a marker of successful treatment.
—Cedric W. Spak, MD (103-10)

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