Anybody who needs regular periodic screening PPDs, like health workers or the institutionalized (nursing homes, prisons, homeless shelters, etc), should receive 2 PPDs the first time, spaced 1-3 weeks apart. Why?
If a person was infected with Mycobacterium tuberculosis (Mtb) a long time ago, their Delayed-Type Hypersensitivity (DTH) response to the PPD may have waned. So the test will be negative. However, the PPD itself will “boost” DTH memory. So the next PPD will be positive, diagnosing Latent TB Infection (LTBI). [note that protective cell-mediated immunity does not wane].
If that 2nd PPD is placed a year later, it’ll look like a “new conversion,” i.e. infection occurred within those 12 months. The first 2 years of LTBI is the highest risk period for reactivation. INH therapy is definitely recommended for New Converters (positive tuberculin test within 2 yrs. of a prior negative).
However, if the 2nd PPD turns positive barely 2 weeks after the first, it’d be a stretch of the imagination to say they “must’ve just been infected.” Rather, they were infected a long time ago, and are usually not a candidate for INH. But they do have LTBI.
Don’t worry that repeated PPDs can cause a positive response; they can’t. In school I did a clinical rotation at NYC’s Rikers Island jail. All inmates got PPDs upon arrival, and there was such a revolving door that many were tested countless times within just a few months. Nobody converted.
And now a secret that most people don’t recognize: there are studies showing a few people with remote Mtb infection boost on the 3rd PPD, or even the 4th. But nobody would ever expect institutions to do 3- or 4-step testing; 2-step covers the majority of “booster” reactions.
Use of the IGRA obviates all of this. But nobody knows if some boosted PPDs may be missed.
DiagnosisDude 