Tuberculin skin test
(TST/ PPD): inject purified protein intradermally, examine for wheal 48–72 hrs later. Interpret based on max diameter of induration, not erythema.
Size of Reaction: Persons Considered to Have ⊕ Test (NEJM 2002;347:1860)
5 mm: HIV ⊕ or immunosupp. (eg, prednisone 15 mg/d × >1 mo) Close contacts of Pt w/ active TB; CXR c/w prior active TB 10 mm: All other populations with ↑ prevalance/risk. Healthcare workers, recent conversion (↑ induration by >10 mm in 2 y). 15 mm: No risk factors
False negative
Faulty application, anergy (including from active TB), acute TB (2–10 wk to convert), acute non-TB mycobacteria (NTM), malignancy
Flase positive
Improper reading, cross-reaction with NTM, BCG vaccination (although usually <10 mm by adulthood)
Booster effect
↑ in duration b/c immunologic boost by prior skin test in prev sensitized individual (by TB, NTM, or BCG). Test ⊖ → ⊕ but not true conversion due to recent infxn. 2nd test true baseline. Can be 1 y after initial test.