Purified Protein Derivative (PPD): Sensitivity and Specificity
adults with active TB
>=10mm threshold for induration
specificity in Japanese adults with a high rate of bCG vaccination. Note that non-TB mycobacterium (e.g. avium) also causes false positives: in US health care workers, in one study 50% of indurations 10-14mm were from M avium (PMID 11769770).
Some centers use a 'booster' or serial PPD strategy whereby a second PPD is placed 1-4 weeks after the first. The idea is to generate a stronger immune reaction in the second test, given that the reaction may be blunted if the initial TB exposure was long ago. This has not been evaluated for specificity.
90% sensitive, 35.4% specific