Delayed hypersensitivity to PPD( purified protein derivative)
0.1 mL of PPD induces a visible and palpable induration (at least 5 mm in diameter) that peaks in 48 to 72 hours
It does not differentiate between infection and disease
False-negative reactions (or skin test anergy)
overwhelming active tuberculous disease.
False-positive reactions may also result from infection by atypical mycobacteria.
only a small fraction of those who contract an infection develop active disease.