What does a positive purified protein derivative (PPD) skin test result indicate?

The PPD (tuberculin) skin test is a Type IV hypersensitivity reaction (delayed-type, cell-mediated). A positive result (induration of 5-15 mm depending on risk group) indicates prior sensitization to Mycobacterium tuberculosis antigens, which occurs either through actual TB infection (latent or active) or through BCG vaccination. A positive PPD does not distinguish between active and latent TB, nor does it confirm active infection. Further evaluation (chest X-ray, sputum culture, IGRA test) is needed to determine the extent of disease. Multiple sclerosis and rheumatoid arthritis are unrelated.