A 72-year-old patient has a positive purified protein derivative (PPD) skin test. What does this type IV reaction indicate?

Correct answer: B. Exposure to Mycobacterium tuberculosis or prior BCG vaccination

• A. The PPD has no relationship to multiple sclerosis. (Interferon beta-1b in the medication list treats MS, but that is a separate finding in the case.)
• B. CORRECT. The PPD (Mantoux) test is a TYPE IV delayed hypersensitivity reaction — T-cell mediated, read at 48–72 hours, with induration produced by sensitised T lymphocytes and macrophages. A positive result means the immune system has PREVIOUSLY ENCOUNTERED mycobacterial antigen: either infection with M. tuberculosis (latent or active) or prior BCG vaccination. It indicates exposure and sensitisation, nothing more.
• C. Rheumatoid arthritis is a type III immune-complex and autoimmune process; it is not assessed by a PPD.
• D. The PPD cannot distinguish LATENT from ACTIVE disease. That distinction requires chest radiography and sputum analysis. Most PPD-positive individuals have latent infection and are not infectious.

Note: Hypersensitivity types worth having cold: I = IgE immediate (anaphylaxis); II = IgG/IgM cytotoxic (haemolytic anaemia, pemphigus); III = immune complex (SLE, serum sickness); IV = T-cell delayed (PPD, contact dermatitis, latex type IV, graft rejection).