Mantoux test (tuberculin skin test)

Principle

Purified protein derivative (PPD) injected into the skin causes a delayed (type IV) hypersensitivity reaction in people sensitised to M. tuberculosis or BCG.

Procedure

• Inject 0.1 mL of PPD (5 TU) intradermally on the forearm to raise a pale wheal.
• Read at 48–72 hours: measure the INDURATION (not redness) across the forearm in millimetres.

Interpretation (common cut-offs)

• 5 mm or more: HIV positive, recent TB contact, immunosuppressed
• 10 mm or more: recent arrivals from high-burden countries, health workers, children under 4
• 15 mm or more: people with no risk factors

Limitations

BCG and non-tuberculous mycobacteria cause false positives; immunosuppression causes false negatives. Interferon-gamma release assays (IGRA) avoid the BCG problem.