Mantoux Test: Principle, Procedure, Interpretation & Clinical Significance

The Mantoux test, also known as the Tuberculin Skin Test (TST), is a widely used diagnostic tool for detecting latent or active tuberculosis infection. It involves the intradermal injection of purified protein derivative (PPD) and subsequent observation of the skin's response.

Principle

The Mantoux test is based on a Type IV hypersensitivity reaction (delayed-type hypersensitivity). When PPD is injected into the skin, individuals previously exposed to Mycobacterium tuberculosis will mount a localized immune response mediated by sensitized T-cells.

Procedure

Using a tuberculin syringe, 0.1 mL of PPD (5 TU) is injected intradermally into the volar surface of the forearm.
A wheal of 6–10 mm in diameter should appear immediately at the injection site.
The site is examined 48 to 72 hours post-injection for induration (not erythema).

Interpretation of Results

≥5 mm induration: Positive in HIV-positive individuals, recent contacts of TB patients, or immunocompromised patients.
≥10 mm induration: Positive in recent immigrants, IV drug users, or patients with certain clinical conditions.
≥15 mm induration: Positive in individuals with no known risk factors.
False positives/negatives: May occur due to BCG vaccination or anergy.

Clinical Significance

The Mantoux test is a cost-effective and accessible method for identifying TB exposure, particularly in high-risk populations. However, it does not differentiate between active and latent TB, requiring further confirmatory testing (e.g., chest X-ray, sputum culture).