Question
Mantoux test.
Answer
The Mantoux test (tuberculin skin test) is the standard in-vivo test for detecting prior sensitization to Mycobacterium tuberculosis antigens, based on a Type IV (delayed-type) hypersensitivity reaction.
Procedure: 0.1 mL of purified protein derivative (PPD), standardized at 5 tuberculin units, is injected intradermally into the volar aspect of the forearm, raising a small wheal; the injection site is examined 48–72 hours later.
Reading: the diameter of induration (palpable hardness, not just erythema/redness, which is not measured) is measured transversely across the forearm in millimetres.
Interpretation (thresholds vary by risk category and national guidelines, but generally):
- ≥5 mm — considered positive in high-risk individuals (HIV-infected, close contacts of active TB cases, immunosuppressed, or those with chest X-ray changes suggestive of old healed TB).
- ≥10 mm — considered positive in intermediate-risk groups (e.g., recent immigrants from high-burden countries, healthcare workers, residents of high-risk congregate settings).
- ≥15 mm — considered positive in low-risk individuals with no known risk factors.
Significance and limitations: a positive test indicates prior sensitization/infection (which may be latent or active) but does not distinguish latent infection from active disease, and does not confirm current infectivity; it can be falsely positive after BCG vaccination (since BCG also induces tuberculin sensitivity) or with exposure to non-tuberculous mycobacteria; it can be falsely negative in severely immunocompromised patients (anergy), very young children, or very early in infection before sensitization has developed. Interferon-Gamma Release Assays (IGRAs) offer better specificity, being unaffected by prior BCG vaccination.
Clinical use: screening for latent TB infection, epidemiological surveys, and as an aid (alongside clinical/radiological findings) in the diagnostic work-up of suspected TB, particularly in children where microbiological confirmation can be difficult.