Mantoux Test (Tuberculin Skin Test / TST)

What It Tests For

The Mantoux test screens for infection with Mycobacterium tuberculosis - it detects both latent TB infection (LTBI) and past/present active TB.

It is a Type IV (delayed-type) hypersensitivity reaction - a cell-mediated immune response involving Th1 cells and sensitized T lymphocytes.

How It Is Performed

Agent injected: PPD (Purified Protein Derivative) - 1 TU of PPD-RT 23 in India; 5 TU PPD-S (standard internationally)
Route: Intradermal, 0.1 mL on the flexor surface of the left forearm, mid-way between elbow and wrist
Needle position: Bevel facing upward, produces a pale wheal of 6-10 mm
Reading time: 48-96 hours; 72 hours (3rd day) is ideal
What is measured: Transverse diameter of induration (not erythema) in millimeters, using a transparent plastic ruler or calipers

Interpretation (CDC Classification)

Induration	Considered Positive In
≥ 5 mm	HIV-positive persons; recent TB contacts; fibrotic chest X-ray changes; organ transplant / immunosuppressed patients (>15 mg/day prednisone ≥1 month)
≥ 10 mm	Recent immigrants (<5 yr) from high-prevalence countries; IV drug users (HIV-negative); lab/healthcare workers; residents of prisons/nursing homes/homeless shelters; high-risk medical conditions (DM, silicosis, CKD, leukemia, lymphoma, gastrectomy, etc.); children <4 years
≥ 15 mm	Persons with no risk factors for TB

Doubtful: 6-9 mm (may reflect M. tuberculosis or atypical mycobacteria)
Negative: <6 mm

Important Points

A positive test = evidence of past or present TB infection (does NOT distinguish latent from active TB)
A negative test does NOT exclude TB - cellular immunity can be suppressed in HIV, malignancy, malnutrition, measles, steroids, and severe bacterial infections (including TB itself)
Strong reactors (≥20 mm) have a higher chance of progressing to active TB
Two-step testing is used for periodic retesting (e.g., healthcare workers) to distinguish a true new infection from a "booster" effect of waned old immunity
Alternative modern test: IGRA (Interferon-Gamma Release Assay) - not affected by BCG vaccination
Park's Textbook of Preventive and Social Medicine
Bailey and Love's Short Practice of Surgery, 28th Edition
Janeway's Immunobiology, 10th Edition

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