What a Positive Tb Skin Test Actually Looks Like: Visual Guide and Induration Breakdown
The standard Mantoux tuberculin skin test involves injecting 0.1 mL of purified protein derivative (PPD) into the inner surface of the forearm. Within seconds, a small, pale wheal forms resembling a mosquito bite, which typically flattens within an hour. The real immune response unfolds over the next two to three days.
When the immune system encounters tuberculin antigens it recognizes, sensitized T-lymphocytes migrate to the injection site. This cellular traffic causes localized vascular permeability, fibrin deposition, and cellular infiltration, generating a firm, rubbery swelling. This hard bump is induration.
Many patients misinterpret extensive skin redness (erythema) as a positive result. Erythema is simply a superficial histamine or cutaneous response. A patient may present with a fiery red circle spanning 30 millimeters across the arm, yet possess zero induration upon palpation. Clinically, that reading is 0 mm, a completely negative test. Conversely, a subtle, flesh-colored elevation measuring 12 millimeters with almost no redness can represent a confirmed positive. Evaluating a TB skin test through a smartphone photograph fails because cameras capture color gradients, not subcutaneous structural density.
