tuberculin skin test
The tuberculin skin test checks whether the immune system has met TB before by giving it a tiny, harmless reminder. A small amount of purified TB protein is injected just under the skin of the forearm; if the body recognises it, immune cells gather there over two or three days and raise a firm, raised bump.
Reading the test means measuring the diameter of the firm swelling (not the redness) at 48 to 72 hours. A larger reaction makes prior TB infection more likely, but the cut-off for calling it “positive” is deliberately adjusted to the person — lower thresholds are used for those at higher risk, such as people with HIV or close contacts of a TB case.
The test detects the immune memory of exposure, not active disease, so a positive result cannot by itself distinguish latent infection from active TB. It has well-known pitfalls: the BCG vaccine and some environmental mycobacteria can cause false positives, while severe immune suppression can cause false negatives. These limits are why blood-based tests are often used as an alternative.
A nurse exposed to a TB patient returns at 72 hours; the area of firm swelling is measured with a ruler and recorded in millimetres to decide if the test is positive.
Measuring induration — the raised firm area, not the redness.
Because prior BCG vaccination can produce a false-positive skin test, an interferon-gamma release assay (a blood test unaffected by BCG) is often preferred in vaccinated people.