a standardised dead extract of cultured TB, injected into the skin to measure the person's immune response to the bacteria. So, if a person has been exposed to the bacteria previously, they should express an immune reaction to the injection, usually a mild swelling or redness around the site. There have been two primary methods of TST: the Mantoux test, and the Heaf test. The Heaf test was discontinued in 2005 because the manufacturer deemed its production to be financially unsustainable, though it was previously preferred in the UK because it was felt to require less training to administer and involved less inter-observer variation in its interpretation than the Mantoux test. The Mantoux test was the preferred test in the US, and is now the most widely used TST globally. Mantoux test See: Mantoux test The Mantoux test is now standardised by the WHO. 0.1 ml of tuberculin (100 units/ml), which delivers a dose of 5 units is given by intradermal injection into the surface of the lower forearm (subcutaneous injection results in false negatives). A waterproof ink mark is drawn around the injection site so as to avoid difficulty finding it later if the level of reaction is small. The test is read 48 to 72 hours later. The area of induration (NOT of erythema) is measured transversely across the forearm (left to right, not up and down) and recorded to the nearest millimetre. Heaf test See:Heaf test The Heaf test was first described in 1951. The test uses a Heaf gun with disposable single-use heads; each head has six needles arranged in a circle. There are standard heads and pediatric heads: the standard head is used on all patients aged 2 years and older; the pediatric head is for infants under the age of 2. For the standard head, the needles protrude 2 mm when the gun is actuated; for the pediatric heads, the needles protrude 1 mm. Skin is cleaned with alcohol, then tuberculin (100,000 units/ml) is evenly smeared on the skin (about 0.1 ml); the gun is then applied to the skin and fired. The excess solution is then wiped off and a waterproof ink mark is drawn around the injection site. The test is read 2 to 7 days later. Grade 0: no reaction, or induration of 3 or less puncture points; Grade 1: induration of four or more puncture points; Grade 2: induration of