<P> A standard dose of 5 tuberculin units (TU - 0.1 ml), according to the CDC, or 2 TU of Statens Serum Institute (SSI) tuberculin RT23 in 0.1 ml solution, according to the NHS, is injected intradermally (between the layers of dermis) and read 48 to 72 hours later . This intradermal injection is termed the Mantoux technique . A person who has been exposed to the bacteria is expected to mount an immune response in the skin containing the bacterial proteins . </P> <P> The reaction is read by measuring the diameter of induration (palpable raised, hardened area) across the forearm (perpendicular to the long axis) in millimeters . If there is no induration, the result should be recorded as "0 mm". Erythema (redness) should not be measured . </P> <P> The results of this test must be interpreted carefully . The person's medical risk factors determine at which increment (5 mm, 10 mm, or 15 mm) of induration the result is considered positive . A positive result indicates TB exposure . </P> <Ul> <Li> 5 mm or more is positive in <Ul> <Li> An HIV - positive person </Li> <Li> Persons with recent contacts with a TB patient </Li> <Li> Persons with nodular or fibrotic changes on chest X-ray consistent with old healed TB </Li> <Li> Patients with organ transplants, and other immunosuppressed patients </Li> </Ul> </Li> <Li> 10 mm or more is positive in <Ul> <Li> Recent arrivals (less than five years) from high - prevalence countries </Li> <Li> Injection drug users </Li> <Li> Residents and employees of high - risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters, etc .) </Li> <Li> Mycobacteriology lab personnel </Li> <Li> Persons with clinical conditions that place them at high risk (e.g., diabetes, prolonged corticosteroid therapy, leukemia, end - stage renal disease, chronic malabsorption syndromes, low body weight, etc .) </Li> <Li> Children less than four years of age, or children and adolescents exposed to adults in high - risk categories </Li> </Ul> </Li> <Li> 15 mm or more is positive in <Ul> <Li> Persons with no known risk factors for TB </Li> </Ul> </Li> </Ul>

Test for tuberculosis involves the intracutaneous injection of purified protein derivative (ppd)