standard dose of 5 tuberculin units (TU - 0.1 ml), according to the CDC, or 2 TU of Statens Serum Institute (SSI) tuberculin 23 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. 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. A positive result indicates TB exposure. 5 mm or more is positive in An HIV-positive person Persons with recent contacts with a TB patient Persons with nodular or fibrotic changes on chest X-ray consistent with old healed TB Patients with organ transplants, and other immunosuppressed patients 10 mm or more is positive in Recent arrivals (less than five years) from high-prevalence countries Injection drug users Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters, etc.) Mycobacteriology lab personnel Persons with clinical conditions that place them at high risk (e.g., diabetes, prolonged coicosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight, etc.) Children less than four years of age, or children and adolescents exposed to adults in high-risk categories 15 mm or more is positive in Persons with no known risk factors for TB A tuberculin test conversion is defined as an increase of 10 mm or more within a two-years period, regardless of age. Alternative criteria include increases of 6, 12, 15 or 18 mm Ref Harrison20th edition pg 1011