CDC classification of tuberculin reaction An induration (palpable raised hardened area of skin) of more than 5–15 mm (depending upon the person's risk factors) to 10 Mantoux units is considered a positive result, indicating TB infection. 5 mm or more is positive in HIV-positive person Recent contacts of TB case Persons with nodular or fibrotic changes on CXR consistent with old healed TB Patients with organ transplants and other immunosuppressed patients 10 mm or more is positive in Recent arrivals (less than 5 years) from high-prevalent 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 corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight, etc.) Children less than 4 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 (Note: Targeted skin testing programs should only be conducted among high-risk groups) A tuberculin test conversion is defined as an increase of 10 mm or more within a 2-year period, regardless of age. BCG vaccine and tuberculin skin test There is disagreement on the use of the Mantoux test on people who have been immunized with BCG. The US recommendation is that in administering and interpreting the Mantoux test, previous BCG vaccination should be ignored; the UK recommendation is that interferon-γ tests should be used to help interpret positive tuberculin tests, also, the UK does not recommend serial tuberculin skin testing in people who have had BCG (a key part of the US strategy). In their guidelines on the use of QuantiFERON Gold the US Centers for Disease Control and Prevention state that whereas Quantiferon Gold is not affected by BCG inoculation tuberculin tests can be affected. In general the US approach is likely to result in more false positives and more unnecessary treatment with potentially toxic drugs; the UK approach is as sensitive in theory and should also be more specific, because of the use of interferon-γ tests. Under the US recommendations, diagnosis and treatment of latent tuberculosis infection (LTBI) is considered for any BCG-vaccinated person whose skin test is 10 mm or greater, if any of these circumstances are present: Was in contact with another person with infectious TB Was born or