Another risk factor for progression to active TB is the degree of tuberculin sensitivity, as measured by the size of induration. A 4-year study of 1.2 million recruits to the US Navy found the risk for progression from LTBI to active TB was greater for sailors whose induration at enlistment measured ≥10 mm than for those whose induration was <10 mm (Comstock et al. 1974a). The investigators found that a history of household exposure to TB further increased the risk for progression to active disease. Tuberculin reactors whose induration measured ≥5 mm at enlistment and who had a history of household exposure to TB were more likely to progress to active TB than sailors who lacked such a history.
HIV infection dramatically increases the risk of both primary TB and reactivation TB (Davies 2005). People with LTBI who become infected with HIV face a 5-10% annual risk of developing reactivation TB (Glynn 1998). HIV-infected people exposed to M. tuberculosis have an approximate 40% risk of acquiring the infection and progressing to the active disease within 3 months. Some other conditions also increase the risk of progression to active TB: disorders associated with defects in cell-mediated immunity, such as hematologic malignancies and lymphatic malignancies; diabetes mellitus; renal dialysis; weight loss; intestinal bypass; and gastrectomy. Medical conditions that increase the risk of TB are silicosis, diabetes mellitus, chronic renal failure, leukemias and lymphomas, carcinoma of the head or neck and lung, loss of