FIGURE 5.2 Incidence of tuberculosis among initial reactors to tuberculin, by age when tuberculosis was first diagnosed. From 1949 to 1951, investigators administered TSTs to nearly 200,000 Puerto Rican children ages 1-19; 82,269 children tested positive. The investigators followed this cohort through June 1969. During that 18- to 20-year period, 1,400 tuberculin-reactors progressed to active TB. Age was the most important risk factor for progression to active disease, the researchers found. This figure illustrates the age distribution of the 1,400 tuberculin-reactors who progressed to active TB. Incidence rates were highest among children less than 4 years old and second-highest among individuals around 20 years old.

SOURCE: Reprinted with permission from Comstock et al. 1974b.

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

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