A major element of the strategy for eliminating TB in the United States is the prevention of active TB through the treatment of persons with latent TB -- those with a positive tuberculin skin test but no evidence of active disease. In this group, TB develops from reactivation of previously controlled infection. Prolonged antibiotic therapy is needed to treat latent TB. Completion rates for a six-month course of self-administered therapy is 3-60 percent, with rates of 20-30 percent in most series. For the currently recommended nine-month course, completion rates are likely to be even lower. The lifetime risk of reactivation TB is estimated at 5-10 percent, an estimate based on a large body of data collected before treatment for latent TB was routinely recommended. A more precise assessment could help to identify those patients at highest risk and motivate them to complete treatment.

Published reports were reviewed to obtain estimates of the TB risk among persons with a positive tuberculin skin test. The author used these data to construct a model to estimate the lifetime TB risk of persons with specific medical conditions.

The results revealed that lifetime risk of reactivation TB is 20 percent or more among most persons with induration of 10mm or more on a tuberculin skin test and either HIV infection or evidence of old, healed TB. The lifetime risk is 10-20 percent among persons with recent conversion of a tuberculin skin test and among persons under age 35 who are receiving infliximab therapy and have induration of 15 mm or more on a tuberculin skin test. Among children age five or younger who have a tuberculin skin test induration of 10mm or more, the risk is 10-20 percent.

"These results have important implications for tuberculosis control," the author concluded. "Persons who are reported to have had a positive tuberculin skin test in the past but whose records do not indicate the size of the reaction, as well as persons whose skin test was read by an untrained reader, should be retested (unless blistering occurred after the earlier test). Such retesting would also identify persons who previously had positive skin tests but have lost their tuberculin reactivity and therefore do not require treatment. Reliable current results of a tuberculin skin test can help clinicians to estimate accurately the risk of reactivation tuberculosis for individual patients. Communication of the existence of a substantial risk will reinforce the importance of adherence to treatment for latent tuberculosis. The population groups that are at the highest risk for reactivation tuberculosis should be targeted with intensive efforts to increase adherence, such as the use of directly observed therapy. The targeting of such efforts will enhance the likelihood that tuberculosis will be eliminated in the United States."

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