Quantifying local TB risk factors and determining the relative cost of treatment of latent TB infection to prevent tuberculosis

RATIONALE: The risk of tuberculosis (TB) disease in tuberculin skin test positive (TST+) persons ranges from 5-15%. National averages of risk mask local variation. OBJECTIVE: Quantify local TB disease risk and the cost of preventing one case of TB disease with treatment of LTBI. METHODS: Saskatchewan persons with a first TST+ recorded from January 1, 1986 to January 31, 2002 were reviewed for progression to TB by ethnic origin, age, sex, and interval from first TST+ to TB. From this risk the cost to prevent one case of TB disease was determined for each group. RESULTS: The cumulative incidence of TB in 11,698 persons after first TST+ was 6.2%. There was variability in incidence density by ethnic origin (0.6-15.2), age (2.4-33.9), sex (4.7-9.2), and recent vs. remote interval to TB disease (1.3-1667). The cost to prevent one case of TB disease in non-Indigenous people >= 20 years with remote infection compared to Indigenous people <20 years with recent infection was higher by a factor of >200. CONCLUSIONS: Progression from first TST+ to TB disease varied by age, ethnicity, and interval from TST+ to TB disease but not by sex. The cost to prevent one case of TB disease varied inversely. Using local epidemiological data is useful and simple to quantify risk of TB disease and cost to achieve best prevention for each dollar spent.