Objective: To investigate the incidence of tuberculosis (TB) in a cohort of contacts and to analyze the variables associated with the disease.
Methods: A prospective analysis was undertaken of all the contact investigations carried out in a health district in the Basque Country in northern Spain between January 1, 1995 and December 31, 2004. The dependent variable was the number of cases of TB detected among the contacts. Independent variables were age, sex, tuberculin skin test result, and the degree of contact. In index cases, additional variables were the site of the disease and smear test result.
Results: Analysis of 5444 contacts of 596 patients with TB yielded 66 secondary cases of TB (40 at the time of the contact investigation and 26 at a later stage); the majority (73%) developed the disease within 12 months. Multivariate analysis revealed a significant relationship between the detection of secondary cases and the following variables: close contact (odds ratio [OR], 3.05; 95% confidence interval [CI], 1.75-5.31), positive smear test (OR, 8.54; 95% CI, 2.06-35.43), induration of 10 mm or larger (OR, 10.18; 95% CI, 4.27-24.26), and age under 30 years (OR, 3.35; 95% CI, 1.88-5.98). The final predictive model constructed on the basis of these 3 variables had a sensitivity of 77.4%, a specificity of 80.3%, and an area under the curve of 0.83 (95% CI, 0.78-0.88).
Conclusions: The contact investigation is a valid strategy for the detection of new cases of TB. Prophylactic treatment should be implemented at an early stage and priority should be given to young contacts of smear positive patients using an induration size of 10 mm or more as a reference.