[Factors predicting new tuberculosis infections and tuberculin conversions in a contact tracing system]

To study new cases of latent tuberculosis (TB) infection, active TB and tuberculin conversions detected by means of a system for tracing the contacts of TB patients and to analyze the factors that predict the appearance of new cases.

We performed a longitudinal, prospective study of all contacts of patients with active TB in the health district of Vigo, Pontevedra, in 2005 and 2006. The total rate of positive tuberculin test results and the percentage of tuberculin conversions at the end of the study were analyzed. The predictive factors for positive results were identified by a multivariate analysis.

A total of 742 contacts of patients with TB were studied. The mean age of contacts was 35 years (SD=19.1) and 406 (54.7%) were women. Most (91.8%) were contacts of pulmonary TB patients, 70.4% were contacts of bacilliferous patients and 40.2% were highly exposed. The study was completed with 540 (72.8%) cases, 280 (51.9%) of which were infected or diseased. Of the 297 contacts who completed the study 12 weeks later, 37 (12.5%) showed tuberculin conversion. The factors independently associated with a positive tuberculin test result were male sex, age >or= 30 years old, and degree of contact (high exposure in contacts and positive sputum smear in the index case); the two latter factors were predictive for tuberculin conversion.

Contact tracing of TB patients was an effective strategy in our area. The factors most closely related to positive tuberculin test results, particularly to conversion, were positive direct microscopy of sputum for the index case and high exposure. Strategies to improve adherence to this kind of study should be sought.