Tuberculin skin test size and risk of tuberculosis development: a large population-based study in contacts.

Setting: Contacts of tuberculosis (TB) cases identified from eight Provincial databases in British Columbia, Canada, between 1990 and 2000.
Objective: To assess the risk of developing TB based on tuberculin skin test (TST) sizes in contacts of TB cases who did not receive treatment for latent TB infection.
Design: Retrospective, population-based cohort study with a 12-year follow-up.
Results: Among 26,542 contacts, 180 individuals developed TB (TB rate 678/100,000). Household contacts with a TST size 0-4 mm had a TB rate of 1014/100,000, those with 5-9 mm a TB rate of 2162/100,000 and those with 10-14 mm a rate of 4478/100,000. Children aged 0-10 years with 0-4 mm had a TB rate of 806/100,000, those with 5-9 mm a TB rate of 5556/100,000 and those with 10-14 mm a rate of 42,424/100,000. Immunosuppressed contacts with TST sizes 0-4 mm had a TB rate of 630/100,000, those with 5-9 mm a TB rate of 1923/100,000, and those with 10-14 mm a rate of 1770/100,000.
Conclusions: TB rates were high for all TST sizes in household contacts, 0-10 year old contacts and immunosuppressed contacts. These contacts may benefit from treatment for latent TB infection, regardless of the size of their TST.