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Among individuals with a positive tuberculin skin test (TST), the risk for active tuberculosis (TB) has been estimated to be as high as 10%. However, this figure is based on studies performed decades ago and may not reflect current conditions.
To update the estimate, investigators conducted a retrospective cohort study using surveillance data from the TB control department of the public health service in Amsterdam from January 2002 through June 2011. Transmission risk was estimated using recent contacts of patients with active TB. TB cases diagnosed in contacts ≤180 days after an index patient's TB diagnosis were considered “coprevalent.” Latent TB infection (LTBI) was determined by TST and chest x-ray in a subset of contacts.
The study involved 610 patients with active pulmonary TB and 9332 of their contacts. Among the 4774 contacts screened for LTBI, 739 showed evidence of infection (coprevalent TB in 57 and LTBI in 681; 1 patient censored). Fourteen of those with LTBI subsequently developed active TB. Among all contacts with evidence of infection, the 5-year risk for active TB was 9.5%; however, among the subgroup with LTBI, risk was only 2.4% for individuals who did not receive preventive TB therapy and 1.4% for those who did. Mycobacterium tuberculosis genotypes were concordant between the index patient and the contact patient in 29 (83%) of 35 instances when such information was available for both.
Sloot R et al. Risk of tuberculosis after recent exposure: A 10-year follow-up study of contacts in Amsterdam. Am J Respir Crit Care Med 2014 Nov 1; 190:1044. (http://dx.doi.org/10.1164/rccm.201406-1159OC)
Comment
These researchers make the important distinction between active TB diagnosed ≤180 days after contact — so-called “coprevalent” disease — and cases subsequently diagnosed in individuals who develop LTBI. Most of the cases of active TB diagnosed ≤5 years after contact fell into the former category. In fact, the risk for TB in contacts with LTBI was lower than in previous estimates, even without preventive therapy.