cohort study of tuberculosis risk
- R Diel, R Loddenkemper, A. Nienhaus
During the past decades, the incidence of tuberculosis (TB) has steadily declined in most west European countries, and many physicians will no longer encounter TB in their patients. Along with the declining practical experience of physicians, the chance for delay in diagnosing TB increases. An delayed diagnosis of TB index cases is the most important risk factor for the nosocomial transmission of Mycobacterium tuberculosis (MTB) , as numerous unprotected contacts among healthcare workers and inpatients may result before the diagnosis of TB is established and respiratory isolation is applied [2, 3]. The clean-up after such a mishap can be complicated and time-consuming, as the individuals concerned need to be identified and contacted. Patients might have been already discharged, or healthcare workers may not be aware of exposure to a TB index case for days or even weeks . However, the difficulty is not over, even when the respective contacts have been identified. As there is no perfect test for the diagnosis of TB infection, the question arises of which test should be used and who should be tested with this assay in order to obtain optimal results. Only close contacts should be tested in order to improve the positive predictive value of the contact screening regardless of the test used. But what is a close contact in a hospital setting? Cumulative exposure to a smear-positive index case for 8 h or more is proposed as an orientation. However, this is merely an inspired guess. In comparison to the tuberculin skin test, the emergence of interferon-c release assays (IGRAs) has facilitated screening after unprotected TB exposure, as there is no booster phenomenon after repeat testing, no need for a second appointment for test reading and no influence of bacillus Calmette–Guérin vaccination on subsequent test results. However, even with IGRAs, many questions remain unanswered. IGRAs are not able to distinguish remote from recent infection and their ability to predict progression to active disease appears to be limited so far. While progression rates of up to 14% have been reported for close contacts with a positive IGRA result [5, 6], two recent systematic reviews and metaanalyses indicate considerably lower progression risks [7, 8]. Therefore, even after a positive IGRA is observed, the question remains regarding what to do next. Of course, active TB needs to be excluded by chest radiograph. But should we propose preventive chemotherapy to all contacts with a positive IGRA result, well knowing that most of these subjects will never develop active TB anyway?