Interpreting DNA fingerprint clusters of Mycobacterium tuberculosis

Many studies of tuberculosis have defined clusters of patients on the basis of shared DNA fingerprint patterns of their Mycobacterium tuberculosis iso lates. Clustering has been equated with recent transmission, and factors as sociated with clustering have been sought as a guide to population subgroup s with high rates of ongoing transmission of M. tuberculosis. Considerable caution should be exercised in conducting and interpreting these studies. G roups of strains may be identical for reasons other than recent transmissio n, depending, for example, on the stability of the marker and the number of strains in the population over time. Cases actually due to recent transmis sion may not be seen as clustered if they are new immigrants to the populat ion or if not all cases in the population are included in the study. The am ount of clustering seen will depend on the duration of the study. Studies s hould give precise information on the study setting, the proportion of case s included, the recruitment period and the definition of clustering used. T he data on clustering should be disaggregated at least by age, sex and immi gration status. To be maximally informative, studies should involve a high proportion of an cases in a population, be conducted in conjunction with co nventional epidemiological investigations of contacts (if possible), and sh ould provide information on tuberculosis incidence in the population and on patients' age, sex, human immunodeficiency virus status, drug resistance a nd social and ethnic group.