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.