[Recent latent tuberculous infection (LTBI) in different sub-groups of contacts]

Screening for latent tuberculous infection (LTBI) in contacts of identified cases of tuberculosis should be based on intention to treat. We know that only LTBI that is attributable to a recent transmission requires preventive treatment in immunocompetent adults. There is no current test that allows accurate dating of LTBI. The objective of this retrospective study with QuantiFERON-TB-Gold In-Tube (QFT) is to estimate the probability of recent LTBI in various groups of contacts. Among 2424 subjects included, 407 (17%) tests were positive; 129 (28%) within the family circle, 69 (19%) within the social environment, 153 (14%) among professional acquaintances and 56 (10%) within the school environment (respectively: OR=0.69 and P=0.04; OR=0.49 and P<0.001; OR=0.51 and P<0.001). After comparison with a basal rate of LTBI, professional and family contacts less than 40 years old, born in a country of incidence lower than 20, had a rate of recent LTBI between 15 and 40% and between 68 and 80%, respectively. For those born in a country of incidence greater than 20, we estimate the rate of recent LTBI between 6 and 18% for professional contacts and between 43 and 50% for family contacts. These results, although approximate and specific to Paris, are important for the clinician who has to decide on preventive treatment and for the public health specialist who has to identify groups of contacts. Finally, to limit the differences in rates of recent LTBI, it would probably be necessary to specify thresholds of accumulated contact time, according to the type of contact, before considering an individual as a contact.