[Screening contacts for latent tuberculosis infection (LTBI) using an interferon test in Paris in 2009]

There are two reasons for screening contacts: one is to identify cases of secondary tuberculosis disease (TB) and the other is to identify new cases of latent tuberculosis infection (LTBI). The tuberculin skin test (TST) and the interferon-gamma-release assay (IGRA) have their limitations when used for the detection of LTBI. They neither allow a definite diagnosis of LTBI nor provide information as to the date of onset. The present study was observational, multi-centre (four centers) and retrospective. Six hundred and one contacts were included. The results of the QFT test showed 88 positive (15 %). Among the 144 index cases, all presented with pulmonary disease and 89 cases were sputum positive. In our series, 101 contacts belonged to the family circle. The four factors that had a significant positive impact on the result of the QFT test were: increasing age, the region of birth of the contact (high incidence areas), both of which may indicate old infection, while contact within the family and sputum positivity of the index case probably indicate recent infection. Only sputum positivity influenced the decision to treat the LTBI. We propose a tool aimed at facilitating the decision making process in QFT positive cases. Estimation of the duration of LTBI should help the physician to decide on the need for preventative treatment as well as a search for factors that increase the risk of progression to TB disease.