[ new blood tests for the diagnosis of tuberculosis infection]

ABSTRACT

The diagnosis of latent tuberculosis infection was based until now on the positivity of the tuberculin skin test. This test poses considerable technical problems of implementation, reading and interpretation. Moreover, it lacks both specificity in cases of previous BCG vaccination and sensitivity in case of immunosuppression. The understanding of cellular immunity with the contribution of molecular biology have helped to develop blood tests that measure in vitro release of interferon Gamma by T cells after stimulation by highly specific antigens of Mycobacterium tuberculosis that are ESAT6 [Early Secretory Antigenic Target 6Kda] and CFP10 [culture filtrate protein 10] and recently the Ag tb 7.7. These antigens are absent in the BCG vaccine and most nontuberculous mycobacteria. Two tests are commercialised, one measures the interferon-secreting cells by ELISpot method: the Enzyme-Linked Immuno Spot. As another measure directly interferon by ELISA Enzyme-Linked Immuno-Sorbent Assay. The current clinical data suggest that these blood tests are more specific than the tuberculin skin test because they did not cross-react with BCG. This high specificity avoids the costs and possible side effects of unnecessary treatment among persons vaccinated with a positive tuberculin test. However in cases of active tubersulosis, the ELISA has the same sensitivity as the TST, while ELISpot appears more sensitive. In patients with negative tuberculin test with a high risk to progress to tuberculosis disease, the sensitivity of these blood tests can be considered a treatment for tuberculosis. The purpose of this paper is to recall the principles of these diagnostic tests, review the evidence on their usefulness, their limitations and their indications