The tuberculin skin test or PPD performed on health professionals evaluates delayed hypersensitivity to a Mycobacterium tuberculosis (Mt) antigen mixture. A very positive or increased area of induration indicates latent tuberculosis. Yet, prescribing a treatment is often difficult because of the test's poor specificity.
Objective: To determine if the T-lymphocytes of the proband exposed to specific Mt antigens to secrete interferon-gamma (IFN-gamma) can be measured; observe an immunizing response to cellular mediation improves T it this specificity?
Patients and methods: The Department of Occupational Health at CHR of Metz performed a Quantiferon TB Gold (tube method, Cellestis) test on three groups of employees distributed according to the anteriority of a very positive IDR (>15mm). The test measures by method ELISA, the quantity of IFN-gamma secreted by the lymphocytes T collected in 1ml of total blood, after in vitro stimulation by antigens ESAT-6, CFP-10 and TB7.7. Group I is composed of 53 people (46 women [W] and 7 men [M]) and related to a recent discovery of October 2006 and June 2007, whereas group II, includes 28 of them (25W and 3M), is made of subjects with known very positive PPD for eight to 27 years. The age (41 years on average) and the sex-ratio are identical for these two groups. Group III is made of employees having made a tuberculosis there is more than 30 years. In the group I, the rate of IFN was supervised at the end of the treatment among patients who received it.
Results: The test Quantiferon TB Gold was positive at 15 subjects of group I (13W and 2M), that is to say 28.3%, and unspecified in one case (1.9%); positive at nine subjects of group II (9W), that is to say 32.1%. These rates of positivity are not significantly different. According to the experts consulted in group I, an antituberculosis treatment was proposed 11 times with 10 effective treatments (one refusal). A proportioning after treatment was carried out among six patients. The rate of IFN remained positive at five of the six supervised patients. This test made it possible to avoid the treatment of the 37 employees with test Quantferon TB negative Gold found over the nine months period of recruitment of group I. The choice not to treat was facilitated.
Conclusion: The test Quantiferon indeed allows to eliminate the false positive of the skin test (1.6) so avoiding useless, expensive treatments and unwanted effects of antituberculosis medicines . On the other hand, the persistent positivity of this test 30 years after one firstly infection or after a contact does not allow to use it at present as certain control of a latent infection, unless having a negative value known about the hiring pa.