[Latent tuberculosis infection in healthcare workers--evaluation of an Interferon-gamma release assay]

The Interferon-gamma release assay (IGRA) is a promising alternative to the tuberculin skin test (TST) for the diagnosis of latent tuberculosis infection (LTBI). But so far only limited experience is available regarding the performance of these IGRAs especially in occupational medicine when used for screening healthcare workers (HCW).

In an ongoing study employees of three German hospitals are screened for LTBI using the TST Mendel-Mantoux and the IGRA (QuantiFERON-TB Gold in Tube) simultaneously. Information on earlier tests which had been performed with the multipuncture tests were obtained by interview. Risk factors for a positive test result were calculated using multivariate logistic regression.

The analysis comprises 161 employees for whom information on all the tests - multipuncture test, TST Mendel-Mantoux and IGRA were available. The multipuncture test was positive in 34.8 %, the TST Mendel-Mantoux in 26.7 % and the IGRA in 12.4 % of the participants. When adjusting for the proportion of positive test results caused by cross-reaction with BCG vaccination, the prevalence of LTBI reduces to 18.8 % in the multipuncture test and 19.2 % in the TST Mendel-Mantoux. The Test results of the IGRA were positively correlated with age while the results of the two in-vivo tests were not.

The multipuncture test and the TST Mendel-Mantoux overestimate the prevalence of LTBI in HCW. A positive result in the TST should be verified by the IGRA before any further actions are recommended. A larger data base is needed in order to confirm that the IGRA is more sensitive in diagnosing a LTBI than the skin tests.