Evaluation of the interferon-gamma release assay in healthcare workers

An in vitro test for the diagnosis of latent TB-infection (LTBI) is now available that has the potential to replace the tuberculin skin test (TST). The new test measures the cell-mediated immune response to TB antigens. Little experience is currently available on the performance of the IGRA when testing HCW for LTBI. Therefore the aim of the study was to compare the IGRA with the TST in testing HCW for LTBI.

The medical staffs of three hospitals were simultaneously tested with TST (RT23) and IGRA (QFT). The study comprises 261 HCW, all exposed to active TB-patients. Information on gender, age, workplace, BCG vaccination and history of both TB and TST were collected using a standardised questionnaire.

The TST was positive in 24.1% and the IGRA in 9.6% of the participants. Agreement between the tests was low in HCW with a BCG-vaccination. A history of an earlier TST was a risk factor for a positive TST but not for a positive IGRA. Out of 48 results positive in the TST but negative in the IGRA 97.8% might be explained by BCG vaccination or an earlier TST. A positive association between age and LTBI was observed for the IGRA but not for the TST results. Out of 25 positive results in the IGRA only 60% were recognized by the TST, too. The HCW with a positive IGRA and a negative TST had the same age (mean 49.1) as the HCW positive in both tests (mean 50.9), which was higher than the age of those negative in both tests (mean 38.8).

The data indicate that the IGRA should replace the TST in serial testing of HCW in high income, low incidence countries. The recommendations to use the IGRA in order to verify a positive TST only should be reconsidered.