[Usefulness of Quantiferon-TB Gold in Tube® in screening for latent tuberculosis infection in health workers]

Abstract

Introduction: Healthcare workers (HW) are considered a risk group for exposure to tuberculosis. Screening for latent tuberculosis infection (LTBI) is mandatory in all HW. The Tuberculin test (TT) has been used up until now for LTBI screening, but gives a high number of false positives, especially in patients vaccinated with BCG. Diagnostic methods based on detection of specific gamma interferon (IGRA) have recently appeared on the market in order to improve these drawbacks, but pose other dilemmas. The aim of this study is to determine the agreement between the two types of test and to carry out a cost-benefit study of the possible diagnostic strategies.

Material and methods: All newly hired HW by the Hospital Universitari Mútua Terrassa between January 2010 and October 2011 we were included in the study, as well as those who had their occupational review. Workers who been in contact with patients admitted with tuberculosis before the initial isolation were also tested. In all cases a parallel TT and serum QuantiFeron-TB Gold-in-Tube(®) (QF-G-IT) assays were performed. TB disease was ruled out in all professional by chest X-ray. The TT was considered positive when it was equal to or more than 10mm and if the QF-G-IT was 0.35 IU/mL. A cost-effectiveness analysis was designed with three possible strategies to detect LTBI in order to find the one with the best cost-benefit.

Results: A total of 226 HW were studied, with a mean age 30.65 ± 16, of whom 44 (19.4%) had previous BCG vaccination history, and 8 (3.5%) unknown. The TT was positive in 33 (14.6%) cases and the QF-G-IT in 17 (7.5%). The values of the TT and QF-G-IT were both positive in 15 cases. In 18 (8%) The TT was positive in 18 (8%) of cases with a negative QF-G-IT value. The agreement between the two tests was 91%, with a Kappa of 0.55. In vaccinated cases, the correlation was 70.5%, with a Kappa of 0.33, while in unvaccinated it was 98.9% with a Kappa of 0.65. The cheapest screening strategies for LTBI diagnosis were those based on TT, but followed closely by the strategy based on TT with reconfirmation of positives with QF-G-IT.

Conclusions: QF-G-IT seems to be a very sensitive technique to detect LTBI and allows false positives due to TT to be detected, particularly in BCG vaccinated HW. In this group QF-G-IT could be the ideal test to detect truly infected staff, and avoid unnecessary chemoprophylaxis. The most cost-benefit strategy was those based in TT with reconfirmation or rejection of positive cases by QF-G-IT.

Keywords: Health workers; Infección latente tuberculosa; Interferon-Gamma release assay; Interferon-gamma release assay; Latent tuberculosis infection; Prueba de la tuberculina; QuantiFeron TB Gold in-Fube; Quantiferon TB Gold in Tube; Trabajador sanitario; Tuberculin test.