IFN-γ Release Assays to Screen Healthcare Workers for Mycobacterium tuberculosis Infection: A Concise Update
IFN-γ Release Assays to Screen Healthcare Workers for Mycobacterium tuberculosis Infection: A Concise Update
TB is a global threat to mankind. Globally, 9 million persons develop active disease attributable to M tuberculosis infection (MTBI) annually. Health-care workers (HCWs) are at increased risk of becoming infected with M tuberculosis through occupational exposure. Periodic screening of health-care workers for MTBI with tuberculin skin test (TST) is a common practice of many hospital infection control programs in the United States (MMWR Recomm Rep. 2005;54[RR17]:1). Interferon gamma (IFN-γ) release assays (IGRAs) are in vitro tests for MTBI that can be used in place of TST to screen health-care workers. Three IGRAs are commercially available for the detection of MTBI in the United States, including the QuantiFERON®- TB Gold In-Tube test (QFT-GIT; Cellestis Ltd; Valencia, CA). However, data on performance of QFT-GIT in such screening programs for healthcare workers in the United States are limited. The study presented at CHEST 2010 (Joshi et al. Chest. 2010; 138:746A) pointed out the limitations of QFT-GIT, including the high number of positive test results despite negative TST history and high reversion rates (40%) on repeat testing that led to a dilemma in clinical decision making to offer MTBI treatment. Subsequently, another study (Gandra et al. Infect Control Hosp Epidemiol. 2010;31[12]:1279) and a meta-analysis (Zwerling et al. Thorax. Published online ahead of print, Jan 12, 2011) have raised similar concerns and proposed caution to interpret positive test results. These large studies in the real world will help guide many health-care institutions that are in the process of implementing IGRAs to replace TST. To conclude, QFT-GIT is not yet completely ready for prime time to replace the TST, and there is a major clinical learning curve ahead to fully understand the QFT-GIT test characteristics in low TB prevalence populations that undergo periodic screening.
Dr. Manish Joshi, FCCP
Steering Committee Member
