False-Positive Interferon Assays for Latent Tuberculosis

Interferon-γ release assays (IGRAs) have many advantages over skin tests for diagnosing latent tuberculosis (TB): They require only one medical encounter, can be repeated without boosting response, and eliminate subjective interpretation of test results. However, IGRAs are not without their own problems.

A large midwestern U.S. hospital replaced skin testing with a commercial IGRA (QuantiFERON-TB Gold In-Tube) for TB screening in 2007. During the subsequent 3 years, 486 of 7374 newly hired employees had baseline positive IGRAs. A consultant suggested repeat testing in 14, because latent TB seemed clinically unlikely; repeat tests were negative in 10. Similarly, among 52 employees with baseline negative IGRAs who converted to positive on rout…