When is an IGRA Conversion False Positive?

Thomas Glück, MD


Journal Watch 

In This Article

Abstract and Introduction


Among hospital employees who underwent serial interferon-γ release assay testing, converters with interferon levels close to the cutoff value often tested negative months later.


In high-income western countries, interferon-γ release assays (IGRAs) have largely replaced the classic tuberculin skin test for diagnosing latent tuberculosis (TB). They appear promising for serial testing, but determining the clinical significance of conversions with low-positive IGRA results has been problematic.

Researchers recently published their experience with repeat testing of healthcare workers (HCWs) at the Cleveland Clinic. From 2007 through 2010, 52 of 1857 HCWs (2.8%) with negative pre-employment IGRA screening results subsequently had positive results on annual testing. Repeat IGRA values were ≤1 IU/mL in 71% of the converters and 0.35 to 0.5 IU/mL — near the cutoff — in 37%; the median value was 0.63 IU/mL. Forty-four percent of the converters had a history of bacille Calmette-Guérin vaccination; none were part of an outbreak investigation or were found to have active TB. Forty-three of the 52 converters were further evaluated, and 26 (61%) were advised to begin isoniazid therapy. Ten who did not receive such therapy (median IGRA value, 0.42 IU/mL; range 0.35–10 IU/mL) repeated the IGRA test 1 to 6 months later, and 8 of them had negative results.