Usefulness of Interferon-γ release assays in the diagnosis of erythema induratum.

BACKGROUND Erythema induratum (EI) manifests as tender indurated nodules on the lower legs. It may be associated with concomitant active tuberculosis (TB) and is considered a hypersensitivity reaction to mycobacterial antigens. However, the results of Mycobacterium cultures are rarely positive, and the tuberculin skin test is of limited usefulness in populations exposed to bacille Calmette-Guérin (BCG). Interferon-γ release assays (IGRAs) are alternatives to the tuberculin skin test and have high specificity. We explored the use of IGRAs as an adjunct in the diagnosis of EI. We describe 5 patients with positive tuberculin skin test results and a history of vaccination against BCG or TB in whom IGRAs supported the diagnosis of EI. OBSERVATIONS All patients were initially seen with tender nodules on the lower legs and a history of BCG vaccination or TB. Tuberculin skin test results were positive, and chest radiographic results were normal. The results of Mycobacterium cultures were negative in all patients, and biopsy specimens were compatible with EI. Interferon-γ release assays were performed in all patients and supported initiation of anti-TB treatment in 4 of 5 patients. Conclusion  Interferon-γ release assays may have value as an adjunct in the diagnosis of EI, particularly in the setting of prior BCG exposure.