Routine Hospital Use of a New Commercial Whole Blood Interferon-γ Assay for the Diagnosis of Tuberculosis Infection

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Abstract
Interferon (IFN)-gamma blood tests may improve the current level of diagnostic accuracy for tuberculosis infection. The QuantiFERON-TB Gold (QFT-Gold) has been used in selected populations and shows higher specificity than the tuberculin skin test (TST). To evaluate the QFT-Gold test in unselected patients and assess the level of agreement with the TST. The test has been routinely performed on whole blood samples in our microbiology laboratory for 8 months. Demographic, clinical, and microbiological data have been collected and correlated to the QFT-Gold results. Of 318 patients tested, 68 (21.4%) gave an indeterminate (low positive mitogen control) QFT-Gold result. Indeterminate results were significantly overrepresented in patients with a negative TST (28.9% vs. 6.6% in TST-positive patients; p < 0.0001, chi2 test) and were more frequent in patients receiving immunosuppressive therapies than in those who were not receiving such treatments (odds ratio, 3.35; 95% confidence interval, 1.84-6.08; p < 0.0001). After excluding indeterminate results, the concordance between QFT-Gold and TST was significantly lower in Bacille Calmette-Guérin-vaccinated individuals (41.5%) than in nonvaccinated individuals (80.3%) (p < 0.0001). In 11 patients with active tuberculosis (5 culture-confirmed), QFT-Gold provided more positive results than the TST (66.7% vs. 33.3%; p = 0.165). The QFT-Gold test is feasible in routine hospital use for the diagnosis of tuberculosis infection. As with the TST, immunosuppression may negatively affect the test's performance, with a significant rate of indeterminate results in the most vulnerable population.