Assay Differentiates Tuberculosis From Nontuberculous Mycobacteriosis

A whole-blood interferon enzyme-linked immunosorbent assay (ELISA) provides a differential diagnosis that distinguishes active tuberculosis (TB) from nontuberculous mycobacteriosis (NTM), according to results reported in the December 15, 2006, issue of Clinical Infectious Diseases.

The QuantiFeron TB-2G (QFT-TB) test's ability to make this differentiation was reported in the December 15, 2006, issue of Clinical Infectious Diseases. The QFT-TB test is an ELISA that measures gamma interferon in whole blood, as an indication of active TB. It usually gives negative results in people who have had a Bacille Calmette Guerin (BCG) vaccination.

Dr. Kobashi and colleagues from the division of respiratory diseases, department of medicine, Kawasaki Medical School, (Kurashiki, Japan) evaluated the usefulness of QFT-TB in differentiating between active tuberculosis and nontuberculous mycobacteriosis using samples from 50 healthy volunteers, 50 patients with active TB, and 100 patients with nontuberculous mycobacteriosis. The tuberculin skin test (TST) and the QFT-TB test were performed on all subjects. The QFT-TB test was performed every two months.

Of the healthy volunteers, 64% had a negative TST result and 94% had a negative QFT-TB test result. Of the patients with active TB, 64% had a positive TST result and 4% had a negative QFT-TB test result. Of the patients with pulmonary Mycobacterium avium complex disease, 60% had a positive TST result and 7% had a positive QFT-TB test result. The QFT-TB test had a mean sensitivity of 86% and a mean specificity of 94%. The QFT-TB test results for patients with active TB transiently decreased during treatment involving antituberculous drugs. The rate of positive QFT-TB test results was 86% at the initiation of treatment, 48% four months later, and 33% 12 months later.

The investigators added, "Because it is possible that the effect of the QFT-TB test may be long lasting after treatment and may not be resolved over time, even with treatment, as in this study, it may not provide any level of certainty regarding cure of infection.”



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