The tuberculosis (TB) interferon-gamma release assay (IGRA) detects the release of interferon-gamma (IFN-γ) in fresh heparinized whole blood samples from test subjects.

In IGRA assays, the blood samples are incubated with mixtures of the two specific Mycobacterium antigens present in Mycobacterium tuberculosis: early secretory antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10).

Scientists at the Chinese Center for Disease Control and Prevention (Beijing, China) enrolled a total of 1,026 participants at three hospitals, including 597 tuberculosis (TB) patients diagnosed clinically (517 patients with pulmonary TB and 80 patients with extrapulmonary TB) and 429 negative controls (244 patients with pulmonary disease but not TB, or with non-tuberculosis mycobacterial lung diseases), and 185 healthy people. Detection performance indicators including sensitivity, specificity, and the Youden index (YI) were used to evaluate performance of commercial IGRA kits.

Blood specimens and sputum samples were tested and analyzed with bacteriological assays (including sputum smear and bacterial culture) and the three commercial cellular immune detection kits: TB.IGRA (Beijing Wantai Biopharm Co., Ltd.; Beijing, China), QB.SPOT (Kinghawk Pharmaceutical Co., Ltd.; Beijing, China), and T-SPOT.TB (Oxford Immunotec; Oxford, UK) simultaneously. Sputum samples from the pulmonary TB patients were subjected to smear acid-fast staining, followed by Mycobacterium culture on Löwenstein–Jensen medium.

Among 597 clinically diagnosed TB patients, the sensitivity of T-SPOT.TB, QB-SPOT, and TB-IGRA was 78.2%, 78.9%, and 74.9%, respectively, with no significant difference in sensitivity among the three kits. For the 80 extrapulmonary TB cases, the sensitivity of T-SPOT.TB, QB-SPOT, and TB-IGRA was 66.3%, 70.0%, and 68.8%, respectively, with no significant difference in sensitivity among the three kits. For the 517 PTB patients, the sensitivity of T-SPOT.TB, QB-SPOT, and TB-IGRA was 81.1%, 80.3%, and 75.8%, respectively. This was compared to 24.7% (125/507) for smear staining and 45.0% (219/487) for bacterial culture. There was a significant difference in sensitivity between the bacterial methods and the three kits .There was a significant difference in sensitivity of T-SPOT.TB and QB-SPOT between PTB and extrapulmonary TB patients, but not for TB-IGRA.

The authors concluded that the data obtained in the current study suggest that the three commercial kits have comparable detection performance in PTB when the bacteriological method is used as the “gold standard.” These kits will also be a very useful aid in the clinical detection and diagnosis of M. tuberculosis infection, especially in patients with smear-negative and culture-negative and extrapulmonary TB. The kits will be beneficial for TB control in China and elsewhere. The study was published online on September 7, 2015 in the International Journal of Infectious Diseases.

Related Links:
Chinese Center for Disease Control and Prevention
Beijing Wantai Biopharm Co., Ltd. 
Oxford Immunotec