Any test for latent tuberculosis that relies on the amount of interferon gamma (IFN-y) released by T cells that have been previously exposed to mycobacterial antigens. One such test, the whole blood assay, is performed by withdrawing about 1 ml of whole blood from the vein of a patient. This blood is heparinized, to prevent clotting, and then mixed with specific mycobacterial antigens and control substances. After overnight incubation, samples of plasma from the blood are tested for INF-y levels. The test is considered positive if INF-y in a sample exposed to mycobacterial antigens is >0.35 IU/ml.