Purposes: To determine the optimum cut-off level of a newly developed method for diagnosing tuberculosis infection based on whole-blood interferon-gamma measurement, and to study the basic characteristics of the method.
Study subjects: 1) A total of 220 young, healthy individuals having no apparent exposure to tuberculosis infection, most of whom have had a vaccination with BCG vaccine. 2) One hundred eighteen tuberculosis patients who were diagnosed by positive Mycobacterium tuberculosis on culture. 3) A group of 75 youngsters exposed to an infectious tuberculosis patient and who showed a strong tuberculin reaction (with erythema diameter of 30 mm or more).
Method: Whole-blood specimens of donors were stimulated with antigens, i.e., ESAT-6 and CFP-10, and then cultured. Plasma concentrations of interferon-gamma discharged were then determined with QuantiFERON-CMI. Correlation between interferon-gamma concentrations in response to ESAT-6 and CFP-10, and their correlation with Mantoux test results were analyzed for various categories of donors. The Receiver Operating Characteristics analysis was performed considering the loss due to misclassification. [
Results and discussion: The optimum cut-off level was determined as 0.35 IU/ml for both ESAT-6 and CFP-10. This gave the test a sensitivity of 89.0% and specificity of 98.1% in detecting tuberculosis infection. The correlation of interferon-gamma response with tuberculin tests among BCG-vaccinated individuals was low, which suggested that the test was not influenced by previous BCG vaccination. The low correlation between ESAT-6 and CFP-10 tests suggested that the simultaneous use of the two tests was beneficial. As in the case of clinical tests in general, the cut-off should be set at a lower level when the test is applied to high prevalence situation and vice versa.