SENSITIVITY AND SPECIFICITY OF A GAMMA-INTERFERON BLOOD-TEST FOR TUBERCULOSIS INFECTION

SETTING : Victoria, Australia. OBJECTIVE: TO determine the sensitivity and specificity of a gamma interferon (IFN-gamma) blood test for tube rculosis infection. DESIGN: Heparinised blood samples from 952 volunte ers were analysed using the QuantiFERON(R)-TB blood test. The levels o f IFN-gamma in whole blood aliquots stimulated in vitro with human, av ian or bovine purified protein derivative (PPD), or phytohaemagglutini n (PHA), were compared to tuberculin reactivity and clinical presentat ion. RESULTS: IFN-gamma (IU/ml) responses were expressed as % PPD/PHA response ratios. The proportion of responders detected by both the IFN -gamma assay and tuberculin skin test (TST) was not significantly diff erent when the % human PPD response ratio was 15%. Using this threshol d, the specificity of the IFN-gamma assay was 98% (407/417 individuals with no known exposure to tuberculosis were negative) and sensitivity was 90% (163/182 untreated TST reactors were positive). The test dete cted positive responses in 83% (10/12) of individuals with proven acti ve disease, 59% (24/41) of those previously treated, 80% (134/168) of those with untreated inactive disease, and 43% (55/128) of those expos ed but TST-negative. CONCLUSION: The measurement of IFN-gamma released from PPD-stimulated blood lymphocytes is a specific, sensitive and ra pid method of detecting Mycobacterium tuberculosis infection. The IFN- gamma assay may be a useful and practical tool for the early diagnosis of tuberculosis infection, especially in immunocompetent individuals.