Setting: Victoria, Australia.
Objective: To determine the sensitivity and specificity of a gamma interferon (IFN-gamma) blood test for tuberculosis infection.
Design: Heparinised blood samples from 952 volunteers were analysed using the QuantiFERON-TB blood test. The levels of IFN-gamma in whole blood aliquots stimulated in vitro with human, avian or bovine purified protein derivative (PPD), or phytohaemagglutinin (PHA), were compared to tuberculin reactivity and clinical presentation.
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 different when the % human PPD response ratio was 15%. Using this threshold, 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 detected positive responses in 83% (10/12) of individuals with proven active disease, 59% (24/41) of those previously treated, 80% (134/168) of those with untreated inactive disease, and 43% (55/128) of those exposed but TST-negative.
Conclusion: The measurement of IFN-gamma released from PPD-stimulated blood lymphocytes is a specific, sensitive and rapid 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.