The Diagnostic Value of Interferon-γ Assay in Patients with Active Tuberculosis

Background: The interferon-gamma assay is reported to have high sensitivity and specificity for making the diagnosis of latent tuberculosis infection.The clinical usefulness of this essay for detecting active tuberculosis has not fully defined.We evaluated the diagnostic value of the commercial interferon-gamma assay kit (QuantiFERON-TB GOLD) for patients with suspected tuberculosis.Methods: From January to August 2007, we recruited 52 patients with suspected tuberculosis infection.We performed chest X-ray, sputum smear, culture, PCR and the QuantiFERON-TB GOLD test.Pleural fluid analysis and pleural biopsy were also done for the patients with pleural effusion.Results: Of the 52 patients we studied, 30 patients had a positive QuantiFERON-TB GOLD test result.35 patients were finally diagnosed with active tuberculosis: twenty-five with a positive QuantiFERON-TB GOLD test and 10 with a negative QuantiFERON-TB GOLD test.The sensitivity of the QuantiFERON-TB GOLD test was 71.4% and the specificity was 64.7%.The positive predictive value was 0.83 and the negative predictive value was 0.50.There was no significant difference of any of the clinical and laboratory characteristics between the two groups of patients except the C-reactive protein (CRP) level.The CRP level was 29.2±27.3mg/dL in the pulmonary tuberculosis patients with a positive QuantiFERON-TB GOLD test and 72.9±67.9mg/dL in the patients with a negative QuantiFERON-TB GOLD test (p＜0.05).Conclusion: The sensitivity and specificity of the QuantiFERON-TB GOLD test were inadequate for making the diagnosis of active tuberculosis.We suggest that the QuantiFERON-TB GOLD test should not be used by itself to exclude the diagnosis of active tuberculosis.The relationship of the QuantiFERON-TB GOLD test and the CRP level in patients with TB would be further investigated.