Interferon gamma levels in pleural fluid for the diagnosis of tuberculosis

To assess the utility of interferon gamma levels, including identification of the best cutoff for the diagnosis of tuberculosis.

We prospectively studied consecutive patients in a tertiary care, university-affiliated hospital who had pleural effusions. Interferon gamma levels were measured blindly by radioimmunoassay. The diagnosis of tuberculosis was established using prespecified standard criteria.

Of the 595 patients with pleural effusions, 82 patients (14%) had tuberculosis. The area under the receiver operating characteristic (ROC) curve for elevated interferon gamma levels in the diagnosis of tuberculosis was 0.99 (95% confidence interval [CI]: 0.97 to 1.00). A cutoff of 3.7 IU/mL yielded a sensitivity of 0.98 (95% CI: 0.91 to 1.00) and a specificity of 0.98 (95% CI: 0.96 to 0.99). The areas under the ROC curves, and the test's sensitivity and specificity, were similar among patients of different ages and by percentage of lymphocytes in the pleural fluid. In 5 of the 28 patients with hematologic malignancies, interferon gamma levels were slightly above the cutoff; no patient with vasculitis or granulomatous diseases had levels higher than 3.7 IU/mL. The 14 immunocompromised patients and the 3 transplantation patients with tuberculosis had interferon gamma levels greater than the cutoff.

Elevated pleural interferon gamma levels (>3.7 IU/mL) are very valuable in diagnosing pleural tuberculosis. Patients with pleural effusion due to hematologic neoplasms occasionally have levels slightly above the cutoff.