Interferon gamma release assay for differentiating tuberculosis among pneumonia cases in acute healthcare setting.

Objectives: Early diagnosis of smear-negative tuberculosis remains challenging. The role of an interferon-gamma release assay (IGRA) in discriminating active pulmonary tuberculosis (PTB) among cases of 'pneumonia' was investigated.

Methods: Consecutive patients admitted to an acute hospital in Hong Kong (intermediate TB burden) during 2006-2008 because of pneumonia and suspected PTB were recruited for IGRA (Quantiferon-TB Gold, QFN-G) study. Diagnosis of tuberculosis was confirmed by mycobacterial culture or histology.

Results: Altogether 179 patients were recruited (median (IQR) age 59 (44-75), 68.7% male); active PTB was confirmed in 63 (35.2%). Among the AFB-smear-negative 'pneumonias' (n = 152), age>50 (OR 0.27, 95% CI 0.09-0.84), absence of weight loss (OR 0.30, 95% CI 0.10-0.88), and negative IGRA (OR 0.08, 95% CI 0.03-0.25) were independently associated with lower risks of PTB. The overall sensitivity, specificity, positive and negative predictive values for the IGRA in diagnosing active PTB were 60%, 87%, 72% and 80% respectively. Among smear-negative 'pneumonias' (n = 152), the performance values of IGRA were 64%, 87%, 62% and 88% respectively; in the absence of characteristic clinical or radiographic features of PTB, the negative predictive value (NPV) improved to 90-95%.

Conclusions: The high NPV of QFN-G among smear-negative 'pneumonias' can be useful for risk stratification in hospitalized patients suspected of PTB. Further investigation on the role of these assays in patient management is warranted.