Radiographic features for predicting smear-negative pulmonary tuberculosis

Background: Diagnosis of smear-negative pulmonary tuberculosis (TB) in some circumstances remains a challenge to physicians especially those working in limited-resource settings. Objective: To investigate and examine radiographic characters as a predictor of true diagnosis of pulmonary tuberculosis in patients with negative sputum smears. Material and Method: This retrospective cross-sectional study was conducted in patients with smear-negative pulmonary TB at Siriraj Hospital between January 2013 and June 2014. Patients with previous TB treatment, HIV co-infection, significant pleural effusion, and corticosteroid therapy equivalent to prednisolone greater than 15 mg/day were excluded. Demographic and clinical data were collected and radiographic features were reviewed and classified as active or inactive TB by a consensus of three independent reviewers. Various diagnostic parameters for true prediction of TB, as defined by culture confirmation and/or radiographic improvement, were then examined. Results: There were 122 patients during the study period, 65 (53%) were male, 27 (22%) were asymptomatic, 20 (16%) had extrapulmonary involvement, and eight (7%) had concomitant diabetes mellitus. TB was confirmed in 92 patients (75%), 72 had positive culture and 20 had radiographic improvement. Miliary nodules and cavitary lesions had high specificity (100% and 100%, respectively) and low sensitivity (9.8% and 13%, respectively) for prediction of true TB. Focal interstitial and alveolar opacities had high positive predictive value (79.5 and 85%) and modest accuracy (62.3 and 47.5%). Conclusion: Given that specific radiographic features are uncommon and non-specific features are common in smear-negative pulmonary tuberculosis, clinicians should supplement clinical symptoms, radiological features, and radiological responses with mycobacterium TB culture to verify diagnosis of TB.