Diagnostic accuracy, agreement, and discordance of Xpert MTB/RIF compared with culture and chest radiography in smear-negative pulmonary tuberculosis

Abstract

Background
Smear-negative pulmonary tuberculosis remains a major diagnostic challenge due to the limited sensitivity of conventional microscopy. Rapid molecular diagnostics, particularly Xpert MTB/RIF, provide a valuable approach for early detection.


Objective
To evaluate the diagnostic accuracy of Xpert MTB/RIF assay in smear-negative pulmonary tuberculosis and to assess its agreement and discordance with mycobacterial culture (reference standard) and chest radiography.


Methods
This prospective cross-sectional study included 50 patients with clinical suspicion of pulmonary tuberculosis and negative sputum smear microscopy. All participants underwent Xpert MTB/RIF testing, mycobacterial culture on Lowenstein–Jensen medium, and chest radiography. Diagnostic performance measures, including sensitivity, specificity, predictive values, and agreement using Cohen’s kappa coefficient, were calculated.


Results

Xpert MTB/RIF detected
Mycobacterium tuberculosis
in 30/50 patients (60%), while culture was positive in 38/50 patients (76%). Compared with culture, Xpert MTB/RIF demonstrated a sensitivity of 76.3% (95% CI: 59.8–88.6) and a specificity of 91.7% (95% CI: 61.5–99.8). The positive predictive value and negative predictive value were 96.7% and 55.0%, respectively, with an overall diagnostic accuracy of 80.0%. Agreement between Xpert MTB/RIF and culture was moderate (Cohen’s kappa = 0.50), with an overall agreement of 80%. Chest radiography showed moderate agreement with both Xpert MTB/RIF (kappa = 0.576) and culture (kappa = 0.478). Discordant results included 9 false-negative and 1 false-positive Xpert MTB/RIF case.



Conclusion
Xpert MTB/RIF represents a rapid adjunctive diagnostic tool for smear-negative pulmonary tuberculosis, characterized by high specificity and moderate sensitivity. While useful as a rule-in test, negative results should be interpreted cautiously, particularly in paucibacillary disease. Integration with microbiological and radiological assessment may provide complementary diagnostic information in clinically suspected cases.