Evaluation of usefulness of pleural fluid/serum alkaline phosphatase in the diagnosis of tubercular pleural effusion

Background: Tuberculosis (TB) remains a common cause of pleural effusions, diagnosed by detecting Mycobacterium tuberculosis in pleural fluid or biopsy specimens through microscopy, culture, or histological demonstration of caseating granulomas and acid-fast bacilli (AFB). In high-burden settings, the diagnosis is frequently inferred. Objective was to assess the usefulness of pleural fluid/serum alkaline phosphatase in the diagnosis of tubercular pleural effusion. Methods: This cross-sectional observational study took place at the department of respiratory medicine, NIDCH, Dhaka, from December 2018 to December 2019. Seventy new cases of pleural effusion meeting specific criteria were enrolled with informed consent obtained. Diagnostic assessments included Gene Xpert, cytology, culture and sensitivity testing, biochemical analyses, and alkaline phosphatase testing on pleural fluid and blood samples. Closed needle pleural biopsies using an Abrams needle were performed for histopathological examination. Data analysis was carried out using SPSS version. Results: In this study of 70 patients, mean age 48.67±17.99 years (range: 17-83 years), with 42.9% aged 51-70 years, males predominated (6:1). Histopathology showed TB in 45.7%. TB patients had lower neutrophil and platelet counts, lower serum alkaline phosphatase, higher ESR, and higher pleural fluid alkaline phosphatase and P/S ALP ratio. The P/S ALP ratio had an AUC of 0.881 (95% CI 0.798-0.964), with a cut-off of 0.49, sensitivity/NPV of 93.8%, and specificity/PPV of 78.9% for diagnosing TB pleural effusion. Conclusions: The pleural fluid/serum alkaline phosphatase ratio is a valuable diagnostic tool for tuberculous pleural effusion. Further validation through a multicenter randomized trial with a larger sample size is recommended.