Retrospective study investigating the utility of pleural fluid ADA in diagnosing pleural TB

<bold>Introduction:</bold> Pleural fluid adenosine deaminase (ADA) has been used as a biomarker to detect tuberculous pleural effusions. However, in the UK ADA is still not routinely used in the investigation of a patient with a pleural effusion. <bold>Methods:</bold> We conducted a single centre retrospective study in London over a 7 month period between April 2023 and November 2023 to investigate the diagnostic utility of pleural ADA in the diagnosis of pleural TB. All patients who had an unexplained unilateral lymphocytic exudative pleural effusion and pleural ADA level sent, were included in the study. <bold>Results:</bold> In total 13 patients had pleural ADA levels sent for analysis. ADA results were reported in a median 18 [1-26] days. 4 patients had pleural ADA > 40 IU/ml. 3/4 patients had microbiologically or histologically confirmed tuberculous pleural effusion. 2/ 4 patients grew Mtb from pleural fluid; 1 of these also had a positive pleural fluid Xpert Ultra result. 1 patient with high pleural ADA had mesothelioma. ADA > 40 IU/ml had a sensitivity of 100% and specificity of 90% in the diagnosis of pleural TB. ADA<40 IU/ml had a negative predictive value (NPV) of 1 in excluding pleural TB diagnosis. Xpert Ultra was sent in 8 of the 13 patients, with a sensitivity of 33% and specificity of 100%. However, the Xpert Ultra result was available in 1 day, with the added advantage of predicting rifampicin resistance. <bold>Conclusion:</bold> Pleural ADA is a relatively inexpensive and readily available test with high sensitivity and specificity in the diagnosis of pleural TB in our cohort, and low pleural ADA helps exclude pleural TB. Although less sensitive Xpert Ultra may have the added benefit of a rapid diagnosis.