Pleural effusion is a common complication of many disease processes. Tuberculosis is still one of the most important causes of exudative effusions.1 Over the last decade many workers have emphasised the importance of estimating adenosine deaminase (ADA) in pleural fluid.2 ADA activity in human biological fluid results from the action of two isoenzymes—namely, ADA₁ and ADA₂—with different affinities on two substrates, 2′ deoxyadenosine and adenosine, respectively. The deaminase ratio (ADA₁/ADA₂) is …