Adenosine Deaminase is a vital enzyme in the metabolism of the purine nucleosides. For the adequate interpretation of adenosine deaminase (ADA), it is of utmost importance to know that the adenosine deaminase is represented by two isoenzymes namely, ADA1 and ADA2. ADA1 isoenzymes are present in every cell but its activity is highest in lymphocytes. ADA2 isoenzyme, on the other hand, is present only in the antigen presenting cells. The most common form of the extrapulmonary TB is pleural effusion or tuberculous pleurisy. Tuberculous pleurisy or pleural effusion usually contains a lower number of mycobacteria, the microscopy sensitivity and pleural fluid culture sensitivity of it is relatively low. The level of ADA is one of the diagnostic measure used to detect the possibility of TB in the patient. The level of ADA increases at the area where infection of mycobacterium is present in order to activate the lymphocytes, i.e., White Blood Cells to respond to the stimulus. In order to detect the pleural effusion the amount of ADA in the pleural fluid is tested.