Adenosine deaminase (also known as adenosine aminohydrolase or ADA) is an enzyme involved in purine metabolism whose primary function in humans is the development and maintenance of the immune system. This test is used to help detect or rule out a Mycobacterium tuberculosis infection in pleural fluid in order to assist in the diagnosis of tuberculosis. The ADA test is not definitive; it is a rapid test and may be elevated even when there are few bacteria present. ADA results may be used to help guide treatment until results from a culture are available.