Adenosine deaminase (ADA) is a purine-degrading enzyme, catalysing adenosine and deoxyadenosine to produce inosine and deoxyinosine in the process. ADA, a product of activated lymphocytes, catalyzes the conversion of adenosine to inosine and is important for normal immune function. The pleural fluid ADA levels are elevated in almost all patients with tuberculous pleuritis but not with other conditions even when associated with lymphocytic effusions.
Despite earlier concerns about false-negative values in HIV-positive patients, ADA remains a sensitive marker for tuberculous pleurisy in patients with HIV. ADA levels can be elevated in other conditions and also in neutrophilic effusions; in one study, ADA above the cutoff (35 U/L) was seen in up to 40% of parapneumonic effusions and in half of effusions due to lymphoma. Because it is a highly sensitive test, the ADA can be a useful test to exclude the diagnosis of tuberculosis when the ADA level is low (<40 U/L).
