The adenosine deaminase (ADA) test is a specialized biochemical analysis used to measure the activity level of the enzyme adenosine deaminase in various body fluids, most commonly in pleural fluid, cerebrospinal fluid, or serum. The test is primarily used in diagnosing diseases characterized by cellular immune activation, such as tuberculosis, especially extrapulmonary forms like tuberculous pleuritis or meningitis. Elevated ADA levels are typically associated with conditions characterized by intense T-cell stimulation, making the test a useful indirect marker of cell-mediated immunity.
Adenosine deaminase plays a vital role in purine metabolism by catalyzing the deamination of adenosine into inosine. This step is crucial for the proliferation and differentiation of lymphocytes, especially T lymphocytes. Increased ADA activity indicates a heightened cellular immune response and is commonly observed in infectious diseases characterized by granulomatous inflammation, such as tuberculosis. High ADA levels are also found in certain lymphoproliferative disorders, autoimmune diseases, and some viral infections. For instance, ADA levels in pleural fluid above certain thresholds can support a presumptive diagnosis of tuberculous pleuritis in regions with high disease prevalence, particularly when other clinical or radiological findings are nonspecific.
On the other hand, low ADA activity is usually considered within normal limits or may occur in immunocompromised states where T-cell function is reduced. In some rare genetic disorders, such as severe combined immunodeficiency (SCID), ADA deficiency is a characteristic finding, resulting in impaired immune responses.
ADA is not specific to a single disease but offers essential supplementary data that can assist in diagnosis, especially when combined with other laboratory and clinical findings. Its importance lies in its ability to detect immune system activation, which helps distinguish between infectious and non-infectious causes of fluid buildup in body cavities.
