An adenosine deaminase (ADA) test on pleural fluid is a diagnostic tool that measures the level of a specific enzyme to investigate certain medical conditions. Adenosine deaminase is an enzyme present in cells throughout the body, where it is involved in processing purines, which are components of cellular energy and genetic material. Pleural fluid is the liquid in the pleural space, the thin area between the pleura—the two layers of tissue lining the lungs and the inside of the chest cavity. This fluid acts as a lubricant for breathing, but certain diseases can cause excess fluid to accumulate, a condition known as a pleural effusion.
The Purpose of the Adenosine Deaminase Test
A physician orders an ADA test on pleural fluid primarily to help diagnose tuberculous pleurisy, a tuberculosis (TB) infection of the pleura. When the immune system responds to Mycobacterium tuberculosis, the bacterium that causes TB, it triggers an inflammatory reaction. This immune response involves the activation of T-lymphocytes, a type of white blood cell, which release a large amount of the ADA enzyme into the pleural space. Consequently, the ADA concentration in the pleural fluid becomes elevated in most cases of tuberculous pleurisy.
Measuring this enzyme provides a rapid diagnostic clue, as directly detecting the bacteria can be difficult and slow. The test is particularly useful in parts of the world where TB is common, as a high ADA level in a patient with corresponding symptoms is a strong indicator of the disease. This can guide a doctor’s decision to start treatment while waiting for definitive culture results, which can take several weeks.
The Pleural Fluid Collection Procedure
The pleural fluid sample is obtained through a procedure called thoracentesis, where a doctor removes fluid from the pleural space. The procedure is done with the patient sitting up and leaning forward, a position that widens the spaces between the ribs for easier access. Before starting, the doctor cleans an area of skin on the back and injects a local anesthetic to numb the site. While the patient may feel pressure, there should be no sharp pain.
Once the area is numb, the doctor inserts a thin needle through the skin, between the ribs, and into the pleural space. Using an attached syringe or collection bag, the doctor withdraws a sample of the fluid. After enough fluid is collected, the needle is removed, and a small bandage is placed over the insertion site. The procedure usually requires only a short period of observation afterward.
Interpreting Test Results
Interpreting the results involves comparing the measured enzyme level, reported in units per liter (U/L), to established reference values. A doctor analyzes the result in the context of the patient’s symptoms, medical history, and other findings as part of a comprehensive evaluation.
A high ADA level is strongly suggestive of tuberculous pleurisy. While cutoff values can vary between laboratories, a result above 40 U/L is considered elevated and suspicious for TB. In regions with a high prevalence of tuberculosis, levels greater than 70 U/L are very likely caused by a TB infection.
Conversely, a low ADA level makes a diagnosis of tuberculous pleurisy much less probable, and a concentration below 40 U/L helps exclude TB as the cause. This can steer the diagnostic focus toward other potential causes of the fluid accumulation. An intermediate result is less conclusive and necessitates further investigation to determine the underlying issue.
Other Causes of Abnormal ADA Levels
Although the ADA test is useful for investigating tuberculosis, other medical conditions can also increase the enzyme’s level in pleural fluid. Certain types of cancer, particularly lymphomas and lung cancer, can trigger an immune response that leads to elevated ADA. Autoimmune diseases, such as rheumatoid arthritis and lupus, can also lead to inflammation of the pleura and a rise in ADA levels. Other infections, including some bacterial pneumonias, can result in moderately increased ADA.
The degree of elevation can offer diagnostic clues, as the ADA levels in these other conditions are often not as high as those found in tuberculous pleurisy. For instance, the median ADA value in malignant effusions is significantly lower than in TB-related effusions. Therefore, the ADA test is primarily used to help confirm or exclude TB and is not the main diagnostic method for these other diseases.