Adenosine deaminase (ADA) is a protein that is produced by cells throughout the body and is associated with the activation of lymphocytes, a type of white blood cell that plays a role in the immune response to infections. Conditions that trigger the immune system, such as an infection by Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB), may cause increased amounts of ADA to be produced in the areas where the bacteria are present. This test measures the amount of adenosine deaminase present in pleural fluid in order to help diagnose a tuberculosis infection of the pleurae.
Pleurae are membranes that cover the chest cavity and the outside of each lung. Small amounts of pleural fluid are continuously produced to lubricate the movement of the lungs against these membranes and the membranes against each other during inhalation and exhalation. A variety of conditions and diseases, including infection, can cause inflammation of the pleurae (pleurisy or pleuritis) and can lead to excessive pleural fluid accumulation (pleural effusion).
Tuberculosis can spread into the lungs and pleurae, causing symptoms such as chest pain, chronic cough, and shortness of breath. Since these symptoms may also be seen with a variety of other conditions, it is important to determine the cause as rapidly as possible in order to properly treat the affected person. Detecting mycobacteria in pleural fluid can be difficult because there may be a large volume of fluid and very low numbers of bacteria present. Though the ADA test is not specific and does not replace the culture for diagnosing TB, it may be positive even when numbers of bacteria are very low and can be used as an adjunct test to help determine whether tuberculosis is the likely source of a person's symptoms.
How is the sample collected for testing?
A sample of pleural fluid is collected by a healthcare practitioner with a syringe and needle using a procedure called thoracentesis. Rarely, other body fluid samples, such as peritoneal or cerebrospinal fluid (CSF), are collected using procedures specific to the fluid type.
How is it used?
The adenosine deaminase (ADA) test is not a diagnostic test, but it may be used along with other tests such as pleural fluid analysis, acid-fast bacillus (AFB) smear and culture, and/or tuberculosis molecular testing to help determine whether a person has a Mycobacterium tuberculosis infection (tuberculosis or TB) of the lining of the lungs (pleurae).
A culture is considered the "gold standard" for diagnosing tuberculosis and guiding treatment, but it may take several days to weeks to complete. Molecular testing and the AFB smear are rapid tests, but they require that a sufficient number of microorganisms be present in the fluid to detect them. Pleural fluid presents a unique problem with detecting M. tuberculosis because there may be a large volume of fluid with a very low number of bacteria present. Though 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.
The ADA test is used as an adjunct test to help rule in or rule out tuberculosis in pleural fluid. Rarely, it may be ordered to detect tuberculosis in other body fluids, such as peritoneal fluid or cerebrospinal fluid (CSF).
When is it ordered?
- Chronic cough, sometimes with bloody sputum
- Fever, chills
- Night sweats
- Unexplained weight loss
- Chest pain
This test may be ordered as one of several tests to help rule in or rule out TB as the cause of a person's symptoms, especially if the individual falls into a high-risk group, such as:
- People with close contact with someone who has active infectious TB
- Immigrants from areas of the world where the incidence of TB is high
- Children younger than 5 years old who have a positive TB screening test
- People who work with or are part of groups with high rates of infection, such as the homeless, IV drug users or confined populations, such as hospitalized patients, prisoners, and residents of nursing homes
- People with weakened immune systems such as:
Testing may be ordered when a healthcare practitioner wants to determine whether a person likely has tuberculosis, in advance of other test results, in order to initiate treatment.
What does the test result mean?
If adenosine deaminase (ADA) is markedly elevated in pleural fluid in a person with signs and symptoms that suggest tuberculosis, then it is likely that the person tested has a M. tuberculosis infection in their pleurae. This is especially true when there is a high prevalence of tuberculosis in the geographic region where a person lives.
When there is a low prevalence of tuberculosis in a region, then a person may have tuberculosis or may have an ADA result that is elevated for another reason, such as cancer (particularly lymphomas), pulmonary embolus, sarcoidosis, or lupus. These other diagnoses are more likely if the ADA result is only mildly or moderately elevated.
A person with a low ADA level is unlikely to have tuberculosis in their pleurae. This does not rule out having the infection in other parts of their body.
Is there anything else I should know?
Can my doctor diagnose tuberculosis without testing my pleural fluid?
Should everyone with suspected tuberculosis have an ADA test performed?
The ADA test is primarily performed when tuberculosis is suspected in the pleurae, and it is not routinely available in all laboratories. It will be performed when a healthcare practitioner determines that it will be useful and timely in helping to diagnose or rule out tuberculosis.
Can my blood be tested for ADA?
What is ADA deficiency?
ADA is an enzyme that converts one byproduct into another byproduct. The first substance is toxic to lymphocytes and must be inactivated by ADA. With ADA deficiency, a rare inherited condition, the body makes insufficient ADA. This leads to the buildup of the toxic byproduct and can cause severe combined immunodeficiency disease (SCID). Infants with this condition have seriously compromised immune systems and may not survive without bone marrow transplantation. For more information, visit the Genetics Home Reference webpage on ADA deficiency.