What is ADA (Adenosine Deaminase)?
ADA, or Adenosine Deaminase, is an enzyme involved in purine metabolism, a process essential for producing energy and building genetic material (DNA and RNA).
It converts adenosine, a nucleoside, into inosine, helping recycle cellular materials and regulate immune activity.
The ADA test measures the activity of this enzyme, mainly in body fluids like pleural fluid (fluid around the lungs).
This test is most commonly used to diagnose tuberculosis (TB) — particularly tuberculous pleuritis, a type of TB that affects the lining of the lungs.
Where is ADA Produced in the Body?
ADA is mainly produced in immune-related tissues, especially lymphocytes, which are white blood cells responsible for fighting infections.
It is also found in:
- Liver
- Small intestine
- Macrophages (a type of immune cell that engulfs bacteria and damaged cells)
These cells release ADA as part of the body’s immune defense mechanism.
When infections like TB occur, immune activity increases, leading to higher ADA levels in affected fluids.
Main Functions and Importance of ADA
The ADA enzyme has important biological and diagnostic roles:
1. Purine Metabolism
ADA breaks down adenosine → inosine, a key step in purine metabolism and energy production.
2. Immune Function
It supports the growth, differentiation, and activation of T-lymphocytes, which are essential for the body’s defense against infections.
3. Diagnostic Marker
High ADA levels in pleural fluid are an important marker for tuberculous pleuritis, making this test a valuable, low-cost tool in TB diagnosis.
Causes of Low ADA Levels
Low ADA levels indicate poor immune activity or reduced enzyme production.
Common causes include:
- Severe Combined Immunodeficiency (SCID):
A rare genetic disorder in infants where ADA deficiency leads to a severely weakened immune system. - Liver Diseases:
Liver damage reduces ADA enzyme synthesis. - Advanced Immunodeficiency Conditions:
Conditions with very low lymphocyte counts, such as late-stage HIV/AIDS.
Symptoms of Low ADA Levels
Low ADA levels themselves cause no symptoms, but related immune problems may cause:
- Frequent infections (respiratory, skin, or gut)
- Poor growth in infants (SCID)
- Weak response to vaccines
- Recurrent fever and prolonged illness
Causes of High ADA Levels
High ADA levels are usually due to infections or diseases that trigger strong immune responses, especially involving T-lymphocytes or macrophages.
Common causes include:
- Tuberculosis (TB) – particularly pleural TB
(most common reason for elevated ADA in pleural fluid) - Other Infections:
- Bacterial pneumonia
- Empyema (pus in the pleural cavity)
- Cancers:
- Lymphoma
- Leukemia
- Autoimmune Diseases:
- Rheumatoid arthritis (RA)
- Systemic lupus erythematosus (SLE)
- Liver Diseases:
- Chronic hepatitis
- Liver cirrhosis
Symptoms of High ADA Levels
ADA itself does not cause symptoms, but high levels are linked to the underlying diseases that cause it:
- Tuberculosis (TB): Persistent cough, fever, night sweats, and weight loss
- Pleural Effusion: Chest pain, difficulty breathing, and fluid buildup around lungs
- Liver Disease: Fatigue, jaundice, and abdominal swelling
- Autoimmune Disease: Joint pain and body aches
Reference Ranges
Reference values may vary slightly depending on the lab and the sample type.
| Sample Type | Normal Range | Clinical Significance |
|---|---|---|
| Serum (blood) | 0 – 40 U/L | Normal enzyme activity |
| Pleural fluid | > 40 U/L | Suggestive of tuberculous pleuritis |
| < 40 U/L | More likely non-TB causes (e.g., cancer, pneumonia) |
🔸 A pleural fluid ADA level above 40 U/L strongly supports the diagnosis of pleural TB.
Sample Type and Handling
| Parameter | Details |
|---|---|
| Sample Type | Serum (blood) or body fluids (pleural, peritoneal, or cerebrospinal fluid) |
| Tube Type | Plain red-top or serum separator tube (SST) |
| Fasting Required | No |
| Special Handling | Samples should be handled quickly and kept cool to preserve enzyme activity. |
How the ADA Test is Performed
- A blood or fluid sample (like pleural fluid) is collected using a sterile needle.
- The sample is sent to the laboratory, where ADA enzyme activity is measured through a biochemical colorimetric assay.
- The results are expressed in Units per Liter (U/L).
- If done on pleural fluid, results are compared to standard cut-off values to help confirm or rule out TB.
Interpreting ADA Test Results
| ADA Level | Possible Interpretation |
|---|---|
| Normal (0–40 U/L in serum) | Normal immune activity |
| >40 U/L in pleural fluid | Suggests tuberculous pleuritis |
| >70 U/L | Strongly supports TB or other severe infections |
| Very low levels | May indicate immune deficiency or liver dysfunction |
⚠️ While a high ADA level supports the diagnosis of TB, it should be correlated with other tests such as chest X-ray, sputum culture, and GeneXpert for confirmation.
Test Preparation
- No fasting is required before the test.
- Inform your doctor if you are taking any antibiotics or TB medications.
- Avoid contamination when collecting pleural or body fluid samples — must be performed under sterile medical supervision.
- Results are best interpreted by a pulmonologist or infectious disease specialist.
When to Consult a Doctor
You should consult a doctor if you have symptoms such as:
- Persistent cough lasting more than 2–3 weeks
- Chest pain or difficulty breathing
- Unexplained fever or night sweats
- Weight loss or fatigue
- Fluid buildup in chest or abdomen
These signs may indicate tuberculosis, liver problems, or immune-related disorders, and an ADA test may help identify the cause.
Important Word Explanations
| Term | Simple Meaning |
|---|---|
| Enzyme | A protein that speeds up chemical reactions in the body. |
| Purine Metabolism | The process of breaking down energy molecules like DNA and ATP. |
| Lymphocytes | White blood cells that help fight infections. |
| Macrophages | Immune cells that digest bacteria and damaged tissue. |
| Pleural Fluid | Fluid present between the layers of the lungs and chest wall. |
| Pleuritis | Inflammation of the lining around the lungs. |
| SCID | Severe Combined Immunodeficiency, a genetic immune disorder. |
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