Coombs Test / Anti-Globulin Test (AGT)
Coombs Test / Anti-Globulin Test (AGT)
Overview
Coombs test is also known as anti-globulin test or AGT. It is either of the two clinical blood tests used in hematology.
- Direct Coombs Test (DCT or also called direct antiglobulin test or DAT)
- Indirect Coombs Test (ICT or also called indirect antiglobulin test or IAT)
Simply, if you have low red blood cells count or anemia, Coombs test is one of the tests that your doctor may order to find out the cause of your anemia.
Why Is It Done?
The purpose of Coombs test is to check out the presence of certain antibodies in the blood. Antibodies are the proteins that our immune system makes for defense against the invader. Sometimes, due to unexplained reason, these proteins or antibodies are produced against our own system or in our case, against the red blood cells.
The purpose of the Coombs test is to find out if you have these antibodies in your blood stream that are attacking and destroying your own red blood cells resulting into the anemia. This condition is labelled as hemolytic anemia.
Direct Coombs test / DCT detects the antibodies that are attached to the surface of the red blood cells. Obviously, the you have to send the sample of blood containing RBCs. Indirect Coombs test / ICT detects the unattached antibodies that are free floating in your blood stream. So, you have to send the serum of the patient.
How Is It Done?
A sample of your blood in required in both types of Coombs test. The blood is drawn through a needle inserted in the vein (venipuncture) of your arm or the hand. The blood is collected and stored in the test tube.
Pre-test Preparations
No special preparations are required for this test. Just be sure to follow the standard protocol regarding hygiene during venipuncture.
Risks
No great risk is there is such type of lab test or procedure. You may feel a tiny prick of needle or small pinching sensation while the sample is drawn. Some rare issues may be:
- Hematoma / collection of blood at the sample site. Do be afraid as it would resolve itself.
- Infections if proper antiseptic measures are not followed.
- Lightheadedness or fainting
- Excessive bleeding from puncture site (only in the patient of bleeding disorders)
Results
Normal
If there is no clumping of RBCs (agglutination), the Coombs test is normal.
Abnormal Direct Coombs Test
The clumping or agglutination of RBCs during the test is abnormal. It indicated that you have antibodies on your RBCs that are causing destructing of your red blood cells, a conditions called hemolysis. Some conditions that results abnormal direct Coombs test:
- Autoimmune hemolytic anemia (AHA – You own immune system mistakenly identifies RBC membrane as a foreign antigen and produces antibodies against it that become attached to the surfaces of red blood cells)
- Drugs toxicity like cephalosporins and nitrofurantoin (anti-biotics), levodopa (for Parkinson’s disease), dapsone (anti-bacterial), NSAIDs e.g. ibuprofen and quinidine (heart medication)
- Transfusion reaction (your immune system attacks the mismatched transfused blood)
- Different blood types between mother and fetus (erythroblastosis fetalis)
- Chronic lymphocytic leukemia / CLL
- Systemic lupus erythematosus / SLE
- Infectious mononucleosis
- Mycoplasma infection
- Syphilis disease
In older adults, Coombs test may be abnormal even without any disease or risk factor.
Abnormal Indirect Coombs Test
Abnormal indirect Coombs test indicates that free floating antibodies are present in your blood stream that are attacking and destroying your red blood cells. Many conditions may lead to abnormal indirect Coombs test:
- Erythroblastosis fetalis (different blood types between mother and fetus)
- Mismatched ABO blood transfusion
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Coombs Test / Anti-Globulin Test (AGT)
