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Coombs Test - Indirect

(ICT)

This test is for

Male, Female

Understanding Coombs Test - Indirect

What is Coombs Test - Indirect?

The Indirect Coombs Test (ICT) is also called as Indirect Antiglobulin Test. It is performed to identify the presence or absence of antibodies in blood directed against antigens found on red blood cells.

ICT test is done at the time of donating blood, tissue, or organ, before receiving blood during the transfusion to determine the compatibility with donated blood, and during pregnancy to determine the risk of incompatibility between mother and fetus.

The Indirect Coombs Test is performed on the principle of the antigen-antibody reaction. It is used to detect very low concentrations of antibodies present in a patient’s plasma. The ICT can also be used for compatibility testing, antibody identification, RBC phenotyping, and titration studies.

What is Coombs Test - Indirect used for?

· At the time of donating blood, tissue, organ, or bone marrow or while getting registered as a potential donor

· Before receiving a blood transfusion or blood components transfusion to determine compatibility with donated blood

· Before pregnancy and during pregnancy to determine the risk of incompatibility between the mother and fetus

· To determine if hemolytic anemia is caused due to an autoimmune disorder

What does Coombs Test - Indirect measure?

The red blood cells have specific antigen markers present on their surface. These marker antigens are glycoproteins and help the body to recognize its own type of red blood cells (RBCs). Antibodies are naturally produced against incompatible RBC antigens when there is exposure to incompatible red blood cells. Three most common antigen markers found on RBCs are A, B, and Rh. The presence or absence of these antigen markers is used to determine the blood group of a person.

Blood transfusion between incompatible blood types causes an antigen-antibody reaction which causes the RBCs to clump together (agglutination) and this results in their destruction. Hence, blood groups of donor and recipient must match to ensure the success of the blood transfusion or organ transplant. Agglutination may also occur due to Rh incompatibility between Rh-negative pregnant woman (who is having Rh antibodies because of previous sensitization) and her Rh-positive child which may cause antibodies in the mother to cross the placenta and attack the RBCs of the fetus. In some autoimmune disorders, antibodies are produced against antigens on the body’s own RBCs. These antibodies may attack the ‘self’ RBCs and cause agglutination.

The Indirect Coombs Test is performed on the principle of the antigen-antibody reaction between incompatible blood groups. The collected blood specimen is treated with a large range of collected or synthetic human RBC antigens and observed under a microscope. If agglutination (clumping) of RBCs is seen upon treatment with a particular antigen, antibodies against that antigen are already present in the blood specimen. Antibodies against RBC antigens A and B occur naturally, while antibodies against other RBC antigens are produced upon exposure to the corresponding antigen. A similar test called the Direct Coombs Test is performed to detect the presence of antibodies (bound to RBCs) against the body’s own RBCs (autoimmune antibodies) that may be the cause of autoimmune hemolytic anemia.

Interpreting Coombs Test - Indirect results

Interpretations

Agglutination: Positive result, antibodies present

No agglutination: Negative result, antibodies absent

Result	Interpretation
Negative	No antibodies detected
Equivocal	Positive in undiluted serum up to 1:16 titre
Positive	Positive in titre of 1:32 or above Positive in increasing titre in subsequent tests

Antibodies are produced in the blood upon exposure to incompatible antigens as during a blood transfusion. Antibodies produced in the first incompatible transfusion produce severe reactions upon subsequent transfusions of the same blood type.

Answers to Patient Concerns & Frequently Asked Questions (FAQs) about Coombs Test - Indirect

Frequently Asked Questions about Coombs Test - Indirect

Q. Is there any risk associated with this test?

There is no risk associated with the test. However, since this test involves a needle prick to withdraw the blood sample, in very rare cases, a patient may experience increased bleeding, hematoma formation (blood collection under the skin), bruising or infection at the site of needle prick.

Q. What other tests can be prescribed by your doctor in case of abnormal Indirect Coombs test result?

Additional test that may be prescribed in case of abnormal Indirect Coombs Test result is Antibody Identification Test. It helps to determine which antibodies are present in the sample that are causing an incompatibility reaction.

Q. What else can the Indirect Coombs Test be used for?

The Indirect Coombs Test can also be used to help diagnose the cause of hemolytic anemia due to autoimmune conditions. In an autoimmune disorder, antibodies are produced against the body’s own RBCs which attack and destroy the RBCs causing hemolytic anemia. Autoimmune hemolytic anemia can be caused by autoimmune diseases like Systemic Lupus Erythematosus, mycoplasma pneumonia infections, or certain cancers like lymphocytic leukemia and lymphoma.

Q. What is the difference between Indirect and Direct Coombs Tests?

Both Indirect and Direct Coombs Test are performed to determine the presence of anti-human RBC antibodies directed against human RBC antigens found on the RBCs. The Indirect Coombs Test is performed to determine antibodies produced against incompatible RBCs obtained from outside the body (as during a blood transfusion or during pregnancy). Whereas, the Direct Coombs Test is performed to detect the presence of antibodies directed against the patient’s own RBCs, which occurs during autoimmune diseases. Also, antibodies detected by the Indirect Coombs Test are present in the serum, while those detected by the Direct Coombs Test are attached to the surface of the RBCs. The Direct Coombs Test is performed to confirm if hemolytic anemia is being caused due to autoimmune disorder. The Indirect Coombs Test is performed primarily to determine compatibility of blood transfusion, or to check for Rh compatibility between a pregnant woman and her child before or during pregnancy.

Q. What is HDFN?

Hemolytic Disease of Fetus and Newborn or HDFN is a condition where the blood type of the fetus is incompatible with the blood type of the mother. The most severe reactions typically occur due to Rh incompatibility, where antibodies are produced in the Rh negative mother against Rh positive fetus. These antibodies cross the placenta and react with the RBC of the fetus, causing HDFN.

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