Direct coombs test procedure (Direct antiglobulin test)

Direct antiglobulin test or direct Coomb’s test is an important test for the diagnosis of haemolytic disease in newborns. Sometimes IgG sensitizes the RBC but it cannot produce agglutination.

Due to the small size of IgG, it is unable to overcome the force that causes rbc to repel one another, resulting in a failing cross-linked bridge.

Direct COOMBS test principle

Direct antiglobulin tests detect human red blood cells which is in vivo sensitized by the antibodies or complements.

Coomb serum contains anti-human globulin and anti-complement antibodies. It agglutinates both of these sensitized red blood cells.

Direct COOMBS test Procedure

Following is the procedure of the direct coombs test.

1. First three times wash the patient’s Red blood cells with normal saline.
2. Add a drop of 3% washed red blood cell suspension in the tube.
3. Add two drops of Coomb’s reagents.
4. Mix and centrifuge for 20 seconds.
5. Look for agglutination.
6. Confirm agglutination by microscope.

Indication of direct antiglobulin test (Direct Coomb’s test)

The following are three main indications of the direct antiglobulin test (coomb’s test).

1. Hemolytic disease of the newborn.
2. Haemolytic Transfusion reaction.
3. Autoimmune haemolytic disease.

What does positive direct Coomb’s test mean?

When there is a positive direct Coomb’s test, it means RBCs are sensitized, it will cause haemolysis in case of blood transfusion.

What does negative direct Coomb’s test mean?

When there is a negative direct coomb test, it means RBCs are not sensitized. It will be safe in case of blood transfusion.

What is the Clinical significance of a direct antiglobulin test?

With the help of DAT or direct antiglobulin test, we can easily understand the causes of haemolysis. If the direct antiglobulin test is positive, it means haemolysis is due to sensitized RBC, but if the direct antiglobulin test is negative if means haemolysis is due to other than sensitized red blood cells.