Direct antiglobulin test or direct coomb’s test is an important test for the diagnosis of haemolytic disease of new born. Some time IgG sensitize the RBC but it cannot produce agglutination.
Due to the small size of IgG, it makes unable to overcome the force that cause rbc to repel one an other, result in fail cross-linked bridge.
Direct antiglobulin test principle
Direct antiglobulin test detect human red blood cell which are in vivo sensitized by the antibodies or complements.
Coomb serum contain anti human globulin and anti complements antibodies. It agglutinate both of these sensitized red blood cells.
Indication of direct antiglobulin test (Direct Coomb’s test)
Followings are three main indications of direct antiglobulin test (coomb’s test).
- Haemolytic disease of new born.
- Haemolytic Transfusion reaction.
- Autoimmune haemolytic disease.
Direct antiglobulin test Procedure
Following is the procedure of direct antiglobulin test.
- First three time wash the patients Red blood cells with normal saline.
- Add a drop of 3% washed red blood cell suspension in tube.
- Add two drops of coomb’s reagents.
- Mix and centrifuge for 20 seconds.
- Look for agglutination.
- Comfirm agglutination by microscope.
What does positive direct coomb’s test mean?
When there is positive direct coomb’s test, it mean RBC are sesitized, it will cause haemolysis in case of blood transfusion..
What does negative direct coomb’s test mean?
When there is negative direct coomb’s test, it mean RBC are not sesitized. It will be safe in case of blood transfusion.
What is the Clinical significance of direct antiglobulin test ?
With the help of DAT or direct antiglobulin test we can easily understand the causes of haemolysis. If direct antiglobulin test is positive, it mean haemolysis is due to sensitized RBC, but if direct antiglobulin test is negative, if means haemolysis is due to other than sensitized red blood cells.