The direct Coombs test (antiglobulin test) is a blood test used to diagnose a type of anemia caused by your immune system. Your immune system is your body’s defense system. It makes proteins called antibodies to attack foreign invaders. In some cases, your immune system can make antibodies against red blood cells.
What Is Direct Coombs Test (Direct Antiglobulin Test)?
The direct Coombs test (sometimes called the direct antiglobulin test) looks for antibodies attached to red blood cells. It is often used to diagnose blood disorders such as autoimmune hemolytic anemia.
The direct Coombs test is used to detect antibodies (IgG or C3) attached to the surface of red blood cells. Many diseases and medications can cause this. These antibodies can destroy red blood cells and cause anemia.
This is a test done on a sample of a newborn baby’s blood, usually in the case of a baby with jaundice. The two most common types of antibody-mediated hemolysis in infants are Rh incompatibility and ABO incompatibility.
Why Direct Coombs test (Antiglobulin test) is Required for newborns?
This is a test done on a sample of a newborn baby’s blood, usually in the case of a baby with jaundice. This test looks for “foreign” antibodies that have already attached to the baby’s red blood cells (erythrocytes), which is a potential cause of hemolysis. This is called “antibody-mediated hemolysis.”
The two most common types of antibody-mediated hemolysis in infants are Rh incompatibility and ABO incompatibility.
Rhesus incompatibility occurs when a mother who is Rh negative (and has natural anti-Rh antibodies in her blood serum) gives birth to a child who is Rh+.
When maternal and fetal blood mix during pregnancy or during childbirth, the mother’s anti-Rh antibodies aggressively attack the baby’s Rh+ red blood cells, attach to the cells, and then lyse them.
ABO incompatibility is caused by the same general mechanism. Type O mothers are more affected because they have anti-A and anti-B antibodies. If the child has type A, B or AB, there is a risk of incompatibility. This is often referred to as ” set-up “.
If maternal and fetal blood mix during pregnancy or delivery, these antibodies can attack the baby’s red blood cells and cause hemolysis. This reaction is generally more serious than Rh incompatibility (severe and potentially fatal if untreated) and usually causes only mild jaundice and anemia.
It should be remembered that a positive Coombs test in the laboratory does not necessarily mean that the child has hyperbilirubinemia. Although the risk of needing phototherapy is certainly higher, there are many factors that affect bilirubin levels, and it is important to evaluate all of these factors in order to make an appropriate treatment decision.
Conversely, active hemolysis can result in a negative Coombs test. Conditions in which the red blood cells are defective in some way (hereditary spherocytosis, G6PD deficiency, etc.) can also lead to severe hyperbilirubinemia, although antibodies are not involved in these processes. The Coombs test will be negative.
The procedure of Direct Coombs’ Test (Antiglobulin test)
- Prepare a 5% suspension of test erythrocytes (RBCs) in isotonic saline.
- Using a clean pipette, add 1 drop of the prepared cell suspension to a small test tube.
- Rinse three times with saline to remove all residual serum.
- After the last wash, drain the water completely.
- Add 2 drops of anti-human serum.
- Mix well and centrifuge at 1500 rpm for 1 minute.
- Agitate the cells gently to resuspend the cells and inspect macroscopically and microscopically for clumps.
What do the results of a direct Coombs test (antiglobulin test) mean?
Test results may vary depending on age, sex, medical history, test method, etc. Test results may not indicate the presence of a problem. Ask your doctor what the test results mean for you.
A direct Coomb’s test (antiglobulin test) tells your doctor whether you or your baby have antibodies against red blood cells. A negative result does not detect antibodies to red blood cells.
In the presence of antibodies to erythrocytes (RBCs), the analysis is considered positive. Test results range from +1 (barely positive) to +4 (highly positive). A positive antiglobulin test can mean:
- Blood transfusion reaction
- Autoimmune hemolytic anemia
- Hemolytic disease of newborns
- Hemolytic anemia in response to medication