Direct Coombs test
The direct 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. This causes red blood cells to break down, a condition called hemolytic anemia.
This blood test tells your doctor whether you have antibodies that have attached to your red blood cells. You may need this test if you have symptoms of hemolytic anemia after a blood transfusion. A baby may need this blood test if the baby's mother makes antibodies against the baby's red blood cells and passes those antibodies to the baby inside the womb. This condition is called hemolytic disease of the newborn.
The most common cause of hemolytic anemia is when your immune system makes antibodies to your own red blood cells by mistake. When your immune system makes antibodies against your own healthy cells, it is called autoimmune disease. Symptoms or signs of hemolytic anemia may include:
Shortness of breath
Blood tests that show low numbers of red blood cells
You may have a blood test that measures the red blood cells in your blood and the amount of oxygen carried by your red blood cells. These tests are called hemoglobin and hematocrit.
A result for a lab test may be affected by many things, including the method the laboratory uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
The direct antiglobulin test tells your doctor whether you or your child has antibodies to red blood cells. A normal blood test will find no antibodies to red blood cells. If there are any antibodies to red blood cells, the test is considered positive. The test results may range from 1+ (barely positive) to 4+ (very positive). A positive antiglobulin test may mean:
A transfusion reaction
Autoimmune hemolytic anemia
Hemolytic disease of the newborn
Hemolytic anemia caused by a drug reaction
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, and a sense of lightheadedness. When the needle is put into your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be sore.
Some drugs can interfere with this test, and some drugs can cause the test to come back positive even though you don't have hemolytic anemia. Drugs that may cause hemolytic anemia include:
Certain blood pressure medications
You don't need any special preparations for this test. Make sure to let your health care provider know about any medications, herbs, or supplements you are taking.