There is a certain type of anemia called autoimmune hemolytic anemia. This type of anemia is a rare condition which results in the development of antibodies against an individual's red blood cells. There are two kinds of blood tests that can detect the presence of these antibodies. These are referred to as the direct Coomb's test and the indirect Coomb's test.
Direct Coomb's test is conducted on a sample of RBCs from a person's body. Here, the test detects antibodies that are already present, whereas, an indirect Coomb's test is conducted on the serum; this would be the liquefied part of the blood. In this case, the test aims to detect antibodies present in the blood stream that could potentially attach themselves to the RBCs, thereby leading to complications in case of blood transfusion or mixing.
This test, as mentioned above, detects antibodies in the red blood cells. This is done to check if antibodies have developed as a result of some disease or a consequence of blood transfusion. A direct Coomb's test is also conducted on an Rh-positive new born baby whose mother is Rh-negative. This test then identifies if the mother's body developed antibodies because of the Rh-positive blood type of the baby, or whether these antibodies were transferred through her placenta to the baby.
This is a regular blood test, and hence, requires no specific preparation. It would be advisable though, to inform the doctor and health care provider regarding any medication you're taking, and also about any discomfort like nausea, excessive bleeding, bruising, or dizziness you have experienced with a blood test in the past.
For this test, the health care provider will strap your arm tight with an elastic band. This will make the vein in your hand (usually on the inside of the elbow, or the back of the hand) swell and be prominent. A needle is then inserted into the vein and blood is drawn into the vial or tube attached to the syringe. As the blood is being collected, the strap on your arm will be loosened. Once the required amount of blood is collected, the needle will be removed and you will be given gauze or cotton to press into the blood-drawn area. If you are prone to dizziness, it's best to sit still for a few moments after the test.
Depending on one's threshold of pain, the test will range from mild discomfort to throbbing soreness. However, blood tests don't cause any lasting damage, and any pain you feel should disappear in a few hours.
A normal direct Coomb's test result is a negative test result. A negative direct Coomb's test implies that your blood does not have any antibodies that have attached themselves to your RBCs (red blood cells).
An abnormal test result, therefore, means a positive direct Coomb's test result. This implies that there are antibodies in your red blood cells. The presence of these antibodies could be due to a variety of reasons; some of them being, autoimmune hemolytic anemia, drug induced hemolytic anemia, mononucleosis, transfusion reaction, syphilis, and lymphocytic leukemia.
Do note that sometimes, the test is also abnormal without any clear reason. This is especially true when this test is conducted on senior citizens.