Blood typing is a method to tell what specific type of blood you have. What type you have depends on whether or not there are certain proteins, called antigens, on your red blood cells.
Blood is often grouped according to the ABO blood typing system. This method breaks blood types down into four types:
Your blood type (or blood group) depends on the types that are been passed down to you from your parents.
Cross matching; Rh typing; ABO blood typing
How the Test is Performed
A blood sample is needed and will be drawn from a vein.
The test to determine your blood group is called ABO typing. Your blood sample is mixed with antibodies against type A and B blood, and the sample is checked to see whether or not the blood cells stick together (agglutinate). If blood cells stick together, it means the blood reacted with one of the antibodies.
The second step is called back typing. The liquid part of your blood without cells (serum) is mixed with blood that is known to be type A and type B. Persons with type A blood have anti-B antibodies, and those with type B blood have anti-A antibodies. Type O blood contains both types of antibodies. These two steps can accurately determine your blood type.
Blood typing is also done to tell whether or not you have a substance called Rh factor on the surface of your red blood cells. If you have this substance, you are considered Rh+ (positive). Those without it are considered Rh- (negative). Rh typing uses a method similar to ABO typing.
How to Prepare for the Test
No special preparation is necessary for this test.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. These soon go away.
Why the Test is Performed
This test is done to determine a person's blood type. Health care providers need to know your blood type when you get a blood transfusion or transplant, because not all blood types are compatible with each other. For example:
If you have type A blood, you can only receive types A and O blood.
If you have type B blood, you can only receive types B and O blood.
If you have type AB blood, you can receive types A, B, AB, and O blood.
If you have type O blood, you can only receive type O blood.
Type O blood can be given to anyone with any blood type. That is why people with type O blood are called universal blood donors.
Blood typing is especially important during pregnancy. If the mother is found to be Rh-, the father should also be tested. If the father has Rh+ blood, the mother needs to receive a treatment to help prevent the development of substances that may harm the unborn baby. See: Rh incompatibility
If you are Rh+, you can receive Rh+ or Rh- blood. If you are Rh-, you can only receive Rh- blood.
If your blood cells stick together when mixed with:
Anti-A serum, you have type A blood
Anti-B serum, you have type B blood
Both anti-A and anti-B serums, you have type AB blood
If your blood cells do not stick together when anti-A and anti-B are added, you have type O blood.
If the blood clumps together only when B cells are added to your sample, you have type A blood.
If the blood clumps together only when A cells are added to your sample, you have type B blood.
If the blood clumps together when either types of cells are added to your sample, you have type O blood.
Lack of blood cells sticking together when your sample is mixed with both types of blood indicates you have type AB blood.
If your blood cells stick together when mixed with anti-Rh serum, you have type Rh-positive blood.
If your blood does not clot when mixed with anti-Rh serum, you have type Rh-negative blood.
There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
Infection (a slight risk any time the skin is broken)
There are many antigens besides the major ones (A, B, and Rh). Many minor ones are not routinely detected during blood typing. If they are not detected, you may still have a reaction when receiving certain types of blood, even if the A, B, and Rh antigens are matched.
A process called cross-matching followed by a Coombs test can help detect these minor antigens and is done before transfusions, except in emergency situations.
Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.