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. on This method breaks blood types down into four categories:

• Type A
• Type B
• Type AB
• Type O

Your blood type (or blood group) depends on the types that are been passed down to you from your parents.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with a germ-killing product. An elastic band is placed around the upper arm to apply pressure, which causes the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected into a tube. During the procedure, the elastic band is removed to restore circulation. Once the blood has been collected, the needle is removed, and a band-aid or gauze is applied.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied if there is any bleeding.

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.

No special preparation is necessary for this test.

Some people have discomfort when the needle is inserted. Others may only feel a tiny prick or stinging sensation. Afterward, there may be some throbbing or a bruise may develop.

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 you can only receive types B and O blood.
• If you have type AB blood, you can only receive types A, B, AB, and O blood.
• If you have type O blood, you can 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 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 B is added, you have type O blood.

Back typing:

• If your blood clumps together when B cells are added to your sample, you have type A blood.
• If your blood clumps together when A cells are added to your sample, you have type B blood.
• If your blood clumps together when both 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.

RH typing:

• 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.

Risks associated with taking blood may include:

• Fainting or feeling light-headed
• Multiple punctures to locate veins
• Excessive bleeding
• Hematoma (blood accumulating under the skin)
• 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.

Goodnough LT. Transfusion medicine. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 183.