Blood typing is a test to classify blood by determining the absence or presence of antigens on the red blood cells and the presence or absence of antibodies to these antigens in the serum.

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 antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

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 to the puncture site if there is any bleeding.

No special preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing or a bruise may develop at the site where the needle was inserted. These symptoms are temporary.

While the general physical makeup of all people is the same, each individual is unique. Every person has identifiers on cells that allow the body to know that those cells belong to the individual. A and B markers are common and important identifiers. The O blood type designates the absence of the A and B markers. Another surface identifier, or antigen, on red blood cells is the Rh factor. Whether or not this antigen is present determines your blood as Rh+ (positive) or Rh- (negative).

The ABO typing process has 2 steps: forward and reverse typing. Initially, your blood is mixed with anti-A serum (serum that contains antibodies against type A blood), then with anti-B serum (serum that contains antibodies against type B blood). A determination of the blood type is based on whether or not the blood cells stick together (agglutinate) in the presence of these serums. The blood cells can stick together only when the anti-A antibody binds to the A antigen or the anti-B antibody binds to the B antigen. A laboratory technician can see the cells sticking together when the blood and serum are mixed in a test tube.

The second step involves placing your serum (the liquid portion of the blood without the cells) with blood that is known to be type A and type B (AB). With the results of these two steps, your blood type can be determined accurately.

Rh typing is similar to ABO typing. Here, your blood is mixed with serum containing anti-Rh antibodies and then observed for agglutination. If this occurs, you have Rh-positive blood. If the blood cells do not stick together, you have Rh-negative blood.

Blood typing is especially important during pregnancy. If the mother is found to have Rh-negative blood, the father should also be tested. If the father has Rh-positive blood, the mother needs to receive a treatment to help prevent antibodies from forming in the her serum. These antibodies may harm a fetus if not treated (see Rh incompatibility).

Blood typing is also necessary before receiving any blood transfusion.