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Ency. home > Test > B > Blood typing

Blood typing   

Overview | Risks | Results

Alternative names:

Cross matching; Rh typing; ABO blood typing

Definition:

A test to classify blood by determining the absence or presence of antigens on the red blood cells, and the presence of antibodies to these antigens in the serum.

How the Test is Performed

Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure, which restricts blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered with gauze or with a bandage to stop any bleeding.

Infant or young child:
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. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to Prepare for the Test

Adults:
No special preparation is necessary for this test.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

How the Test will Feel

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.

Why the Test is Performed

While the general physical make-up of 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 him or her. A and B 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 this antigen is present or not identifies your blood as Rh+ (positive) or Rh- (negative).

The ABO typing process has two 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 tests, 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, then you have Rh-positive blood. If the blood cells do not stick together, then you have Rh-negative blood.

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

Ency. home > Test > B > Blood typing

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