Cross matching

Alternative names
Blood typing; Rh typing; ABO blood typing

Definition

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.

How the test is performed

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.

How to prepare for the test
No special preparation is necessary for this test.

Infants and young children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

     
  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

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 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.

Normal Values

     
  • Forward typing:       o If your blood cells stick together when mixed with anti-A serum, you have type A blood.       o If your blood cells stick together when mixed with anti-B serum, you have type B blood.       o If your blood cells stick together when they are mixed with both anti-A and anti-B serums, you have type AB blood.       o If your blood cells do not stick together in the presence of either serum, you have type O blood.       o If your blood cells stick together when mixed with anti-Rh serum, you have type Rh-positive blood.       o If your blood does not clot when mixed with anti-Rh serum, you have type Rh-negative blood.  
  • Reverse typing:       o Blood cell stickiness that occurs when B cells are mixed with your serum indicates you have type A blood.       o Blood cell stickiness that occurs when A cells are mixed with your serum indicates you have type B blood.       o Blood cell stickiness that occurs when your serum is mixed with both types of cells indicates you have type O blood.       o Lack of blood cells sticking together when your serum is mixed with both types of blood indicates you have type AB blood.  
  • Transfusions:       o If you have type A blood, you can receive blood transfusions from people with types A and O blood.       o If you have type B blood, you can receive blood transfusions from people with types B and O blood.       o If you have type AB blood, you can receive blood transfusions from people with types A, B, AB, and O blood.       o If you have type O blood, you can receive blood transfusions from people with type O blood only.       o If you have Rh-positive blood, you can receive transfusions from people with types Rh-positive and Rh-negative blood.       o If you have Rh-negative blood, you can receive transfusions from people with type Rh-negative blood only.

Notify your health care provider if you have had past transfusion reactions and be sure to tell them about any recent administration of blood products.

What abnormal results mean
There are no abnormal values.

What the risks are

     
  • 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)

Special considerations

There are many antigens besides the major ones (A, B, and Rh). Many minor antigens are not routinely detected during blood typing. If allowed to go unrecognized, they can initiate a blood transfusion reaction, usually of less magnitude than that of a major blood group incompatibility.

These minor antigens can be detected by cross-matching, which consists of incubating the recipient’s serum with the donor’s red blood cells (RBCs) in a saline solution followed by the addition of Coombs serum.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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