Crossmatch

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Also known as: Compatibility testing; XM
Formal name: Crossmatch

At a Glance

Why Get Tested?

To enable the transfusion laboratory to select and issue the most appropriate and compatible blood for a transfusion.

When to Get Tested?

If your doctor indicates that transfusion with blood or blood components may be required as part of your medical treatment.

Sample Required?

A blood sample drawn from a vein in the arm.

Frequency of testing?
These tests will be performed prior to receiving a blood transfusion. Where medical treatment requires ongoing transfusions over a period of time the crossmatch will be repeated every 72 hours in order to reduce the possibility of a transfusion reaction.

The Test Sample

What is being tested?

A crossmatch is a group of tests undertaken before a person requires a blood transfusion. The transfusion laboratory completes a group and screen to establish the person's blood group and identify any possible red cell antibodies that may be present in their blood. Once completed, the transfusion laboratory then matches these results to compatible donor blood that can be safely transfused to the person.

Red blood cells have chemical structures (most commonly proteins or carbohydrates) on the outside surface of a red blood cell. These chemical structures usually have a defined function such as determining the shape of the cell or the transport of chemicals into, or out of, the red blood cell. In addition, they determine a person's blood group.

There are over 200 different blood groups known. The most important blood groups are the ABO and the RhD blood groups. However other blood groups can also become important. There are inherited blood group differences between people. When a person is exposed to blood groups that are different to their own the immune system may respond by producing a blood group antibody. This is very similar to the way we produce antibodies against germs (bacteria and viruses). The transfusion laboratory performs an antibody screen to detect and identify any blood group antibodies as these can cause transfusion reactions or haemolytic disease of the newborn (HDN).

If a transfusion is required it is important that the transfusion laboratory provides compatible blood. The blood selected for transfusion will usually lack the blood group that the person has an antibody against. A crossmatch is a final test that is performed to reduce the likelihood of a transfusion reaction. This test may be performed using laboratory tests or may be undertaken by a computer program (computer crossmatch) using results obtained from blood groups and antibody screens.

Further information on blood groups, antibody screens, crossmatching and how the tests are performed can be found in Inside the lab: Blood typing.

How is the sample collected for testing?

The crossmatch is performed on a blood sample taken by a needle placed in a vein in the arm.

The Test

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