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Blood Type and Crossmatch

Does this test have other names?

Blood typing, crossmatching

What is this test?

This is a set of tests that looks for harmful interactions between your blood and donor blood. The tests are done before a blood transfusion.

Blood typing is the first step. This test finds out whether you have blood type A, AB, B, or O. Your blood is also tested to find out whether your Rh type is negative or positive. It's important for your doctor to know your blood type in order to select a donor blood that's compatible before doing the crossmatch.

An intermediate step between blood typing and crossmatching is called a recipient antibody screen. This test checks for unexpected antibodies in your blood. If unexpected antibodies are found, this can speed up the selection of compatible donor blood.

Crossmatching is a way for your doctor to test your blood against a donor's blood to make sure they are fully compatible. Crossmatching takes 45 minutes to an hour. It's essentially a trial transfusion done in test tubes to see exactly how your blood will react with potential donor blood.

It's important for donor blood to match your own as closely as possible. Otherwise, your immune system might create antibodies against the donor blood cells. In this case, your immune system correctly views the donor cells as foreign, but incorrectly views them harmful. This can lead to a dangerous and possibly fatal reaction.

Why do I need this test?

You might have this test if you need or may need a blood transfusion. For example, you might need a blood transfusion if you have an acute hemorrhage that causes a severe loss of red blood cells.

You may also have this test if you are having certain medical procedures that could cause significant blood loss, such as a cesarean section, a renal biopsy, or a cholecystectomy.

You may have a partial crossmatch if you are in critical need of blood, and your doctor decides that waiting for a full test could be more dangerous for you. If your situation is too urgent to wait for even a partial crossmatch, type O blood may be used. Type O blood has the highest probable compatibility with other blood types.         

What other tests might I have along with this test?

Your doctor may also order an antibody screen.       

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

This test does not have a "normal" result. The goal of blood typing and crossmatching is to find a compatible blood type for transfusion.

If your crossmatch finds no antibodies, you have a very low possibility that your blood type will be incompatible with the donor type.

If your crossmatch comes back positive, it means it's likely that antibodies were found. In this case, the antibodies must be isolated in a lab to find out how significant they are. Not all antibodies cause donor blood to be incompatible with your type, but when they are, using blood from that particular donor will most likely be ruled out.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

A crossmatch that's done more than three days before a transfusion could have inaccurate results.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.