If you need a blood transfusion or transplant, your doctor can use blood typing and crossmatching to learn if your blood is compatible with donor blood or organs.
Blood typing reveals what type of blood you have. This depends on the presence of certain antigens on your red blood cells (RBCs). Antigens are proteins that trigger your immune system to produce antibodies. There are four main types of blood:
- type A blood, which contains type-A antigens
- type B blood, which contains type-B antigens
- type AB blood, which contains type-A and type-B antigens
- type O blood, which contains neither type-A nor type-B antigens
Your blood will also be classified as Rh positive (+) or Rh negative (-), based on the presence or absence of a particular protein on your RBCs, known as rhesus factor.
Crossmatching is a test used to check for harmful interactions between your blood and specific donor blood or organs. It can help your doctor predict how your body will react to those donor materials.
Your doctor uses blood typing and crossmatching to learn if donor blood or organs are compatible with your blood. Incompatible donor blood or organs can cause harmful interactions. Your immune system may attack the donor material, leading to dangerous and even fatal reactions.
Your doctor may order blood typing, crossmatching, or both if:
- you’re scheduled to receive a blood transfusion or organ transplant
- you’re scheduled to undergo a medical procedure where you face the risk of significant blood loss
- you have certain medical conditions, such as severe anemia or a bleeding disorder
Your doctor may also order blood typing if you’re pregnant. If your developing fetus has a different blood type than you, it raises their risk of developing a type of anemia called hemolytic disease.
Blood typing helps your doctor determine what type of donor blood is compatible with your own. Some blood types contain antibodies that trigger immune reactions against other blood types. In general:
- if you have type A blood, you should only receive types A or O blood
- if you have type B blood, you should only receive types B or O blood
- if you have type AB blood, you can receive types A, B, AB, or O blood
- if you have type O blood, you should only receive type O blood
If you have type AB blood, you’re known as a “universal recipient,” and can receive any ABO category of donor blood. If you have type O blood, you’re known as a “universal donor,” and anyone can receive type O blood. Type O blood is often used in emergencies when there isn’t enough time to perform blood typing tests.
Crossmatching can also help reveal if specific donor blood or organs are compatible with your own. In addition to anti-B and anti-A antibodies, other types of antibodies may be present in your blood that negatively interact with donor materials.
To perform blood typing and crossmatching, your doctor will collect a sample of your blood to send to a laboratory for testing.
Collecting the sample
A trained health care practitioner can draw a sample of your blood at your doctor’s office, blood bank, or other sites. They’ll use a needle to draw the sample from one of your veins, usually on the inside of your elbow.
They’ll likely start by disinfecting the area with an antiseptic. An elastic band will be placed around the upper part of your arm, causing your vein to swell up with blood. A needle that they gently inserted into your vein will collect a sample of your blood in a tube.
Once they’ve collected enough blood, the practitioner will remove the needle and unwrap the band from your arm. The puncture site will be cleaned, and if needed, bandaged. Your blood sample will then be labeled and sent to a laboratory for testing.
Blood typing the sample
In the laboratory, a technician can conduct several tests to type your blood.
They will mix some of your blood with commercially prepared anti-A and anti-B antibodies. If your blood cells agglutinate, or clump together, it means your blood has reacted with one of the antibodies.
Next, the technician will perform back typing. This calls for some of your blood to be mixed with type A and type B blood. Your blood with then be checked for signs of reaction.
Following that, the technician will perform Rh typing. This is when they mix some of your blood with antibodies against Rh factor. Signs of any reaction will be noted.
Crossmatching the sample
To crossmatch your blood against donor blood or organs, the technician will mix a sample of your blood with a sample of the donor material. Again, they’ll check for signs of reaction.
Depending on the results of your blood typing, your blood will be classified as type A, B, AB, or O. It will also be classified as Rh+ or Rh-. There is no “normal” or “abnormal” blood type.
The results of your crossmatching test will help your doctor assess if it’s safe for you to receive specific donor blood or organs.
If your blood cells clump only when mixed with:
- anti-A antibodies, you have type A blood
- anti-B antibodies, you have type B blood
- either anti-A or anti-B antibodies, you have type AB blood
If your blood cells don’t clump when mixed with either anti-A or anti-B antibodies, you have type O blood.
If your blood cells clump only when mixed with:
- type B blood, you have type A blood
- type A blood, you have type B blood
- type A or B blood, you have type O blood
If your blood cells don’t clump when mixed with either type A or B blood, you have type AB blood.
If your blood cells clump when mixed with anti-Rh antibodies, you have Rh+ blood. If they don’t clump, you have Rh- blood.
If your blood cells clump when mixed with a donor sample, the donor blood or organ is incompatible with your blood.
Blood draws are generally safe for most people, but they do pose some risks. You may experience some discomfort or pain when the needle is inserted. You may also develop bleeding, bruising, or infection at the puncture site.
In most cases, the potential benefits of blood typing and crossmatching outweigh the risks. Talk to your doctor to learn more about the procedure. They can also help you understand your test results and recommend appropriate follow-up steps.