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If you need to have a blood transfusion it is essential that you are given blood that is similar or compatible to your own to prevent reactions to the transfusion and potentially a severe illness. Your blood group needs to be identified and your blood tested to see whether you have any antibodies to blood group antigens. The major blood groups are ABO and RhD. When the laboratory has this information it can decide which is the right type of blood to give you.


What is being tested?

A blood group is commonly performed as part of a group of tests. These tests include ABO and RhD blood groups, a blood group antibody screen and a crossmatch.

Blood groups are chemical structures (most commonly proteins or carbohydrates) on the outside surface of the 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 ones are the ABO and the RhD blood groups. These are routinely determined prior to transfusion and during pregnancy. These two blood group systems also determine the eight most common blood types: A Positive, A Negative, O Positive, O Negative, B Positive, B Negative, AB Positive and AB Negative.

Further information on blood groups and how the tests are performed can be found in the Feature article on Blood banking: Blood typing.

The ABO blood groups are extremely important when receiving a blood transfusion. There are some blood groups (e.g. group O Negative, the ‘universal donor’) that can be given to almost any other person. However other blood groups (e.g. group AB) must only be transfused to people of the same blood group in order to prevent a potentially fatal transfusion reaction.

How is it used?

Blood groups are used to select appropriate blood for transfusion and to ensure that a patient is transfused with blood from the same (or a compatible) blood group.

When is it requested?

The RhD group is also used during and immediately after pregnancy (antenatal group and screen), to determine whether the mother might require an injection of anti-D immunoglobulin. This is used to prevent haemolytic disease of the newborn (HDN), a condition which may lead to severe anaemia, brain damage and even death of the baby.

What does the result mean?

The result determines your blood group. This is used by laboratory staff to select appropriate blood for transfusion and to determine whether pregnant women may require an injection of anti-D immunoglobulin.

Common questions

  • Can a blood group be used for paternity or parentage testing to determine the father of a child?

Although blood groups are inherited from our parents there are many possible variations that can result.  These depend on the genetic information that each parent has. This means that blood groups are usually of limited value in parentage testing. There are more specific tests (such as DNA testing) that can determine paternity, with a much greater level of certainty. If you require this testing you should consult your local doctor to find out how this can be done.

  • Sometimes you see a television show where people have a rare blood group that requires family members to donate blood.  How often does this happen?

Like many things on television and in the movies, blood groups and blood transfusion may not be portrayed accurately. It is extremely rare that a family member would be asked to donate blood for someone. There is usually sufficient blood available for all patients, and there are protocols in place to provide access to special or rare blood when it is required. In some cases, donating blood for a family member could also be dangerous. Family members may feel pressured to donate, and so may conceal important medical history that could result in danger to either themselves or the recipient.

Last Updated: Thursday, 1st June 2023

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