Summary

• This test can show what blood type you have and whether you are Rhesus (RhD) factor negative or positive.
• This information is important if you are about to have a blood transfusion and when you are pregnant. 
• When you have a transfusion, you need to be given blood that is compatible with your blood.
• In pregnancy, if you are RhD-negative and your baby is RhD-postive, this can harm the baby however treatment is available. Only RhD-negative women and their babies are at risk. 

What are blood groups and why are they important?

Blood is made up of different cells including red blood cells, white blood cells and platelets suspended in a liquid called plasma. All cells have protein markers called antigens on their surface.  

The surface of every red blood cell is coated with these marker antigens. Everyone has their own set of antigens which are inherited from their parents. There are about 30 blood group types, but A, B, AB and O are the most common.

If you are having a blood transfusion, your blood will be tested to identify which antigens you have. This is because your immune system can attack transfused blood from a donor if their antigens are different to yours.

Your immune system protects you by making antibodies to fight off substances that are not from your own body that could be potentially harmful. This includes infections like viruses and bacteria and toxic substances.  It does this by targeting marker antigens. Blood cells with marker antigens on their surfaces that are different from your own can also be attacked.

Testing will show which of the A, B, AB or O blood groups you have. It also shows whether you are RhD-positive or negative. This is another blood grouping. With this information, the lab will perform a test called a crossmatch to decide on which blood will be safe to give to you. 

Blood groups

Rhesus (RhD) factor

Each of these blood types is further classified according to its Rhesus (RhD) factor. RhD is another antigen on the surface of red blood cells, and you are either RhD positive or negative.

If you are RhD-positive your blood cells have RhD antigens. If you are RhD-negative, your blood cells do not have the RhD antigen.

• O positive blood is the most common group in Australia. 
• O negative blood does not have A or B markers and is RhD-negative. 
• O positive blood does not have A or B markers but is RhD-positive.

Why get tested?

Blood transfusion

If your medical team advises that transfusion with blood or blood components may be required as part of your treatment, a blood group antibody screen will be done.

Blood compatibility in pregnancy

If you are pregnant, and your baby has inherited a type of RhD factor from their father that is different from yours, the baby can be harmed. RhD incompatibility is not common. Only pregnant women who are RhD-negative are at risk and most people are RhD-positive.

When the blood from  a RhD-positive baby gets into the bloodstream of an RhD-negative mother her body will recognise that the RhD-positive blood is not hers. Her immune system will attack it by making anti-RhD antibodies.

RhD incompatibility is usually not a problem if this is your first pregnancy. This is because your baby's blood would not normally enter your own blood system. It is during the birth that your baby’s blood is most likely to mix with yours.

In this first pregnancy, your body does not have a chance to make a lot of antibodies and so there is unlikely to be a strong immune reaction.  However, if treatment is not given and if in future you carry another baby with different RhD factor to your own, your antibodies will recognise the RhD proteins on the baby's blood cells as foreign and attack those cells.

This is known as haemolytic or RhD disease of the newborn. This is a serious type of anaemia in which the baby’s red blood cells are destroyed faster than the body can replace them.

You will be given RhD immune-globulin treatment during your pregnancy to protect your baby from  the antibodies. This acts like a vaccine.

Having the test

Sample

Blood.

Preparation

None.

Your results

Your test results will show whether you are type A, B, AB or O and if you are Rh negative or positive.

Crossmatch

In blood transfusion, once your blood type is confirmed and antigens identified, a crossmatch test is performed. This is the final check to ensure that any blood transfused to you is compatible, safe, and avoids a transfusion reaction. It involves taking a sample of the donor blood that has been selected for your transfusion and checking there is no reaction with any antigens that have been identified in your blood.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms.   It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.