Red blood cell (RBC) antibody screen
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
An RBC antibody screen is a blood test that looks for antibodies that could attack red blood cells. It is done routinely before a blood transfusion and during pregnancy to keep you and your baby safe.
Who needs this test
You will usually have this test in the following situations.
Before a blood transfusion: your blood is checked for antibodies before you receive donor blood, or if you might need donor blood. This helps the team find blood that is compatible with yours.
During pregnancy: all pregnant women have this test as part of routine antenatal care. Your midwife or doctor uses it to check for antibodies that could harm your baby’s red blood cells.
The blood group and antibody screen are tested at booking and at 28 weeks’ gestation to identify the ABO and D group and to detect red cell antibodies that have the potential to cause harm.
In pregnant women with no potentially harmful antibodies present at 28 weeks’ gestation, no further routine antenatal blood grouping or antibody screening is necessary
After a blood transfusion: If you have had a transfusion reaction in the past, this test may also be used to investigate what happened.
Preparing for your test
You do not need to do anything special before this test. A healthcare professional will take a small blood sample from a vein in your arm. This usually takes just a few minutes.
On average, results take about 7 working days, though some specialist tests may take longer.
Understanding your results
What the test measures
Your red blood cells (RBCs) have proteins on their surface called antigens. These antigens make up your blood group. There are nearly 400 known red blood cell antigens organised into 48 blood group systems. The most important systems are ABO and Rh (you may hear this referred to as ‘Rhesus’, although technically Rh is the appropriate term on its own).
An antibody is a protein made by your immune system. Your immune system makes antibodies when it encounters something it does not recognise, such as a foreign red blood cell antigen. If these antibodies enter your bloodstream again — through a transfusion or in pregnancy — they can attack and destroy red blood cells that carry the matching antigen.
This test checks for these antibodies in your blood. It does not look for the naturally occurring ABO antibodies, as these are always present and are already accounted for when your blood is grouped.
Your body develops RBC antibodies when it is exposed to red blood cells that are different from your own. This can happen through a blood transfusion or during pregnancy, when a small amount of your baby’s blood may enter your bloodstream.
The first exposure usually does not cause problems straight away. But the second time your body encounters the same foreign antigen, it responds much more strongly. This can cause haemolysis — the breakdown of red blood cells.
What your results mean
If your result is negative
A negative result means no unexpected red blood cell antibodies were found. This is the expected result for most people.
If you are pregnant and Rh negative (RhD–) with a negative result, your midwife or doctor will offer you an injection of anti‑D immunoglobulin. This injection prevents your immune system from developing antibodies to RhD-positive blood. It is given:
- usually as a single dose regimen given around 28 weeks, or two dose regimen given at around 28 and 34 weeks
- Following any ‘sensitising events’ when there is a risk of the mother’s immune system being exposed to fetal blood, such as following a miscarriage.
- after delivery if your baby is RhD positive.
If your result is positive
A positive result means one or more RBC antibodies were found in your blood. This means your immune system has previously been exposed to a red blood cell antigen that your own red blood cells do not carry.
A positive result does not always mean there is an immediate health problem. But it does mean further investigation is needed. Your doctor will arrange an antibody identification test to find out exactly which antibodies are present.
Common antibodies found include those from the Kell, Duffy, Kidd, MNS and Rh blood group systems. Some antibodies are more likely to cause problems than others, and your hospital laboratory will assess how significant they are.
If you need a blood transfusion and your result is positive
Compatible donor blood must be found — blood that does not carry the antigen your antibody reacts to. This may take longer to source than for a routine transfusion. The blood bank team will work to find the right blood for you.
Once you have an RBC antibody, it can remain in your system at low levels for years. Even if it becomes undetectable in future tests, it will always be recorded on your blood bank record. This ensures you are always given safe, compatible blood for any future transfusion.
If you are pregnant and your result is positive
If antibodies are found during pregnancy, your doctor will identify which antibodies are present. Some antibodies can cross the placenta and attack your baby’s red blood cells. This is called Haemolytic Disease of the Fetus and Newborn (HDFN).
The most serious cause of HDFN is anti-RhD antibody. If you have already developed anti-RhD antibodies, the anti‑D injection will not be effective in preventing HDFN. Your pregnancy will be monitored more closely. Your doctor will work with you to manage your pregnancy safely.
Other antibodies found in pregnancy may be less serious. Your doctor will explain what this means for you and your baby specifically.
Questions to ask your doctor
What happens next
If your result is negative and you are having a transfusion, the blood bank will confirm you have compatible donor blood ready for you.
If your result is positive, the next step is an RBC antibody identification test. This test identifies exactly which antibody is present. Once identified, the blood bank team or your maternity team will decide on the best plan for you.
If you have a positive result and are pregnant, your care team will monitor your pregnancy more closely. Depending on which antibody is found, your baby may need extra monitoring before and after birth.
If you have a positive result, you should be given written information about the antibody that has been found. You should provide this information if you ever need a blood transfusion in the future, particularly if you are at a hospital you have not been to before, as they will not necessarily be aware of this result.
What can affect your results
Several factors can affect the test or what it finds:
- Previous blood transfusions: each transfusion exposes you to a different combination of red blood cell antigens. The more transfusions you have had, the more likely you are to have developed antibodies.
- Pregnancy: carrying a baby can expose you to red blood cell antigens that are different from your own, particularly if your baby has inherited blood group antigens from their father, which are different to your own
- Timing: an antibody may be present at very low levels and sometimes fall below the detection limit of the test. It will still be treated as present if it has ever been detected before.
- Autoimmune conditions and some medicines: rarely, the immune system may produce antibodies against a person’s own red blood cells. This can happen with conditions such as lupus, lymphoma, or chronic lymphocytic leukaemia, or with certain medicines such as penicillin.
Other tests you might need
If your antibody screen is positive, you will usually need an RBC antibody identification test. This pinpoints exactly which antibody is present.
Your doctor may also request a direct antiglobulin test (DAT). This checks whether antibodies are already attached to your red blood cells. It is used if a transfusion reaction is suspected or to investigate certain types of anaemia.
Blood typing is always done at the same time as an antibody screen so that compatible blood can be selected for transfusion.
About haemolytic disease of the fetus and newborn (HDFN)
HDFN occurs when a mother’s antibodies cross the placenta and attack her baby’s red blood cells. It can range from mild to severe. The most common cause is Rh incompatibility. It is now much rarer thanks to anti‑D immunoglobulin injections given in pregnancy. For more information, visit
For more information, visit Rh disease (NHS).