Blood group test (blood type): ABO and Rh factor
Also known as: ABO
About 48% of people in the UK have blood group O (NHS); this is a frequency in the population, not a medical reference range
- Updated
- 2026-09-10
- Every number with its source
What a blood group test is
A blood group test determines two properties together: the ABO group and the Rh (RhD) factor. The ABO system is based on proteins (antigens) on the surface of red blood cells, and four groups are distinguished by them: O, A, B and AB. The Rh factor shows whether an additional surface protein is present on the red blood cells: if it is present, the blood is considered Rh positive; if it is absent, Rh negative.
In the laboratory, the blood sample is mixed with antibodies against the A and B antigens and it is observed whether the cells clump together (agglutinate); in a second step, the serum is tested against red blood cells of known groups. The blood group and the Rh factor are inherited and do not change throughout life, so the test is usually done only once.
The result matters most in two situations: before a blood transfusion or an organ transplant, so that a compatible donor can be chosen, and during pregnancy, when an Rh negative mother is expecting an Rh positive baby. Additional care may then be needed to prevent anaemia and jaundice in the newborn.
How common the groups are (NHS)
| Group | Frequency, % | Source | |
|---|---|---|---|
| Blood group O (frequency in the population) About 48% of people in the UK have blood group O (NHS); this is a frequency in the population, not a medical reference range | about 48 | NHS Reviewed 2023-05-10 | |
| Rh (RhD) positive people About 85% of people in the UK are RhD positive, and the remaining roughly 15% are RhD negative (NHS) | about 85 | NHS Reviewed 2023-05-10 | |
| O Rh positive (O+) — the most common combination O+ is the most common combination of blood group and Rh factor in the UK — about 35% of the population (NHS) | about 35 | NHS Reviewed 2023-05-10 | |
These figures show how common each group is in the population, not a medical reference range. The exact frequencies differ depending on the population and the region.
Universal donor and universal recipient
Group O Rh negative is considered the universal donor, because it is suitable for most recipients in emergencies, when the blood group is not yet known. Group AB Rh positive is considered the universal recipient, because it can receive blood from all ABO groups.
Why the Rh factor matters in pregnancy
If an Rh negative mother is expecting an Rh positive baby, her body may produce antibodies against the fetus's red blood cells. This can cause anaemia or jaundice in the newborn, so such a case is monitored and discussed with a doctor. The Rh status is established for all pregnant women in early pregnancy.
Can the blood group change?
No. The blood group and the Rh factor are inherited and do not change throughout life, so this test is usually done only once. Even so, pregnant women are always given a blood group test, and before a blood transfusion the patient's blood is tested against a sample of donor cells.
Related tests
Related tests: full blood count, red blood cell count, haemoglobin, haematocrit and beta-hCG.
When the test is done
- In early pregnancy — for all pregnant women, in order to establish the Rh status and any possible incompatibility between mother and fetus
- Before planned or emergency surgery, when a blood transfusion may be needed
- Before an organ or bone marrow transplant
- When registering as a blood donor
- After injury or major blood loss, when an urgent transfusion is needed
- As part of a pre-operative or general health check
How to prepare
- No special preparation is needed — fasting is not required
- It is worth knowing your previously determined blood group, if that information exists — it can speed up the doctor's work
- If you have had a blood transfusion before, or a pregnancy with Rh incompatibility, it is worth telling your doctor about it
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Frequently asked questions
›What is a blood group and the Rh factor?
The blood group (the ABO system: O, A, B, AB) and the Rh factor (positive or negative) are inherited properties, determined by proteins (antigens) on the surface of red blood cells. They matter when choosing compatible blood for a transfusion and when assessing risk during pregnancy.
›How is a blood group test done?
In the laboratory, the blood sample is mixed with antibodies against the A and B antigens and it is observed whether the cells clump together. In a second step, the serum is tested against red blood cells of known ABO groups, and the Rh (RhD) status is established by a separate test.
›Can a blood group change during a lifetime?
No, the blood group and the Rh factor are inherited and do not change throughout life, so this test is usually done only once. Even so, pregnant women are always given a blood group test, and before a blood transfusion the patient's blood is tested against a sample of donor cells.
›Why does the Rh factor matter during pregnancy?
If an Rh negative mother is expecting an Rh positive baby, her body may produce antibodies against the fetus's red blood cells. This can cause anaemia or jaundice in the newborn, so such a case is monitored and discussed with a doctor.
›What is a universal donor and a universal recipient?
Group O Rh negative is considered the universal donor, because it is suitable for most recipients in emergencies, when the blood group is not yet known. Group AB Rh positive is considered the universal recipient, because it can receive blood from all ABO groups.
›How common are the blood groups and Rh types among people?
According to NHS data, about 48% of people in the UK have group O and about 85% are Rh (RhD) positive; the most common combination is O Rh positive, which about 35% of people in the UK have. The exact frequencies differ depending on the population and the region.
Sources
- NHS: Blood groups — 2026-09-06
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.