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Blood Test Tracker

ABO Blood Group

Your ABO blood group (A, B, AB or O), set by markers on your red blood cells. It stays the same for life and matters for blood transfusions and pregnancy.

AI-verified against cited sources. Awaiting expert reviewAlso called ABOLOINC 883-9

What it is

Your red blood cells can carry two kinds of markers on their surface, called A and B. If you have only A, your group is A. If you have only B, it is B. If you have both, it is AB, and if you have neither, it is O. The lab finds your group by mixing your red cells with test reagents, and then by checking your plasma against known A and B red cells, so each result double-checks the other. Many reports print your Rh type (positive or negative) right next to it, such as "A Pos", but Rh is a separate test.

Why it matters

Your ABO group, together with your Rh type, decides which donor blood is safe for you in a transfusion. People with type O blood (especially O negative) are called universal red cell donors, and people with type AB can receive red cells from any group. It is also part of routine blood work in pregnancy. Your group can also nudge other lab results. In one large study of blood donors, people with type O had lower von Willebrand factor levels (average about 75 U/dL) than people with type A (about 106), B (about 117) or AB (about 123), so clinicians may keep blood group in mind when they read von Willebrand factor and factor VIII results.

What can affect your result

  • Your genes set your blood group at conception, so age, food, fasting and time of day do not change it
  • A stem cell or bone marrow transplant from a donor with a different group can change your group over time
  • Recent transfusions of blood from a different group can make results harder to read
  • Rare variants, such as the Bombay phenotype (no H marker), need special matching, because those people can react to common donor blood
  • Newborns are tested a different way than adults
  • A stem cell or bone marrow transplant from a donor with a different blood group can change your group over the following months.
  • A recent transfusion of blood from a different group can make the result harder to read.
  • If the red cell (forward) result and the plasma (reverse) result disagree, the lab does extra testing to sort it out before it reports a final group.
  • Newborns are tested with a different test than adults (Mayo Clinic Laboratories lists a separate newborn ABO/Rh test).

Your blood group does not change over time, apart from after a stem cell transplant from a donor with a different group.

Questions you could ask

  • Is my blood type written in my medical record?
  • Does my blood type matter for any other test I am having, such as clotting tests?
  • If I am pregnant or planning a pregnancy, does my blood type and Rh type change any follow-up?

Questions people ask

Can my blood type change?

Almost never. Your blood group is set by your genes and stays the same for life. The main exception is a stem cell or bone marrow transplant from a donor with a different group, which can change it over several months.

How common is each blood group?

In the US, roughly 44 in 100 people are type O, 42 are type A, 10 are type B and 4 are type AB, according to figures from Stanford Blood Center. The mix is different in other countries and between ancestry groups.

Why does my lab report say "Pos" or "Neg" next to my blood type?

That is your Rh type, which is tested at the same time as your ABO group. Blood Test Tracker shows Rh as its own result.

Show the science and sources

For clinicians and researchers

ABO grouping combines forward typing (A and B antigens on the red cells, tested with anti-A and anti-B reagents) and reverse typing (anti-A and anti-B in the plasma, tested against known A and B red cells). When the two do not agree, it is an ABO discrepancy, which is worked up with further testing, including molecular testing in some cases, before a group is reported (Meny 2017). Mayo Clinic Laboratories reports ABO and Rh by hemagglutination under LOINC 882-1, with the reference result "ABO and Rh blood group antigens identified", and lists a separate test for ABO/Rh problems and one for newborns. The group is stable for life except after allogeneic stem cell transplantation: in 263 ABO-incompatible transplants, conversion to the donor's type took a median of about 91 to 108 days, depending on the direction of the mismatch (Li 2023). ABO also affects hemostasis. In 1,117 blood donors, mean vWF:Ag was 74.8 U/dL in group O, 105.9 in A, 116.9 in B and 123.3 in AB (Gill 1987), and group O is also linked to lower factor VIII. A meta-analysis of 38 studies with 10,305 venous thromboembolism cases found a pooled odds ratio of 2.09 for non-O versus O groups (Dentali 2012). Stanford Blood Center lists US frequencies of about 44% O, 42% A, 10% B and 4% AB (Rh positive and negative combined); the mix differs by ancestry. People who lack the H antigen (Bombay phenotype, Oh) have natural anti-H that can cause severe hemolytic transfusion reactions (Scharberg 2016).

LOINC 883-9 (primary), 882-1, 57743-7. Specimen: whole-blood.

Sources

  1. Blood typing (Medical Encyclopedia), MedlinePlus, National Library of Medicine
  2. Blood Types, Stanford Blood Center
  3. ABO/Rh, Blood (test ABOMR), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  4. ABO/Rh Newborn, Blood (test ABONR), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  5. The effect of ABO blood group on the diagnosis of von Willebrand disease, Blood (1987)
  6. Non-O blood type is the commonest genetic risk factor for VTE: results from a meta-analysis of the literature, Seminars in Thrombosis and Hemostasis (2012)
  7. Recognizing and resolving ABO discrepancies, Immunohematology (2017)
  8. The H blood group system, Immunohematology (2016)
  9. A study of blood group conversion in patients with ABO incompatible hematopoietic stem cell transplantation: a decade survey, Transfusion and Apheresis Science (2023)
  10. LOINC 883-9 and 882-1 with answer lists (NLM Clinical Tables API), National Library of Medicine / Regenstrief Institute

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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