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  • Also Known As:
  • Blood Group
  • Rh Factor
  • Formal Name:
  • ABO Group and Rh Type
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At a Glance

Why Get Tested?

To determine your ABO blood group and Rh type

When To Get Tested?

When you need a transfusion of blood or blood components; when you donate blood at a collection facility or donate an organ, tissue, or bone marrow for transplantation; before or during a woman’s pregnancy to determine the risk of Rh incompatibility with the fetus

Sample Required?

A sample of blood is drawn from a vein or from the tip of the finger (fingerstick). In newborns, blood from the umbilical cord or a small amount of blood from a heelstick may be used for testing.

Test Preparation Needed?


What is being tested?

Blood types are based on the markers (specific carbohydrates or proteins) or antigens on the surface of red blood cells (RBCs). Two major antigens or surface identifiers on human RBCs are the A and B antigens. Another important surface antigen is called Rh. Blood typing detects the presence or absence of these antigens to determine a person’s ABO blood group and Rh type.

People whose red blood cells have A antigens are in blood group A, those with B antigens are group B, those with both A and B antigens are in group AB, and those who do not have either of these markers are in blood group O.

If the Rh protein is present on the red blood cells, a person’s blood type is Rh+ (positive); if it is absent, the person’s blood is type Rh- (negative).

Our bodies naturally produce antibodies against the A and B antigens that we do not have on our red blood cells. For example, a person who is blood type A will have anti-B antibodies directed against the B antigens on red blood cells and someone who is type B will have anti-A antibodies directed against the A antigens. People with type AB blood have neither of these antibodies, while those with type O blood have both.

The following table indicates the type of antibodies a person is expected to have based on their blood type.

a person with blood type … will have antibodies to …
A B antigen
B A antigen
AB Neither antigen
O A and B antigens

These antibodies are useful for determining a person’s blood type and help determine the types of blood that he or she can safely receive (compatibility). If a person who is group A with antibodies directed against the B antigen, for example, were to be transfused with blood that is type B, his or her own antibodies would target and destroy the transfused red blood cells, causing severe, potentially fatal complications. Thus, it is critical to match a person’s blood type with the blood that is to be transfused.

Unlike antibodies to A and B antigens, antibodies to Rh are not produced naturally. That is, Rh antibodies develop only after a person who does not have Rh factor on his or her red blood cells (Rh negative) is exposed to Rh positive red blood cells. This can happen during pregnancy or birth when an Rh-negative woman is pregnant with an Rh-positive baby, or sometimes when an Rh-negative person is transfused with Rh-positive blood. In either case, the first exposure to the Rh antigen may not result in a strong response against the Rh positive cells, but subsequent exposures may cause severe reactions.

Common Questions

How is it used?

Blood typing is used to determine an individual’s blood group, to establish whether a person is blood group A, B, AB, or O and whether he or she is Rh positive or Rh negative.

Blood typing may be used to:

  • Ensure compatibility between the blood type of a person who requires a transfusion of blood or blood components and the ABO and Rh type of the unit of blood that will be transfused. Blood typing is performed in conjunction with other tests such as an RBC antibody screen and a crossmatch to determine what type of blood or blood components the person can safely receive. A potentially fatal transfusion reaction can occur if a unit of blood containing an ABO antigen to which the blood recipient has an antibody is transfused to the recipient. For example, people with blood group O have both anti-A and anti-B antibodies in their blood. If a unit of blood that is group A, B, or AB is transfused to this person, the antibodies in the recipient’s blood will react with the red blood cells, destroying them and causing potentially serious complications.Similarly, if an Rh-negative individual is transfused with Rh-positive blood, it is likely that the person will produce antibodies against Rh-positive blood. Although this situation does not cause problems for the recipient during the current transfusion, a future transfusion with Rh-positive blood could result in a serious transfusion reaction.
  • Determine compatibility between a pregnant woman and her developing baby (fetus). Rh typing is especially important during pregnancy because a mother and her fetus could be incompatible. If the mother is Rh negative but the father is Rh positive, the fetus may be positive for the Rh antigen. As a result, the mother’s body could develop antibodies against the Rh antigen. The antibodies may cross the placenta and cause destruction of the baby’s red blood cells, resulting in a condition known as hemolytic disease of the fetus and newborn (HDFN). To prevent development of Rh antibodies, an Rh-negative mother is treated with an injection of Rh immune globulin during her pregnancy and again after delivery if the baby is Rh-positive. The Rh immune globulin binds to and “masks” the fetus’s Rh antigen during pregnancy and delivery and prevents the mother from developing antibodies against the Rh antigen.
  • Determine the blood group of potential blood donors at a collection facility. Units of blood collected from donors are blood typed and then appropriately labeled so they can be used for people that require a specific ABO group and Rh type.
  • Determine the blood group of potential donors and recipients of organs, tissues, or bone marrow, as part of a workup for a transplant procedure. Along with HLA testing, ABO blood typing is used to identify and match organ and tissue transplant donors with recipients who have the same or an acceptable number of matching HLA genes and antigens.

When is it ordered?

ABO grouping and Rh typing are performed on all donated blood. They are also performed when people require blood transfusion. Conditions or situations that may warrant a transfusion include:

  • Severe anemia and conditions causing anemia such as sickle cell disease and thalassemia
  • Bleeding during or after surgery
  • Injury or trauma
  • Excessive blood loss
  • Cancer and the effects of chemotherapy
  • Bleeding disorders such as hemophilia

Testing is ordered when a woman becomes pregnant to determine whether she is Rh negative or positive. All newborn babies of Rh-negative mothers are typed for ABO and Rh soon after birth to determine if the mother needs to receive Rh immune globulin.

Blood typing may be ordered when a person becomes a candidate for an organ, tissue, or bone marrow transplant, or when a person wishes to become a donor. It is one of the first of many tests used when determining whether a potential donor and recipient are compatible.

Sometimes blood typing may be done as part of the process for determining whether someone could be a blood relative.

What does the test result mean?

The results of blood typing will determine if a person is type A, B, AB, or O and if he or she is Rh negative or positive. The results will tell the healthcare provider what blood or blood components will be safe for the person to receive.

The following table shows what types of blood patients can safely receive, based on their individual blood type.

Blood Group and Rh Type of Patient Safe (Compatible) Blood Types for RBC Transfusion*
A positive A positive, A negative, O positive, O negative
A negative A negative, O negative
B positive B positive, B negative, O positive, O negative
B negative B negative, O negative
AB positive AB positive, AB negative, A positive, A negative, B positive, B negative, O positive, O negative
AB negative AB negative, A negative, B negative, O negative
O positive O positive, O negative
O negative O negative

*These apply for RBC transfusions only; when transfusing plasma products and platelets, the compatible choices are different. (See the article on Transfusion Medicine for more on this.)

Blood typing results will show whether a pregnant woman is Rh positive or negative. This information will indicate whether she may be a candidate for receiving Rh immune globulin, which would prevent her from developing antibodies against her fetus’ blood cells.

Typing of donated blood is important because this information allows healthcare practitioners to determine which patients are compatible and can safely receive that blood.

Similarly, when a donor organ, tissue, or bone marrow is compatible with the intended recipient, it is less likely to be rejected in the immediate post-transplant period.

Is there anything else I should know?

The following table summarizes the approximate distribution of blood groups and types in the U.S. population:

Blood Type Rh POSTIVE Rh NEGATIVE total
O 39% 9% 48%
A 30% 6% 36%
B 9% 2% 11%
AB 4% 1% 5%
TOTAL 82% 18% 100%

Although Rh incompatibility has more severe consequences, one of the most common causes of hemolytic disease of the fetus and newborn (HDFN) is actually an incompatibility between the mother’s and baby’s ABO blood groups, not the Rh factor. However, ABO grouping cannot be used to predict whether HDFN will occur because antibodies to the ABO blood groups are naturally occurring.

Besides A and B, many other antigens exist. Having a rare blood type is especially problematic if you need repeated transfusions, as sickle cell anemia and thalassemia patients do. If blood transfusions are not closely matched to blood types of patients, they may suffer transfusion reactions. Such reactions are less likely if donors and recipients are from the same racial or ethnic groups. Molecular testing techniques that analyze a person’s genes may be used to predict the presence of unusual or uncommon red cell antigens.

Do I need to know my blood type?

If you need a transfusion, healthcare practitioners at your health care facility will determine your blood type before they give you any blood. The only time you would not get a blood type test is in an extreme emergency and there is not enough time to type your blood. In this case, you would receive group O since this blood type does not have any A or B antigens that can potentially cause a hemolytic transfusion reaction. The use of Rh-negative blood depends upon the situation and the supply of O negative blood in the area.

Who are universal blood donors and recipients?

Universal blood donors have type O, Rh negative blood. This means they have no A or B antigens or Rh factor on their red blood cells to which the recipient’s antibodies can react. Recipients of their red blood cells have little risk of a hemolytic transfusion reaction due to ABO or Rh incompatibility.

Universal recipients have type AB, Rh positive blood. They recognize A, B and Rh antigens as “self” and can receive red blood cells of any ABO or Rh type with no risk of a serious hemolytic transfusion reaction due to ABO or Rh incompatibility.

Besides ABO and Rh, are there other types of red blood cell antigens?

Yes, numerous other antigens can be present on the surface of red blood cells. These other RBC blood group antigens include, for example, Kell, Kidd, Duffy, and other Rh antigens. The body does not produce antibodies to these antigens unless it is exposed to these antigens through blood transfusion or during pregnancy. These antibodies are not detected during routine blood typing but may be found with an RBC antibody screen.

View Sources

Sources Used in Current Review

© 2018 AABB. Blood FAQ. Available online at http://www.aabb.org/tm/Pages/bloodfaq.aspx. Accessed October 2018.

© 2018 The American National Red Cross. Understanding Your Blood Type. Available online at https://www.redcrossblood.org/donate-blood/how-to-donate/types-of-blood-donations/blood-types.html. Accessed October 2018.

© 2018 The American National Red Cross. Molecular Testing. Available online at https://www.redcrossblood.org/biomedical-services/blood-diagnostic-testing/molecular-testing.html. Accessed October 2018.

(March 13, 2018) MedlinePlus. Rh Incompatibility. Available online at https://medlineplus.gov/rhincompatibility.html. Accessed October 2018.

Sources Used in Previous Reviews

Pagana, Kathleen D. & Pagana, Timothy J. (© 2006). Mosby’s Manual of Diagnostic and Laboratory Test 3rd Edition: Mosby, Elsevier, Saint Louis, MO., Pp. 141-145.

Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Ch. 34.

MedlinePlus Medical Encyclopedia. Blood Typing (Online information, accessed October 2007). Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003345.htm.

(September 2007) National Heart, Lung and Blood Institute. Diseases and Conditions Index: Blood Transfusion (Online information). Available online at http://www.nhlbi.nih.gov/health/dci/Diseases/bt/bt_whatis.html. Accessed November 2007.

(June 2005) Nemours Foundation. Kids Health: What You Need to Know in an Emergency (Online information). Available online at http://kidshealth.org/parent/firstaid_safe/home/healthin.html. Accessed October 2007.

(July 2007) AABB. About Blood and Cellular Therapies: Blood FAQ (Online information). Available online at http://www.aabb.org/Content/About_Blood/FAQ/bloodfaq.htm. Accessed October 2007.

Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Ch. 34.

(February 5, 2010) MedlinePlus Medical Encyclopedia: Blood Typing (Online information). Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003345.htm. Accessed July 2011.

(July September 20097) National Heart, Lung and Blood Institute. Diseases and Conditions Index. Blood Transfusion (Online information). Available online at http://www.nhlbi.nih.gov/health/dci/Diseases/bt/bt_whatis.html. Accessed July 2011.

(©2011July 2007) AABB. About Blood and Cellular Therapies: Blood FAQ. Available online at http://www.aabb.org/Content/About_Blood/FAQ/bloodfaq.htm. Accessed July 2011.

(©2011) National American Red Cross. Learn About Blood. Available online at http://www.redcrossblood.org/learn-about-blood. Accessed July 2011.

(November 20, 2009) Sandler G. Transfusion Reactions. Medscape Reference. Available online at http://emedicine.medscape.com/article/206885-overview. Accessed July 2011.

(November 4, 2009) Salem L. Rh Incompatibility. Medscape Reference. Available online at http://emedicine.medscape.com/article/797150-overview. Accessed July 2011.

Blood Types. American Red Cross. Available online at http://www.redcrossblood.org/learn-about-blood/blood-types. Copyright 2014. Accessed October 24, 2014.

Testing and Procedures: Blood Transfusion. Mayo Clinic. Available online at http://www.mayoclinic.org/tests-procedures/blood-transfusion/basics/risks/prc-20021256. Last updated April 26, 2012. Accessed October 24, 2014.

Rare Donor Program. Lifeshare Blood Centers. Available online at http://www.lifeshare.org/services/rare-donor-program. Copyright 2014. Accessed October 27, 2014.

Blood Types: Distribution Stats, Interesting Facts. Blood Banker. Available online at http://bloodbanker.com/plasma/plasma-donation/blood-donation-terms/. Accessed October 28, 2014.

(Updated Oct 29, 2014) Gonsorcik V. ABO Grouping. Medscape Reference. Available online at http://emedicine.medscape.com/article/1731198-overview. Accessed October 2014.

(Updated Nov 6, 2013) Gonsorcik V. Rh Typing. Medscape Reference. Available online at http://emedicine.medscape.com/article/1731214-overview. Accessed October 2014.


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