Lab result explainedBlood count (CBC)Last updated November 11, 2026

Blood type test: how ABO and Rh typing works, what Rh negative means, compatibility and how to find out your blood type

Blood type test (ABO/Rh) · Type and screen (ABO/Rh typing with antibody screen) · reported in Reported as a blood group letter (A, B, AB or O) plus Rh factor (positive or negative), with an antibody screen reported as negative or positive; no numeric units

A blood type test identifies the inherited markers (antigens) on the surface of your red blood cells. The two systems that matter most are ABO, which gives you group A, B, AB or O, and Rh, which makes you Rh positive or Rh negative depending on whether you carry the D antigen. Combined, these produce the eight familiar types, from O positive (the most common in the United States, about 37 percent of people) to AB negative (the rarest, under 1 percent). About 15 percent of Americans are Rh negative, with the proportion highest in people of European ancestry and lowest in those of East Asian ancestry.

The test matters because your plasma contains antibodies against the ABO antigens you lack: a group A person has anti-B, a group O person has both anti-A and anti-B. Transfusing incompatible red cells triggers immediate, potentially fatal destruction of the donor cells, which is why every unit of blood is matched to the recipient and why hospitals never rely on a type you report from memory, a card or a home kit. In pregnancy, an Rh-negative mother carrying an Rh-positive baby can form anti-D antibodies that attack a later baby's red cells; a blood type and antibody screen at the first prenatal visit is how this is anticipated and prevented with Rh immune globulin.

Most people find out their blood type when they donate blood, during pregnancy, before surgery, or from a prior hospital record; it is not part of a routine physical and is not on a standard CBC. Home finger-prick kits can give an accurate ABO result in careful hands but make more errors with the Rh factor, and no clinician will act on one. This page explains how typing and the antibody screen work, what each result means, compatibility for donation and transfusion, what Rh negative means in pregnancy, and what blood type does and does not say about your health.

ABO/Rh at a glance

What it is: a test that identifies your ABO group (A, B, AB or O) and Rh factor (positive or negative) from antigens on your red cells and the matching antibodies in your plasma; usually paired with an antibody screen before transfusion or in pregnancy.
US frequencies: O+ about 37%, A+ 36%, B+ 8.5%, AB+ 3.4%, O− 6.6%, A− 6.3%, B− 1.5%, AB− 0.6%. Around 15% of people are Rh negative.
Compatibility: O negative red cells can go to anyone (universal red cell donor); AB positive can receive red cells from anyone (universal recipient); AB plasma can go to anyone. Within ABO you can receive your own group or O.
Rh negative in pregnancy: an antibody screen is done at the first visit and at 28 weeks, and Rh immune globulin is given at 28 weeks and within 72 hours of delivering an Rh-positive baby (and after miscarriage, bleeding or invasive procedures) to prevent sensitization.
How to find out your blood type: donate blood (free, result by mail or app), ask for prenatal or surgical records, or request a type and screen from a clinician or lab; home kits are informational only.
Blood type does not change, except after a bone marrow (stem cell) transplant, and no diet, supplement or lifestyle factor alters it. The 'blood type diet' has no supporting evidence.

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What ABO/Rh measures

ABO typing has two halves that must agree. In forward typing, your red cells are mixed with reagent anti-A and anti-B antibodies; clumping (agglutination) with anti-A means you have the A antigen, with anti-B the B antigen, with both AB, and with neither O. In reverse typing, your plasma is mixed with known A and B red cells to detect the naturally occurring antibodies you should have: anti-B if you are group A, anti-A if group B, both if group O, neither if AB. Rh typing mixes your cells with anti-D; clumping means Rh positive. When a result is weak or absent, labs perform a 'weak D' test or genotyping, and a person who types as weak D may be treated as Rh negative for transfusion and pregnancy purposes to be safe.

A 'type and screen' adds an antibody screen (indirect antiglobulin test), in which your plasma is tested against a panel of reagent red cells carrying the clinically important antigens of the Rh (D, C, c, E, e), Kell, Duffy, Kidd, MNS and other systems. A positive screen means you have an 'unexpected' antibody, usually from a previous transfusion or pregnancy, and the lab then identifies which one so that antigen-negative blood can be found. About 1 to 2 percent of hospital patients and up to 1 percent of pregnant women have a positive screen. A crossmatch, in which your plasma is tested directly against a donor unit, is the final check before transfusion.

Modern labs run these tests on automated gel or solid-phase platforms with results in 30 to 60 minutes, and for safety most hospitals require two separately drawn samples, or a historical record, before issuing anything other than group O red cells. Results are reported as the group and Rh factor (for example 'A positive' or 'O Rh(D) negative'), with the antibody screen as negative or positive plus the antibody identified. Molecular genotyping of RHD and other blood group genes is used for weak D results, for patients who have been recently transfused, and increasingly to predict the fetal Rh type from cell-free DNA in the mother's blood.

Normal range for ABO/Rh

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (Reported as a blood group letter (A, B, AB or O) plus Rh factor (positive or negative), with an antibody screen reported as negative or positive; no numeric units)	Notes
O positive	About 37% of the US population	The most common type. Can receive O+ or O− red cells; can donate red cells to any Rh-positive recipient (A+, B+, AB+, O+).
A positive	About 36%	Can receive A+, A−, O+ or O− red cells; can donate to A+ and AB+.
B positive	About 8.5%	Can receive B+, B−, O+ or O−; can donate to B+ and AB+. More common in people of Asian and African ancestry.
AB positive	About 3.4%	Universal red cell recipient (can receive any ABO/Rh type). Can donate red cells only to AB+. AB plasma is the universal plasma donor.
O negative	About 6.6%	Universal red cell donor, used in emergencies before the patient's type is known. Can receive only O− red cells.
A negative	About 6.3%	Can receive A− or O−; can donate to A+, A−, AB+ and AB−.
B negative	About 1.5%	Can receive B− or O−; can donate to B+, B−, AB+ and AB−.
AB negative	About 0.6%	The rarest of the eight. Can receive any Rh-negative red cells (A−, B−, AB−, O−); can donate red cells to AB+ and AB−.
Rh negative by ancestry	About 15–17% European, 7–8% African American, 5–7% Hispanic, 1–2% East Asian ancestry	Rh negative is an inherited absence of the D antigen, not a disease. It matters for transfusion and pregnancy. Labs and blood services report frequencies slightly differently.

Rh negative, or a positive antibody screen: the results that need action

A blood type has no 'high' or 'low'; every type is normal. The results that change what happens next are being Rh negative and having a positive antibody screen. Being Rh negative means your red cells lack the D antigen. It has no effect on your day-to-day health, but if Rh-positive red cells enter your bloodstream, through transfusion or, far more commonly, through small leaks of a baby's blood across the placenta during pregnancy and delivery, your immune system can make anti-D antibodies. Once formed, these persist for life and cross the placenta in later pregnancies, where they destroy the red cells of an Rh-positive fetus (hemolytic disease of the fetus and newborn), causing anemia, jaundice, heart failure and, if untreated, death.

This is almost entirely preventable. About 15 percent of pregnant women in the US are Rh negative, and roughly 60 percent of their babies will be Rh positive if the father is Rh positive. Guidelines call for an ABO/Rh type and antibody screen at the first prenatal visit, a repeat screen at 28 weeks, a dose of Rh immune globulin (anti-D, 300 micrograms) at 28 weeks, and another within 72 hours of delivering an Rh-positive baby, as well as after miscarriage, ectopic pregnancy, abortion, amniocentesis, chorionic villus sampling, external cephalic version, significant bleeding or abdominal trauma. Since routine prophylaxis began, sensitization has fallen from about 13 percent of at-risk pregnancies to under 0.2 percent. If you are Rh negative and your partner is confirmed Rh negative, the baby cannot be Rh positive and prophylaxis is unnecessary; cell-free fetal DNA testing of the mother's blood can also determine the baby's Rh type from about 10 to 12 weeks.

A positive antibody screen means your plasma contains an antibody to a red cell antigen you lack, acquired through a previous transfusion, pregnancy or transplant. The lab identifies it (anti-D, anti-E, anti-c, anti-K and anti-Fya are among the most common and clinically significant) and its strength is measured as a titer. For transfusion, it means units negative for that antigen must be found, which can take hours to days for rare combinations. In pregnancy, a significant antibody with a rising titer (a critical titer is usually 1:16 or above for most antibodies, and any titer for anti-K) leads to referral to maternal-fetal medicine for fetal monitoring by Doppler ultrasound. Some antibodies found on screening, such as anti-Lewis, anti-M at low titer and cold antibodies, are not clinically significant and need no action; the lab report will say which.

Common causes

Inheritance (Rh negative)
Rh negative results from inheriting two copies of a non-functional or deleted RHD gene, one from each parent; two Rh-negative parents always have Rh-negative children, while two Rh-positive parents can have an Rh-negative child if both carry the recessive variant. It is simply a normal inherited trait.
Weak D and partial D variants
About 0.2 to 1 percent of people express the D antigen weakly or in an altered form and may type as positive on one method and negative on another. Guidelines recommend RHD genotyping; some weak D types can safely be treated as Rh positive, while partial D individuals are managed as Rh negative because they can still form anti-D.
Previous pregnancy (positive antibody screen)
Fetal red cells cross into the mother's circulation in most pregnancies, especially at delivery; without prophylaxis, an Rh-negative woman carrying an Rh-positive baby has a roughly 13 to 16 percent chance of developing anti-D. Antibodies to other antigens (E, c, K) can also form this way.
Previous transfusion
Each transfusion of red cells carries a 1 to 2 percent chance of forming an antibody to a donor antigen you lack; people with sickle cell disease or thalassemia who are transfused regularly form antibodies much more often, which is why extended matching is used for them.
Rh immune globulin given recently
A dose of anti-D in pregnancy produces a weakly positive antibody screen for up to 12 weeks; this is passive antibody from the injection, not sensitization, and the lab distinguishes it by the low titer and the timing.
ABO incompatibility between mother and baby
A group O mother carrying a group A or B baby can pass naturally occurring anti-A or anti-B across the placenta, causing mild jaundice in about 1 in 10 such babies in the first days of life; it is far milder than Rh disease and does not require prophylaxis.
Autoantibodies and cold agglutinins
In autoimmune hemolytic anemia the antibody screen is positive against all cells and the direct antiglobulin (Coombs) test is positive; cold agglutinins, often after mycoplasma or EBV infection, can cause a positive screen and ABO typing discrepancies at room temperature.
Bone marrow or stem cell transplant
After a transplant from a donor of a different ABO or Rh type, the recipient's red cells gradually switch to the donor's type over months, while plasma antibodies may lag; typing during this transition shows mixed results and transfusion support follows a specific protocol.
Medications and treatments that interfere with typing
Daratumumab (a myeloma drug that binds CD38 on reagent red cells) makes antibody screens falsely positive for months; high-dose intravenous immunoglobulin, rouleaux from myeloma proteins and recent transfusion of a different ABO type all cause discrepancies that the lab must resolve.

Symptoms that can go with it

None; being Rh negative or having a red cell antibody causes no symptoms in you
In an affected fetus or newborn: jaundice in the first 24 hours of life, anemia, pale skin, lethargy, poor feeding or, in severe cases, swelling (hydrops) seen on ultrasound
During an incompatible transfusion: fever, chills, back or flank pain, dark urine, a feeling of dread, breathlessness or a sudden drop in blood pressure within minutes, which is an emergency
Days to two weeks after a transfusion: unexplained fever, jaundice, dark urine or a falling hemoglobin, which can indicate a delayed hemolytic reaction from a previously undetected antibody
In a pregnancy affected by a significant antibody: reduced fetal movements or a fast fetal heart rate on monitoring, which prompt urgent review

Read it alongside

Antibody screen (indirect antiglobulin test) — the companion test that finds unexpected red cell antibodies before transfusion and in pregnancy; a type without a screen is incomplete for either purpose.
Direct antiglobulin (Coombs) test — detects antibody already attached to red cells; positive in hemolytic disease of the newborn, autoimmune hemolytic anemia and transfusion reactions.
Antibody titer — the strength of a clinically significant antibody in pregnancy; a critical titer (commonly 1:16, or any titer for anti-K) triggers fetal surveillance.
Hemoglobin and hematocrit — the reason most transfusions are ordered; a type and screen is done when transfusion is possible, for example before surgery or when hemoglobin is under 7 to 8 g/dL.
Bilirubin and reticulocyte count — in a newborn of an Rh-negative or group O mother, a rising bilirubin and high reticulocytes indicate active hemolysis.
Partner's blood type or cell-free fetal DNA — if the father is confirmed Rh negative, or fetal DNA shows an Rh-negative baby, Rh immune globulin is not needed.
Kell antigen status — Kell-negative women of childbearing potential are given Kell-negative blood, because anti-K causes severe fetal anemia at low titers.
Crossmatch — the final compatibility check of a specific donor unit against your plasma before it is issued.

What usually happens next

If you have just learned your type for information, nothing further is needed; keep a record, and if it came from a home kit, do not rely on it for any medical decision because hospitals will retype you.
If you are pregnant and Rh negative, expect an antibody screen now and at 28 weeks, Rh immune globulin at 28 weeks and within 72 hours of delivery if the baby is Rh positive, and a dose after any bleeding, miscarriage or invasive procedure; ask whether your partner's type or a cell-free fetal DNA test could show prophylaxis is unnecessary.
If your antibody screen is positive, ask which antibody it is, whether it is clinically significant, what the titer is, and whether you should carry an antibody card; in pregnancy a significant antibody means referral to maternal-fetal medicine for titers every 2 to 4 weeks and Doppler monitoring.
Before surgery, a type and screen is usually drawn within 72 hours of the procedure; if you have been transfused or pregnant in the past 3 months the sample must be fresh, because new antibodies can appear in that window.
If you want to know your type and have no medical reason for a test, donating blood is the simplest route: blood services type every donation and report it to you, usually within a few weeks.
If you have been told you are weak D or that two results disagreed, ask for RHD genotyping; it settles whether you should be treated as Rh positive or negative for transfusion and pregnancy.

Diet, lifestyle and natural levers with evidence

What can move ABO/Rh once the cause is known. Nothing here replaces finding the cause.

Nothing changes your blood type, and the 'blood type diet' does not work

Strong evidence

Your ABO and Rh type is fixed by your genes and does not change with diet, supplements, illness or age; the only thing that changes it is a bone marrow or stem cell transplant, after which you gradually acquire the donor's type. The popular 'eat right for your type' diet claims that each group should follow a different eating pattern. In a study of 1,455 adults, following any of the recommended diets was associated with modest improvements in cardiometabolic markers, but the improvements were the same regardless of the person's actual blood type, meaning the type-specific premise is unsupported, and a systematic review found no trial evidence for it.

Things that make typing harder: tell the lab about transfusions, transplants and certain drugs

Strong evidence

A transfusion within the past 3 months can leave a mixture of cell types that confuses forward typing, and a recent stem cell transplant can mean your cells and plasma give conflicting results for months. Daratumumab causes a positive antibody screen for up to 6 months after the last dose, high-dose intravenous immunoglobulin can transfer anti-A, anti-B or anti-D passively, and cold agglutinins after a mycoplasma or EBV infection cause clumping that mimics an antigen. Infants under 4 months have not yet made their own ABO antibodies, so reverse typing is not used. None of these change your true type; they change how carefully the lab has to work to find it.

Home blood-type kits: technique determines accuracy

Moderate evidence

Finger-prick card kits use dried anti-A, anti-B and anti-D reagents and work on the same agglutination principle as the lab. Evaluations in trained hands report agreement with laboratory ABO typing above 95 to 99 percent, with most errors in the Rh result: weak D individuals and under-mixed or under-read cards produce false 'Rh negative' results, and dried or overheated reagents fail. Use enough blood, mix each well separately with its own stick, read within the stated time under good light, and treat the result as a curiosity rather than a medical record. A free blood donation or a $10 to $40 laboratory type and screen is more reliable.

Never use a home result to decide whether you need Rh immune globulin in pregnancy or to inform a transfusion; both are always based on a hospital laboratory test.

When to see a clinician promptly

During or within hours of a transfusion: fever, chills, back or flank pain, dark or red urine, breathlessness, hives, chest tightness or a sudden feeling of dread: tell staff immediately; an acute hemolytic reaction is a medical emergency.
If you are Rh negative and pregnant and have any vaginal bleeding, abdominal trauma (including a car accident or fall), a miscarriage or an invasive procedure: Rh immune globulin is most effective within 72 hours, so same-day contact is needed.
A newborn with jaundice in the first 24 hours of life, or a baby who is very pale, floppy or feeding poorly, when the mother is Rh negative or group O: possible hemolytic disease, needs urgent assessment.
A positive antibody screen in pregnancy with a rising titer, or any anti-K antibody, for referral to maternal-fetal medicine.
Unexplained fever, jaundice, dark urine or a falling hemoglobin 3 to 14 days after a transfusion, which can be a delayed hemolytic reaction.
If you have been told you have a red cell antibody and are due to have surgery, let the surgical team know well in advance so compatible blood can be found.

Frequently asked questions

How can I find out my blood type?

The simplest free way is to donate blood: blood services type every donation and send you the result within a few weeks. Otherwise, check prenatal or surgical records, where a type and screen is routine, or ask a clinician or a direct-access lab for an ABO/Rh test, which typically costs $10 to $40. Blood type is not on a standard CBC or metabolic panel, so it will not appear on routine results unless it was specifically ordered. Home kits can give an answer but are not accepted for medical purposes.

Are at-home blood type tests accurate?

Reasonably accurate for ABO when the instructions are followed carefully (agreement with the lab above 95 to 99 percent in evaluations), less so for Rh, where weak D variants and under-mixed or faint reactions produce false negatives. The reagents also degrade if the kit has been stored in heat. Treat the result as informational. A hospital will always type you itself before a transfusion, and Rh decisions in pregnancy are made only on a laboratory result.

What does Rh negative mean?

It means your red cells lack the Rh D antigen, an inherited trait found in about 15 percent of Americans. It has no effect on your health. It matters in two situations: you should receive only Rh-negative red cells in a transfusion to avoid forming anti-D antibodies, and in pregnancy, if your baby is Rh positive, you are given Rh immune globulin at 28 weeks and after delivery (and after any bleeding, miscarriage or invasive procedure) so that you do not form antibodies that could harm a future Rh-positive baby.

What is the universal donor and universal recipient blood type?

O negative is the universal red cell donor: its cells carry neither A, B nor D antigens, so they can be given to anyone, which is why emergency departments keep O negative on hand for patients whose type is not yet known. AB positive is the universal red cell recipient, because AB positive people have no anti-A, anti-B or anti-D antibodies. For plasma it is reversed: AB plasma contains no ABO antibodies and can be given to anyone, while O plasma can go only to group O recipients.

Which blood types are compatible for transfusion?

For red cells, you can receive your own ABO group or group O, and if you are Rh negative you should receive Rh-negative cells. So A+ can receive A+, A−, O+ and O−; B+ can receive B+, B−, O+ and O−; AB+ can receive anything; O− can receive only O−. A positive antibody screen adds further restrictions, because units must also lack the antigen your antibody targets. Compatibility for plasma and platelets follows different rules, and the lab handles all of this with a crossmatch before any unit is issued.

Can two parents with the same blood type have a child with a different one?

Yes. Two group A parents can have a group O child if both carry a hidden O allele, and two Rh-positive parents can have an Rh-negative child if both carry the recessive Rh-negative gene. Two group O parents can only have group O children, and two Rh-negative parents can only have Rh-negative children. Blood type is a rough guide to parentage at best and is never used to establish it; DNA testing is.

Does my blood type affect my health or disease risk?

Modestly, in a few areas. People with non-O types have about 1.5 to 2 times the risk of venous blood clots compared with group O, because they carry higher levels of von Willebrand factor, and slightly higher rates of coronary disease and some cancers such as pancreatic cancer; group O has a slightly higher risk of peptic ulcers and of bleeding. These are small effects relative to smoking, blood pressure, weight and family history, and they do not change any screening or treatment recommendation. There is no good evidence that blood type should determine your diet.

Can your blood type change?

Not under normal circumstances. The only way to acquire a different type is a bone marrow or stem cell transplant from a donor of a different group, after which your new marrow makes red cells of the donor's type over several months. Apparent 'changes' between tests are almost always a clerical error, a home kit misread, a weak D variant typed differently by two methods, or temporary interference from a recent transfusion, pregnancy antibodies or a medication such as daratumumab. If two results disagree, a reference laboratory can resolve it.

References

Blood typing. MedlinePlus, US National Library of Medicine (2023)View source
Rh incompatibility. MedlinePlus Medical Encyclopedia, US National Library of Medicine (2023)
Rh factor blood test. Mayo Clinic (2024)View source
Dean L. Blood Groups and Red Cell Antigens. National Center for Biotechnology Information, US National Library of Medicine (2005)View source
American College of Obstetricians and Gynecologists. Practice Bulletin No. 181: Prevention of Rh D alloimmunization. Obstetrics and Gynecology (2017)
Pretransfusion testing. Merck Manual Professional Version (2024)
Garratty G, Glynn SA, McEntire R. ABO and Rh(D) phenotype frequencies of different racial/ethnic groups in the United States. Transfusion (2004)
Sandler SG, Flegel WA, Westhoff CM, et al.. It's time to phase in RHD genotyping for patients with a serologic weak D phenotype. Transfusion (2015)
Wang J, García-Bailo B, Nielsen DE, El-Sohemy A. ABO genotype, 'blood-type' diet and cardiometabolic risk factors. PLoS ONE (2014)
Cusack L, De Buck E, Compernolle V, Vandekerckhove P. Blood type diets lack supporting evidence: a systematic review. American Journal of Clinical Nutrition (2013)
Dentali F, Sironi AP, Ageno W, et al.. 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)
Cohn CS, Delaney M, Johnson ST, Katz LM, eds.. Technical Manual, 20th edition. AABB (2020)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.