Blood Type Compatibility Calculator

Check donor and recipient compatibility for red cells, plasma or platelets using the ABO and RhD rules, and see exactly which antibody would react if the pairing fails. Use it as a study aid for understanding universal donor and universal recipient patterns.

Introduction to ABO and Rh transfusion compatibility

Safe transfusion depends on matching molecules on the surface of red blood cells against antibodies circulating in plasma. The most significant of those molecules belong to the ABO system, which sorts blood into groups A, B, AB and O according to which antigens sit on the cell membrane. A second classification, the Rh system, records whether the D antigen is present (+) or absent (−). Combining the two yields the eight common blood types this calculator works with. Karl Landsteiner described the ABO groups in 1901 and the D antigen was characterised in 1940; together those two discoveries turned transfusion from a gamble into routine medicine.

The immune system is built to recognise and attack antigens it did not grow up with. A person with group A blood naturally produces anti-B antibodies; a group B person produces anti-A; a group O person produces both; and a group AB person normally produces neither. Those naturally occurring ABO antibodies are present without any prior transfusion, which is what makes an ABO mistake so immediately dangerous. When incompatible red cells enter the circulation, the recipient's antibodies bind the donor cells, bridge them into visible clumps — agglutination — and trigger complement-mediated destruction. That acute haemolytic reaction is the single most feared event in transfusion practice, and avoiding it is what compatibility testing exists to do.

Conceptually, compatibility is a set relationship: $A\cap B=\varnothing$, where $A$ is the set of antigens the donated material carries and $B$ is the set of antibodies waiting in the recipient. An empty intersection means nothing binds and the transfusion proceeds; a non-empty intersection means agglutination. That single line explains why group O negative red cells are the universal red-cell donor: their antigen set is empty, so the intersection with any recipient's antibody set is empty too.

Why the rule inverts between red cells, plasma and platelets

Whole blood is rarely transfused today. Donations are separated into components, and the compatibility question changes with the component because the hazardous ingredient changes. A unit of packed red cells is mostly membrane-bound antigen with very little plasma, so the danger runs donor antigen against recipient antibody. A unit of plasma is the mirror image: no red cells at all, but a full complement of the donor's anti-A and anti-B, so the danger runs donor antibody against recipient antigen. Reverse the ingredient and you reverse the answer. Group O is the universal red-cell donor and the worst plasma donor; group AB is the universal plasma donor and the worst red-cell donor.

Platelet concentrates sit between the two, because platelets carry A and B antigens of their own and are suspended in donor plasma. A major ABO mismatch — recipient antibody against donor platelet antigen — blunts the post-transfusion platelet increment. A minor ABO mismatch — donor plasma antibody against the recipient's own red cells — risks haemolysis, especially from a high-titre group O unit. Neither is an absolute bar: ABO-identical platelets are preferred, but mismatched platelets are given every day when identical units are unavailable, which is why this calculator reports platelets as a caution rather than a flat refusal.

Red-cell donor to recipient matrix

The matrix below answers all sixty-four red-cell pairings at once. Read a row to see everyone a donor can supply, or read a column to see everyone a patient can receive from. Note the two extremes: the O− row is all yes, and the AB+ column is all yes.

How to use the donor, recipient and component menus

Set Donor type to the blood group printed on the unit and Recipient type to the patient's confirmed group. Direction matters: giving A+ blood to an O− patient is a different question from giving O− blood to an A+ patient, and the calculator always treats the pair as donor → recipient. Then choose the Component, because that is what decides which rule applies. Finally, tick Recipient has childbearing potential if avoiding anti-D immunisation is a priority; that flag tightens the platelet result for a D-positive unit going into a D-negative recipient.

Press Check compatibility and the result panel returns a verdict of compatible, compatible with caution, or not compatible, together with the specific ABO reasoning, the specific RhD reasoning, and the full list of donor types this patient could accept for that component. Reset returns every control to its default. Because all eight types and all three components are encoded in the page, the tool runs entirely offline in your browser.

The antigen-antibody formula behind a yes or no

Write $\mathrm{Ag}$ for a set of antigens and $\mathrm{Ab}$ for a set of antibodies. Group A cells carry $\mathrm{Ag}=\left\{A\right\}$, group AB cells carry both A and B, group O cells carry none, and a positive Rh type adds D. Plasma antibodies are the complement of the cell antigens: group A plasma holds anti-B, group B plasma holds anti-A, group O plasma holds both, group AB plasma holds neither.

For red cells the test is that the donor's antigens must not meet a matching recipient antibody, which is equivalent to requiring that the donor's ABO antigen set be a subset of the recipient's:

Formula: Ag_donor ∩ Ab_recipient = ∅

${\mathrm{Ag}}_{\mathrm{donor}}\cap {\mathrm{Ab}}_{\mathrm{recipient}}=\varnothing$

For plasma the same equation runs the other way, because it is now the donor's antibodies meeting the recipient's antigens:

Formula: Ab_donor ∩ Ag_recipient = ∅

${\mathrm{Ab}}_{\mathrm{donor}}\cap {\mathrm{Ag}}_{\mathrm{recipient}}=\varnothing$

The RhD rule for cellular components is a separate implication rather than an intersection, because anti-D is not naturally occurring — it appears only after exposure. Routine practice therefore refuses to create that exposure:

Formula: D ∈ Ag_donor ⇒ D ∈ Ag_recipient

$D\in {\mathrm{Ag}}_{\mathrm{donor}}\Rightarrow D\in {\mathrm{Ag}}_{\mathrm{recipient}}$

A pairing is reported compatible only when the applicable ABO test and the applicable RhD test both pass. Expanding the ABO test gives the familiar lists: for red cells, O donates to everyone, A donates to A and AB, B donates to B and AB, and AB donates only to AB. For plasma, AB donates to everyone, A donates to A and O, B donates to B and O, and O donates only to O.

Worked example: one A-positive patient, three components

Take a single patient of group A positive, and walk one component at a time with a group O negative unit in hand.

Red cells. The O− unit carries no A, no B and no D. The patient's plasma holds anti-B. The intersection of {} with {anti-B} is empty, and the unit is D-negative going into a D-positive patient, which the implication above permits. Verdict: compatible. This is the emergency-release case every hospital relies on.

Plasma. Now the same O donor is the worst possible choice. Group O plasma carries both anti-A and anti-B; the patient's red cells carry A. The intersection of {anti-A, anti-B} with {A, D} contains anti-A, so the unit would attack the patient's own circulating red cells. Verdict: not compatible. The right plasma for an A+ patient is group A or group AB, and RhD does not enter the decision at all.

Platelets. Group O platelets into an A+ patient is a minor ABO mismatch: the suspending plasma carries anti-A against the patient's A antigen. There is no major mismatch, because the patient has no anti-O to attack. Verdict: acceptable with caution, and a volume-reduced or low-titre unit would be preferred. Swap the pairing round — group A platelets into a group O patient — and you get a major mismatch instead, where the patient's anti-A shortens the transfused platelets' life and the count barely rises.

One patient, one donor unit, three different answers. That is the whole lesson of this page in a single example.

Limitations and assumptions: where the ABO/Rh rule stops and the lab begins

This tool models only the two dominant antigen systems, and it assumes a straightforward serological picture. Real transfusion medicine layers a great deal on top. An antibody screen and a physical crossmatch mix the actual donor and recipient samples to catch alloantibodies against minor systems such as Kell, Duffy, Kidd and MNS, which typing alone cannot see. A patient's transfusion and pregnancy history can reveal prior sensitisation. Weak D and partial D phenotypes blur the Rh line, so a person typed as D-positive by one method may need D-negative units. Neonates, patients on massive transfusion protocols, and recipients of ABO-mismatched stem-cell grafts all follow separate protocols. Anti-A and anti-B titres vary enormously between donors, which is why "high-titre negative" labelling exists for group O platelets and plasma.

The assumptions baked into the answers here are: naturally occurring ABO antibodies are present and clinically significant; anti-D is present only after exposure, so it is treated as a risk to prevent rather than a reaction to predict; plasma is treated as acellular; and platelet units are treated as platelets suspended in donor plasma with a small residual red-cell load. Treat every answer as the textbook baseline for teaching, never as clearance for an actual transfusion. That decision belongs to a crossmatch and a clinician.

Blood type compatibility questions people ask

Sources: the ABO and RhD rules, component-specific selection and the universal donor and recipient patterns used here follow American Red Cross — Blood types, the NHS Handbook of Transfusion Medicine (5th edition) chapters on red cell, plasma and platelet selection, the AABB Standards for Blood Banks and Transfusion Services, and Blood Groups and Red Cell Antigens (NCBI Bookshelf). This page is an educational aid only; actual transfusion decisions require laboratory typing, an antibody screen, a crossmatch and clinician oversight.