ABO and RhD Compatibility

The ABO system is defined by A and B antigens on red cells and by naturally occurring anti-A and anti-B antibodies in the plasma of people who lack the corresponding antigen. These IgM antibodies cause immediate intravascular haemolysis when incompatible red cells are given, and ABO incompatible transfusion is the most dangerous preventable transfusion error. The RhD system matters because RhD positive cells readily immunise RhD negative recipients, and anti-D then causes haemolytic disease of the foetus and newborn and haemolytic transfusion reactions.

Group distribution. Group O people have anti-A and anti-B, group A have anti-B, group B have anti-A, and group AB have neither. Group O is the universal red cell donor and group AB the universal plasma donor.

Risk context. Each product has different compatibility rules, because the risk comes from antigens on the donor cells (for red cells) or antibodies in the donor plasma (for plasma, platelets and cryoprecipitate). Applying red cell rules to plasma is a recognised and dangerous error.

Special groups. Neonates and children have lower tolerance of passive anti-A and anti-B and need special rules. Women of childbearing potential need RhD negative products where possible. RhD negative donors are uncommon in many East African populations, so RhD negative units are scarce and allocation matters.

Red cell compatibility (donor red cells must lack antigens the recipient has antibodies against).

Plasma compatibility (donor plasma must lack antibodies against recipient red cells).

Platelet compatibility. ABO identical platelets are preferred, because platelets express some ABO antigen and carry plasma. ABO non-identical platelets are acceptable when identical ones are unavailable, but group O platelets given to non-O recipients carry passive anti-A and anti-B, especially when the titre is high, and this matters most in neonates, children and small adults. Plasma reduced or low titre products reduce this risk.

RhD positive platelets given to RhD negative recipients carry a risk of immunisation through red cell contamination, so anti-D immunoglobulin is given to women of childbearing potential when unavoidable.

Cryoprecipitate compatibility. ABO matching is preferred but not always required in adults because of the small plasma volume. Matching matters more in neonates and small children. RhD does not need to match, because cryoprecipitate has negligible red cells.

RhD rules.

• RhD negative recipients receive RhD negative red cells.

• RhD negative girls and women of childbearing potential should not receive RhD positive red cells except in life-threatening shortage; if given, anti-D immunoglobulin and specialist advice are needed.

• Males and women beyond childbearing age with RhD negative status may receive RhD positive red cells in emergencies when RhD negative supplies are scarce.

• Weak D and partial D variants are managed by laboratory policy; in pregnancy and for women of childbearing potential, partial D is usually treated as RhD negative.

Emergency products. Group O red cells for any recipient and AB plasma, or group A plasma with low anti-B titre in centres that use it, for any recipient. Low titre group O whole blood is used in some systems.

Red flags. ABO discrepancies between current and historical results, a mixed field, recent bone marrow or organ transplant with different donor group, and unidentified patients.

Practical steps.

1. Confirm the recipient's ABO and RhD group on a current sample, and compare it with any historical record.

2. Choose products by their own compatibility table, not by a single rule.

3. Use ABO identical products wherever possible for platelets.

4. Protect RhD negative stock for women of childbearing potential, children and patients with anti-D.

5. In stem cell or organ transplant, follow the transplant centre's group plan.

Special groups.

• Neonates: group O or the infant's own group for red cells, with attention to maternal antibodies; plasma and platelets of the infant's own group or AB, low titre.

• Pregnancy: RhD negative women receive RhD negative products, with anti-D prophylaxis as needed.

• Rare phenotypes such as the Bombay phenotype need special donors and early laboratory referral.

Monitoring. The final identification check at the bedside is the main safeguard.

Complications. ABO incompatible red cells cause acute haemolytic reactions; incompatible plasma or platelets can cause haemolysis in the recipient; RhD alloimmunisation causes later haemolytic disease and reactions.

Referral and escalation. Blood bank medical staff for discrepancies, rare groups and unplanned RhD positive exposure in RhD negative women.

Disposition. Document the group, the products given and any exposure to non-identical products.