Protocols for switching the ABO groups of donor red cells are needed so that patient anti-A or anti-B (or both) do not hemolyse ABO-incompatible donor cells.
| Patient Group | Possible Donor Group |
| A | A or O |
| B | B or O |
| O | Only O |
| AB * | A or B or O |
* If AB donor red cells are unavailable, group A units are preferable (first choice) because here we are concerned with ABO antibodies in the donor plasma: anti-B is usually a weaker antibody than anti-A. After group A, group B donor units are preferable to group O units, because group O units would contain anti-A, anti-B and anti-A,B in the plasma.
NOTE: If a group A, group B or group AB patient has been
transfused with group O donor RBC in an
emergency, and RBC of the donor's ABO group
subsequently becomes available, the decision
whether or not to transfuse is based on the
presence or absence of passively acquired
donor anti-A and anti-B in the patient. If an
IAT crossmatch with the patient's serum and
RBC of the patient's ABO group is
incompatible, continue to transfuse group O
donor RBC.
Also note that patients who have received
donor RBC of a different ABO group (usually
group O) may present with unusual ABO
groupings in which mixed-field agglutination is present.
To illustrate:
| anti-A | anti-B | A1 cells | B cells |
| 2+ * | -- | -- | 4+ |
* mixed-field agglutination (MFA) microscopically
This may be an example of a group A Rh positive patient
who has received many transfusions of group O red cells.
The MFA is caused by 2 red cell populations (group O and
group A).
| Switching ABO Groups (RBC) | Switching Rh(D) Groups
|