How to Choose the Correct Rh Blood Type - Part 2

Last week, I discussed what many might think is a basic question in the world of blood banking – How do I choose the correct ABO blood type for my patient? This blog is a continuation of that article by explaining how you choose the correct Rh blood type for your patient.

Matching for Rh is different than matching for ABO, as anti-D antibodies are not naturally occurring. Therefore, there should be no concern for transfusing plasma with anti-D antibodies as all donations are screened to make sure no significant alloantibodies are present. This means that the purpose of matching for Rh is only to prevent alloimmunization to the D antigen – which happens in about 20-30% of Rh negative hospitalized patients who receive Rh positive RBCs.  This is most important for Rh negative women of childbearing age as the formation of an anti-D antibody could potentially impact the health of an Rh positive fetus in future pregnancies. Of course, any patient who has formed anti-D antibodies (due to previous exposure) should receive Rh negative RBCs to avoid hemolytic transfusion reaction. Remember, we must protect the red blood cells that are transfused!

Another difference in matching for Rh type is that the <2mL RBC volume limit (for requiring an ABO crossmatch) no longer applies. Any amount of RBCs in a yellow product could cause potential Rh alloimmunization –though it may be very low risk depending on the amount of RBCs. The only yellow product which may contain residual RBCs are platelets (platelets themselves do not express Rh antigens) – and they have very, very few residual RBCs (on the microliter scale)! As you can imagine, this is very low risk for Rh alloimmunization, and generally platelets can be transfused irrespective of their Rh type (depending on availability). Women of childbearing age have the option of receiving RhIG in order to eliminate the potential risk of Rh alloimmunization. Thawed plasma and cryoprecipitate have been frozen and, therefore, have no residual intact RBCs – posing almost no risk of alloimmunization.

References:

Standard for Blood Banks and Transfusion Services, 32nd ed. April 1, 2020.

AABB Technical Manual, 19th ed. 2017. Edited by Fung M, Eder AF, Spitalnik SL and Westhoff CM.