This is a teaching document I give to new staff, medical technology students, pathology and residents. Very often I get the question, “Why can’t I just do the antiglobulin phase crossmatch first and then phenotype the RBC unit?” Or: “Why do I have to add reagents in a particular order?”
My practice has always been to select an antigen-negative RBC unit first, then do the antiglobulin-phase AHG crossmatch. This way I know that the unit was definitely phenotyped before release. Likewise, the blood bank computer now only offers antigen-negative units for allocation and then crossmatching if there is a clinically significant antibody.
In a manual setting without a blood bank computer system, performing the AHG crossmatch may yield a negative result, even if the unit is antigen-positive. With storage, some antigenic expression is weakened so it may not be detected at the time of crossmatch. Yet, there may still be enough antigen present to cause hemolysis. Not detected does not necessarily mean not present!!
I expect that many inexperienced staff may be tempted to forego the antigen typing if the AHG crossmatch is negative.
This is an analogous logic to the question, “Do I add the cells or the antiserum/plasma/serum first for the reaction?” If you add the cells first, you may forget to add the patient’s plasma/serum or a typing antiserum and you might not be able to detect the omission by looking at the tube or gel. Actually, I once recommended to one vendor that it color the typing antiserum so it was conspicuously showing on the gel.
I was taught that this is a matter of discipline to ensure that all steps are performed.
However, for every practice, there has to be flexibility. If there is no typing reagent or if it is very expensive or in short supply, one may have no choice but to do the AHG crossmatching first. Often there is still another option: one can often preliminarily screen units first before using a rare reagent—examples:
- Check if patient is group O first and antibody screen panreactive in suspected anti-H.
- Check P1 typing first if there is a suspected anti-PP1Pk.
- Check the antibody screen for panreactivity first for antibodies of high-incidence or prevalence antigens.