This is an important procedure that must take place when blood products, specifically red cell concentrates, are ordered for a patient.
Here is a summary of the process:
- The patient’s ABO and Rh groups first need to be determined by identification of antigens on the red cells (called ‘forward’ grouping) and confirmation of corresponding antibodies in their plasma (called ‘reverse’ grouping).
- A broad screen for any abnormal antibodies in the donor’s plasma is then performed. In the event that the antibody screen is positive, additional blood specimens will be requested from the patient in order to identify the specific antibody that is present.
- Based on the results in the first two steps, the blood bank staff will select a compatible red cell product from the refrigerator.
- A cross-match procedure is then run on an automated analyser. Simply speaking, this involves mixing some red cells from the selected product with a small amount of the patient’s plasma. The purpose of this is to perform a ‘mini trial’ to check that there are no antibodies in the plasma that could react with antigens on the red cells. If this happens, the machine will detect agglutination in the sample and alert the technologist of a compatibility issue.
- If the cross-match is successful, the red cell product will be labelled with the patient’s details, secured in a hamper with a cable tie and issued from the designated Blood Bank.
The reason that cross-match samples can only be up to 48 hours old is that there is potential of degradation of patient antibodies in their sample if the blood is older than this. While it can be frustrating and cause a delay to the issue of the blood product, it is definitely in the patient’s best interest to receive a product that is properly matched to avoid potentially life-threatening incompatible transfusions.
For more information about cross-matching and other blood banking procedures, please contact Meridah Mwase, Head – Blood Banks (meridah@wcbs.org.za).