eli5 Do blood clinics test the blood before giving the donation to a patient?
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Blood transfusions were almost-always tested for compatibility before transfusion. A drop of the blood to be transfused was mixed with a drop of the recipient’s blood just before transfusion to confirm that the blood is compatible. This is called cross-match.
This type of test was performed before all blood transfusions, unless a dire emergency (for example major trauma), where group matched blood (if blood group known) or O negative blood (if blood group unknown) would be used without cross matching.
These days, however, blood group and compatibility analysis has improved, and computer databases are more robust and stricter electronic tracking with barcodes or similar is in place. This means that in most cases, electronic cross matching is now performed, where the computer database verifies all the compatibility issues. However, in some cases an electronic cross match isn’t always possible – for example, if someone has already had a lot of previous transfusions, so a proper cross match test will need to be performed in this type of case.
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