Clinical Laboratory - Transfusion Medicine

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Blood Compatibility Testing

Blood Type

A "type" includes a "front type" and a "back type". The "front type" determines which antigens ("flags") in the ABO blood group system are on the patient's red blood cells as follows:

Antigen Blood Type
A antigen only Type A
B antigen only Type B
A and B antigens Type AB
Neither A or B Type O

The "back type" identifies the isohemagglutinin (naturally occurring antibody) in the patient's serum and should correspond to the antigens found on the red blood cells as follows:

Antibody Blood Type
anti-B Type A
anti-A Type B
anti-A and anti-B Type O
Neither anti-A or anti-B Type AB

Blood Type O is considered to be a "Universal Donor" in that it lacks both major ABO antigens, while AB is the "Universal Recipient" in that it has both major antigens but lacks antibodies to A and B.

In addition, RBC's are Rh typed and identified as "D" positive or negative.


The patient's blood is typed for the blood type and then a "screen" looks for unexpected red cell alloantibodies which may form following pregnancy or prior transfusions. If the screen is positive, the antibody needs to be identified. The physician is also notified. Antibody identification can be complicated and take more than a day to complete.

"Type and Screen"

This procedure is ordered when it is unlikely that blood will be needed emergently. There are no donor units specifically matched for the patient and reserved for the patient. However, the patient's blood type is identified, and a screen will have identified any potential antibodies that could complicated obtaining blood. A crossmatch to find compatible units can be done more easily following a "type and screen."

"Type and Cross"

A "type and cross" is ordered when it is likely that blood will be needed. Compatibility testing between patient and donor units is performed least 2 units are crossmatched for the patient and resrved specifically for that patient. These units cannot be used for anyone else. If they are not used, then they can go back into the inventory for use by others.

  • A full crossmatch procedure takes about 45 minutes to complete and cannot be shortened.

  • Units are refrigerated until used.

  • A unit of blood must be properly labelled and the label MUST be checked before use.

Every unit crossmatched to a patient is removed from the general inventory and reserved for the patient for 72 hours. Units which are crossmatched unnecessarily will deplete Blood Bank inventories and can result in blood shortages. Blood shortages can result in cancellation of elective surgical procedures.

Blood will ordinarily not be released for transfusion until compatibility testing is completed. However, under emergency conditions, blood products may be released without a crossmatch if the patient is in danger of dying if transfusion is delayed. In such cases, if the patient's blood type is not known, then group O Rh negative (O neg) blood can be released without compatibility testing. In cases in which the patient's blood type is reliably known, then type-specific blood of the same ABO and Rh group may be released.

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