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Type and Cross-Match

Blood typing is the process of determining the blood type and rH factor of a sample of blood. 

Cross-matching involves finding the best donor for a patient prior to blood transfusion. In addition to the blood type and rH, minor blood groups are also evaluated. Rarely is there a perfect "match" but usually the match is close enough to preclude any clinically-significant transfusion reactions.

In an emergency, O Negative blood (the "universal donor") can be given without much risk of an immediate reaction. If O Negative blood is not available, O Positive can safely be given to men. Women receiving O Positive blood (if they are O negative) may develop antibodies against the rH factor, which could prove dangerous to the fetus during a future pregnancy. This must be weighed against the need for transfusion.