When blood is required in an emergency, how quickly it can be provided and degree of compatibility will depend on whether pre transfusion testing has been completed, and whether the recipient's antibody screen is negative.

In extreme or life-threatening situations where pretransfusion testing is not possible or cannot be completed in time it may be necessary to transfuse uncrossmatched blood. This is a clinical decision determined by the level of urgency and whether the immediate correction of blood loss or anaemia outweighs the potential risk of acute or delayed immune haemolysis due to potentially incompatible blood. It is important that a specimen is collected for subsequent pretransfusion testing before blood products are administered. 

In emergency situations it is important for individuals requesting a transfusion to clearly inform the laboratory staff of the urgency of transfusion.
 

Typical timeframes for the provision of blood in an emergency situation*

Note: *Dependent on laboratory workflow; **If a red cell antibody is detected there may be a delay while the antibody is identified and compatible red cells found. It may be necessary to provide ABO & RhD compatible but otherwise serologically incompatible red cells until further investigations are completed.

Reference

Australian and New Zealand Society of Blood Transfusion Inc. Guidelines for Transfusion and Imuunohaematology Laboratory Practice, 1st edition. Sydney, Australia, 2016.