I apologize this is a dumb question. Why should there be 3 days between type and screens if a patient requires blood?  What can happen in 3 days? Thank you in advance, I appreciate your time and knowledge 🙌🏽👍🏽

The timing for fresh samples is somewhat different in the UK than in, for example, the USA.  The timings, and the reasons for these timings, are set out in paragraph 3.7 of the BCSH Guideline "Guidelines for pre-transfusion compatibility procedures in blood transfusion laboratories" (written by Claire Milkins, Jenny Berryman, Carol Cantwell, Chris Elliott, Richard Haggas, Joan Jones, Megan Rowley, Mark Williams and Nay Win, for, and on behalf of the BCSH, and published in Transfusion Medicine 2013; 23: 3-35.  doi: 10.1111/j.1365-3148.2012.01199.x), with the Key Recommendation of this paragraph being, "Serological studies should be performed using blood collected no more than 3 days in advance of the actual transfusion when the patient has been transfused or pregnant within the preceding 3 months."

As I understand it, this timing was based on the work originally carried out by Professor Patrick Mollison many years ago, who found that a new specificity of an alloantibody, or a nascent antibody that has become undetectable by normal serological techniques, can appear (or reappear) in the plasma of an individual within three days, after stimulation.  The problem is that not all patients know whether or not, or when, they have been transfused, or may deny it for religious reasons (I once cross-matched for a patient who had been a life-long Jehovah's Witness, who had an anti-Fya in his plasma that had a titre well in excess of 128).

I hope that helps.