Incrementing Primary Key

Dear List - Thanks for all your excellent help. I am setting up a database for medical research, which will be conducted at various sites. The sites will be identified by a letter {A,B,C ....}. The medical record number [primary key] will start at 1001 and increment by one(1) for each patient at each site; ie, A 1001, A1002, B1001, B1002 ...... How do I do this? Do I need a separate database for each site? Ethan