Whenever I need to have a test done outside of my own doctors (Mayo and my primary) I ask them to fax over results (actual scan also) so that the exams are in the same database location. It can be easier if you request at the time you are being seen there because otherwise when you call back to get it done, they may ask you to fill out permission forms etc., which is more work. Good idea to travel with the fax numbers of your doctors too.
In the past I have noticed that scans done at my doctor's will say "nothing to compare to", so good to have any previous ones in their database so they can see changes.
Big challenge to get all the medical information in one place. Sometimes they can look at another institution's records, but it depends. We can only hope for that day in the future where all our medical records will automatically be stored in one place.