I was told that it makes a difference what the patient or their records claim, because it can accelerate obtaining the correct result. Since, it was alleged to

I was told that it makes a difference what the patient or their records claim, because it can accelerate obtaining the correct result. Since, it was alleged to me, you don't do a test for "what's your blood type", but rather something more like "is your blood type A-?". Therefore, if they say "oh, I'm O+", you can just run the test "is this person O+?". Is that not the case?