"When having a blood test the patient should be asked what medication they are taking"

I visited Royal Preston Hospital yesterday with my mother-in-law. She was having blood taken. When the nurse finished and told her to let go of the cotton pad the blood spurted from her arm. My mother-in-law said "that's because I'm on warfarin". The nurse said "you should have told me that because I have to use a smaller needle". Why should we have to know that, the nurse is supposed to be the expert. Shouldn't she have asked if any meds were being taken? I remained tight lipped whilst my mother-in-law apologised to the nurse! It was clean and the rest of the procedure involved good hand hygiene and the nurse cleaned the skin before taking the blood. Just concerned that there needs to be a set procedure in place to establish if medication is being taken.