We always do several measurements each time and take an average. One of mine is terrified of people other than me. Needless to say going to the vet is very traumatic for him.
We are measuring his because he has an elevated level of protein in his urine. His BUN and Creatinine are normal, so is his thyroid. He is 10, though, so either disease could be there in an early stage.
His BP is averaging 150-155 right now and we are debating whether to just go ahead and start him on enalapril. His has had measurements as his as 190.
My CRF kitty has always been well under 160. We monitor hers just as a precaution.