instead of the rectal exam he went straight to a blood test?

I went to my GP/doctor today as I have been having trouble urinating,weak flow etc...I had done some research so guessed I had a Prostate issue, he agreed but instead of performing the expected physical rectal exam he went straight to a blood test saying that the physical exam may affect and give inaccurate results of a blood test if it is done soon after the rectal exam, is this normal? it contradicts everything I have read...i.e Physical exam then depending on the findings of that, a blood test.