The problem isn't with your usage of "opinion," it's the use of "arbitrary."
ARBITRARY - adjective; based on random choice or personal whim, rather than any reason or system
Certainly you should have an opinion on a medical matter, but even without a "concise set of standards of fitness" being handed to you, is your opinion really based upon random choice or personal whim? Or is it based upon your training and experience as a physician and what an examination tells you about the examinee's physical condition? If you were examining the person to participate in some other physical activity, like swimming or jogging, would you need a USG body to lay out standards for you in order for you to render a reasonable medical opinion?
IMHO, you should be rendering an
informed opinion, not an arbitrary one.