Hi Russ, The creatinine levels that experts consider to be normal can vary among different hospitals and laboratories. That may explain the differences you’re seeing with your clinic vs what Mayo has on their website.
Mayo Clinic has an upper limit for males at 1.35 and 1.04 for women.

My local clinic has a different parameter with the high end at 1.11 for women. I was bothered initially when I’d see my numbers vary between both clinics. It was a non-issue.
Our creatine can fluctuate daily. Hydration can change those numbers, among other things. Also taken into account with the health of your kidney would be the BUN and GFR numbers.

Creatinine is a byproduct of normal muscle function. It is a metabolite of creatine phosphate, which the muscles use as an energy source.
The greater someone’s muscle mass, the higher their creatinine levels. For this reason, males tend to have higher creatinine levels than females.
From my experience, with your 1.31 you can relax…if you were at Mayo, you’d be in healthy range.

By the way, welcome to Mayo Clinic Connect! May I ask what brought you to our forum? Are you concerned about your kidneys or just wondering about your creatine numbers from a blood draw?