Hi @mimi1234, I add my welcome. I expanded the title of this helpful topic to reflect your question: "Is this age-related decline in GFR or kidney disease?"

I found this helpful article
- Glomerular Filtration Rate (GFR) Calculator and FAQ https://www.upmc.com/services/transplant/kidney-pancreas/glomerular-filtration-rate-calculato

Excerpt:
GFR gets lower with age, even if you don't have kidney disease. But no matter how old you are, a GFR below 60 for three months or more means you have kidney disease."

So it looks like your doctor is monitoring you to see whether the lower value of 52 is a one-time reading or if this is a new trend that may indicate kidney disease. Because you are otherwise in good health, your doctor is not jumping to conclusion based on one test.

The lower value may be due to other temporary factors like increased muscle mass, reduced fluid intake and medications such as NSAIDs and ACE inhibitors.

I agree with @gingerw, this is good time to make small changes for your health, like stay hydrated, reduce salt intake, and stay active.

What physical activities do you like to do?