i have had high blood pressure since the age of 19. at age 65 I was diagnosed with chronic kidney disease stage 3. this was caused by high blood pressure. since my diagnosis, my egfr has fluctuated up and down.im now 72.
I don't know for certain whether or not a colonoscopy would affect your egfr. i would ask your doctor that question to be sure.

you are absolutely right. you want your egfr to be as high as possible and your blood pressure to run in the normal range.

My eGFR was 90 and drooped to 11 almost overnight and currently 32. The main reason doctors end up in medical practice lawsuits is they don't listen to their patients. 80% of males over 70 have an enlarged prostate and I am 76. But they have never examined my prostate to determine the size and one will not contradict another. I am urinating three times over what Mayo says is the average amount and they continue to say I need a TURP, transurethral resection of the prostate to relieve difficulty urinating and urinate more and it is my Kidneys. I complained to the State Medical Board, Insurance and Federal Medicare

Yes, my eGFR has been nearly 100 for years even past age 50. Was 96 early last year, then this past November it went down 20 points. The only difference was taking high dose B12 and that colonoscopy a couple of days prior to the blood test. Freaked me out. My BP has never been high, but I take a small dose BP med and it stays around 112/70. I have type 1 diabetes and have had really good control over this last year. Never poor control. I avoid things like ibuprofen, pain pills, etc. Very moderate alcohol, good diet, exercise……lost a lot of weight, see nutritionist….. I can’t think of how I could promote my health any more. Intend to retest soon,