I have had eGFRs vary up and down over my 22 year MGUS odyssey. Dehydration pushes creatinine up and the eGFR down. My lowest value was 57 after a 3 day winter road trip to Minnesota when I drank almost no water. I had an extremely painful
Hip that I was on my way to have replaced. I did not want to get out of the car cause it was freezing cold and my hip was excruciating painful.
That said, dehydration affects a lot of lab values including hemoglobin, hematocrit, glucose, etc..
Do a google search and it is appalling how some diagnoses that depend on reasonably hydrated blood can be affected.
By the way, my eGFR fully recovered by the next blood draw!!