Reason for GFR level to fluctuate from 20 to 57?

I have no medical training, so take this for what it's worth--just one experience to give you something to think about. My son's GFR is usually 80. He passed several kidney stones in the last week, during which time they measured GFR again. It was lower while he was trying to pass the stones. The Dr very quickly explained that the number fluctuated for him because the stone was causing the urine to back up into the kidneys.

@aihtnyc & @jsdms welcome both of you to Mayo Clinic Connect. Regarding the GFR, these values can vary greatly for a number of reasons (hydration, other conditions & ailments, the particular lab that is used to perform the tests ….. ) One GFR level is not as important as considering the overall trend over time. If you have a need of tracking the GFR it is best to stay with one provider / lab and get the draw done when all other factors are as consistent as possible. If you don’t mind sharing, what are your reasons for following the GFR?

Like jsdms, I have no formal medical training so please understand that my comments come from personal experience. I have had Chronic Kidney disease since 2003 (although it was not mentioned by any of my doctors until I asked about the low eGFR reading on my annual blood test in 2019. I then went back and looked at my blood test results as far back as I have them which is 2003) My Creatinine and eGFR have been consistent with CKD Stage 3a for 20 years, during which time my eGFR test results have fluctuated between 47 and 61.
The lower readings occurred when I was dehydrated or had some other source of illness at the time of the test. The eGFR 47 reading occurred when I had a bladder infection. The 61 reading occurred when I was recently in the hospital recovering from a bicycle accident in which I broke both my left arm and left leg. While in the hospital daily blood tests were done and the five readings for my eGFR were:
Sep 11, 2023 61 mL/min/1.73m*2
Sep 10, 2023 56 mL/min/1.73m*2
Sep 9, 2023 60 mL/min/1.73m*2
Sep 8, 2023 59 mL/min/1.73m*2
Sep 7, 2023 59 mL/min/1.73m*2

You can see from these results that a variance of 5 ml occurred between Sep 10 - Sep 11. The tests were done every morning before breakfast, and I was on an IV drip so my hydration was relatively stable. I am guessing that the low reading on Sep 10 was due to some alarming news that my doctor gave me on the afternoon of Sept 9th. Stress can also affect your kidney function. Thankfully the alarming news proved to be unfounded with further testing.

At my follow-up appointment with my primary care physician on Oct 17, 2023
my eGFR reading was 55 which is where it is most often during my regular blood tests.

I have read that an acute kidney injury (AKI) can cause a sudden significant drop in eGFR, and once the cause of injury has been addressed the kidney function as shown by eGFR improves - but usually not to the level of function that was standard before the AKI.

I hope you will find this information helpful in answering your question about eGFR levels.

@aihtnyc as you have read, there can be reasons for fluctuations in eGFR [kidney function]. Dehydration, having an infection, reaction to a medication, an underlying comorbidity that is acting up can all be factors. Can you recall if any of these have happened during these test results?
Ginger