I recently did two Comprehensive Metabolic Panels back-to-back within 4 days. The first test was day 1 of fast. 2nd Test was the day after breaking a 3 day fast the previous night. 1st test, kidney function was normal. The 2nd test, the serum creatine spiked about 80% and apparently threw the eGFR <60, indicating abnormal kidney function. Apparently, the literature indicates prolonged fasting can spike the creatine readings. Protein in the urine for the 2nd test was trace normal, so no kidney dysfunction confirmation of the abnormal creatine reading. This result has sort of blunted my enthusiasm for extending fasting past 24 hours. At least, put some time between a fast and lab work.
Sounds like you ran in to this exact issue
The assertion is that creatinine is overestimated in the common clinical lab method due to the increased acetoacetate levels from prolonged fasting. eGFR uses the incorrect creatinine value and ends up being overestimated. They note that creatinine levels tested through an alternate method showed no increase. There’s not much evidence presented to back this up, but it is a possible alternate explanation for what you were seeing.
I am running into a similar issue but with my liver–working theory is that my extended fasting is causing my mildly elevated liver enzyme values to spike. I really like my 48-72s but I am having to stop while I do more tests. Unfortunately kidneys are far less resilient then livers so I think you may have less wiggle room than I do. Do you have a lot going on healthwise?