Earlier quoted context omitted.
I think you might be thinking of BUN. For creatinine, there is peer-reviewed data demonstrating that in people without renal disease, increasing the protein in your diet tends to improve your eGFR, not decrease it (which is what happened here). So for him, it either fluctuated or was a marker of true kidney disease, but in any case this is not just a marker of adequate protein intake.
Neither. I'm talking of creatinine levels in the blood, and my doctor was also very clear on that, and that it wasn't normally a concern at the levels she would expect to see from protein supplementation, but if it was too high, she'd need to establish filtration rate to rule out renal problems. Also, while you may very well be right that GFR would improve, the little bits of reading I've had time to do indicates tha…
Also, creatinine is measured in serum to estimate gfr. Urine creatinine is more used as part of a test to look for nephrotic syndrome.
Creatinine is produced constantly, filtered and not resorbed, without active secretion, which is why it is an excellent proxy for glomerular filtration rate.