Cystatin C tests should be the gold standard for determining kidney function. I’ve had several major surgeries and “gold standard” was always used by medical professionals. I would think gold standard for who? Patients panic when they see fluctuations in creatinine kidney function. I did. I’d think did I drink enough water, exercise too much that made my muscles excrete creatinine, did I stress the kidneys from something or other? That’s not how it works. Yes, eat wisely, exercise, drink water. Ok. But to hang your emotions on every time you get a blood draw? Find the stability. C Cystatin seems to provide stability.