Cystatin C vs creatinine for tracking kidney on peptides

Classic LBM confound. Serum creatinine is a muscle-derived metabolite — if you gain 6 lbs of LBM your creatinine goes up with zero change in kidney function. Pull a Cystatin C, which is NOT muscle-mass dependent. If your Cystatin-C-based eGFR is normal, kidneys are fine.

Creatinine of 1.08 at 6 lbs LBM gain is basically textbook for 'no kidney problem, just more muscle'.

Ordered Cystatin C. If it's clean I'll send my PCP the paper trail.

This is why I tell every GH/tesa user to get Cystatin C BEFORE the cycle. Then you have a non-muscle-confounded baseline to compare against.

Cystatin C has its own noise (CV ~5-7%) but it's not muscle-confounded and it also responds to acute kidney changes faster than creatinine. Best single marker for your use case.

Also — BUN:creatinine ratio is often a better quick-look than creatinine alone. If BUN is proportionate you're probably just muscular. If BUN stays put while creatinine climbs, it's more likely real.

KDIGO guidelines explicitly recommend Cystatin C confirmation when eGFR-creatinine is between 60-89 without other markers of kidney disease. Your case is literally what the guideline is for.

UPDATE: Cystatin C back. 0.86 mg/L. Cystatin-C-eGFR is 102. So kidneys are actually BETTER than the creatinine-based eGFR suggested. LBM confound confirmed. Sending my PCP the literature.

As expected. Glad your PCP is getting educated alongside you.

This thread actually answers a question I've had for a while. I'm only a few months in myself and my creatinine bumped up similar to OP's but nobody mentioned getting cystatin C done. Guess I'm gonna ask my doc about it since apparently thats literally what KDIGO says to do for this exact situation. Thanks for the data on BUN ratio too, that's an easy check I can just look at on my normal panel.