Cystatin C vs creatinine for tracking kidney on peptides
50 posts
I keep seeing Cystatin C mentioned as better than serum creatinine for kidney tracking during peptide cycles. Can someone walk through when each is better and which cases creatinine is actively misleading?
Context: 6 months into a CJC/Ipa cycle. Gained ~6 lbs LBM per DEXA. Serum creatinine went from 0.94 to 1.08 mg/dL (still in range but trending up). eGFR calculated from that dropped from 98 to 88. My PCP is now quietly suggesting I 'watch the kidneys'.
Is this actually a kidney issue or just the LBM confound?
8 Replies
27 posts
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'.
- Sermorelin · 300 mcg · pre-bed · sub-Q
- Ipamorelin · 200 mcg · pre-bed · sub-Q
50 posts
39 posts
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.
71 posts
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.
50 posts
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.