Cystatin C

Kidney Function marker

Cystatin C

Category: Kidney Function
Unit: mg/L

Small protein filtered by the kidneys. Unlike creatinine, Cystatin C is not affected by muscle mass, making it a more accurate kidney function marker for muscular individuals.

PED Notes

CRITICAL: The gold-standard kidney marker for athletes with high muscle mass. Creatinine-based eGFR is unreliable in muscular individuals because creatinine scales with muscle mass, giving falsely 'elevated' readings and falsely low eGFR. Cystatin C-based eGFR removes this confounder entirely. If creatinine is elevated but Cystatin C is normal, kidney function is fine — the creatinine elevation is from muscle mass. Request this test whenever creatinine or eGFR results are ambiguous. Especially important when using nephrotoxic compounds (Trenbolone, high-dose orals) or chronic NSAID use.

When high

Why Cystatin C Matters for Athletes:

  • Unlike creatinine, Cystatin C is NOT affected by muscle mass, diet, or exercise
  • Provides accurate kidney function assessment when creatinine is unreliable
  • Request Cystatin C-based eGFR calculation from your lab for the most accurate GFR

If Elevated (>1.15 mg/L):

  • Indicates genuine kidney function impairment (unlike creatinine, this is NOT a false positive from muscle)
  • Evaluate current compound use — trenbolone and high-dose oral AAS are most nephrotoxic
  • Assess NSAID use (ibuprofen, naproxen) — chronic use damages kidneys
  • Check blood pressure (hypertension damages kidneys over time)

Supplements:

  • Astragalus -- 500-1000mg/day (renoprotective properties)
  • Omega-3 Fish Oil -- 2-4g/day (anti-inflammatory, renal protective)
  • CoQ10 -- 100-200mg/day (antioxidant support for renal cells)

Pharmacological options (confirmed eGFR decline):

  • Telmisartan -- 40-80mg/day; ARB; first-line renoprotective agent; reduces intraglomerular pressure and slows CKD progression (RENAAL, IRMA-2 evidence); PPAR-gamma activity is a bonus for AAS users
  • Losartan -- 50-100mg/day; alternative ARB if telmisartan is not tolerated
  • Lisinopril / Ramipril -- ACE inhibitors; 5-10mg and 2.5-10mg/day respectively; equally renoprotective but monitor for cough and hyperkalaemia; do not combine ACE + ARB (additive hyperkalaemia/AKI risk)
  • Empagliflozin / Dapagliflozin -- 10-25mg/day; SGLT2 inhibitor; EMPA-KIDNEY and DAPA-CKD trials confirmed renoprotection independent of diabetes status; emerging standard-of-care for CKD in athletes with concurrent metabolic dysfunction
  • All require physician oversight; baseline potassium and eGFR before starting

Creatine caveat for AAS users:

  • Creatine monohydrate and high muscle mass raise serum creatinine (falsely suggesting reduced eGFR on creatinine-based calculations) but do NOT affect cystatin-C
  • Cystatin-C is the more accurate marker for AAS users and creatine supplement users; use cystatin-C-based eGFR (CKD-EPI-CysC) when available
  • Do not stop creatine supplementation solely to improve eGFR; use cystatin-C instead

Lifestyle:

  • Hydrate well (3-4L/day)
  • Limit NSAID use — use paracetamol or topical alternatives for pain
  • Monitor blood pressure (target <130/80)
  • Reduce AAS dose or discontinue if eGFR (cystatin-C based) is declining; trenbolone and high-dose orals are the most nephrotoxic compounds
  • If persistently elevated, nephrology referral recommended

History Chart

Reading History

Frequently Asked Questions

Reference Ranges

Standard Range

0.55 - 1.15 mg/L

Statistics