In a 39-year-old male with low muscle mass, normal serum creatinine (0.9 mg/dL) but elevated cystatin‑C (1.16 mg/L), no history of diabetes, hypertension, NSAID use, or autoimmune disease, normal urine and albumin‑to‑creatinine ratio, and renal ultrasound showing slight increased cortical echogenicity with normal corticomedullary differentiation and size, and a remote history of lithium use stopped two years ago, how should I proceed and what could be causing the elevated cystatin‑C?

Elevated Cystatin-C with Normal Creatinine in Low Muscle Mass Patient

In this 39-year-old male with low muscle mass, the elevated cystatin-C (1.16 mg/L) likely reflects his true kidney function more accurately than the normal creatinine (0.9 mg/dL), which is falsely reassuring due to reduced muscle mass—proceed with confirmatory testing using combined creatinine-cystatin C eGFR equations and monitor for early chronic kidney disease.

Understanding the Discrepancy

The creatinine-cystatin C mismatch in your patient is a classic scenario where creatinine underestimates kidney dysfunction:

• Creatinine is heavily influenced by muscle mass 1, 2, 3. In patients with low muscle mass, serum creatinine remains artificially low even when GFR is declining. Studies demonstrate that serum creatinine correlates strongly with lean body mass (r > 0.7), while cystatin C shows minimal association with muscle mass 1, 3.

• Cystatin C is produced by all nucleated cells at a constant rate and is less affected by muscle mass, diet, age, or sex 4. The 2024 KDIGO guidelines specifically recommend cystatin C for confirmatory testing when creatinine-based eGFR is expected to be inaccurate due to extremes of muscle mass 5, 6.

• Your patient's slight increase in cortical echogenicity suggests early parenchymal changes that may not yet be reflected in creatinine but are captured by cystatin C 7.

Calculate Combined eGFR

Use the CKD-EPI creatinine-cystatin C equation (eGFRcr-cys) to get the most accurate assessment 8, 5:

• The 2024 KDIGO guidelines (the most recent and authoritative source) recommend using eGFRcr-cys in clinical situations when eGFRcr is less accurate, specifically including patients with low muscle mass 5, 6.

• This combined equation will likely reveal an eGFR in the 45-60 mL/min/1.73 m² range (CKD stage 3a), given the elevated cystatin C of 1.16 mg/L (reference 0.6-0.95 mg/L) 7, 9.

• Studies show that when creatinine and cystatin C are discordant, the combined equation provides superior accuracy and better predicts adverse outcomes than either marker alone 10, 8.

Potential Causes of Elevated Cystatin-C

Beyond reduced GFR, consider these non-GFR factors that can elevate cystatin C:

Primary consideration:

• Early CKD from remote lithium exposure 11. Lithium causes chronic tubulointerstitial nephropathy with glomerular and interstitial fibrosis, which can progress even after discontinuation. The FDA label warns that morphologic changes with glomerular and interstitial fibrosis have been reported in patients on chronic lithium therapy 11. Your patient's increased cortical echogenicity supports this.

Other factors that can elevate cystatin C (less likely given your patient's profile):

• Thyroid dysfunction (check TSH if not done)
• Corticosteroid use (none reported)
• Smoking status (inquire)
• Obesity (not mentioned)
• Malignancy (no indication)
• Inflammation/high CRP (can check) 4

Clinical Management Plan

Immediate steps:

1. Calculate eGFRcr-cys using the 2021 CKD-EPI equation (race-neutral version) 10, 5. This will provide your baseline true GFR.

2. Repeat testing in 3 months to establish chronicity per KDIGO criteria 6. Two measurements >3 months apart with eGFR <60 mL/min/1.73 m² confirms CKD.

3. Check additional markers:

   • TSH (thyroid dysfunction affects cystatin C)
   • Uric acid (often elevated in lithium-induced nephropathy)
   • Fasting glucose and HbA1c (rule out diabetes)
   • Lipid panel (cardiovascular risk stratification)

Monitoring frequency:

• If eGFRcr-cys confirms stage 3a CKD (45-59 mL/min/1.73 m²) with normal ACR, monitor every 12 months per KDIGO staging grid 8.

• If eGFRcr-cys shows stage 3b (30-44 mL/min/1.73 m²), increase monitoring to every 6 months 8.

Long-term considerations:

• Avoid nephrotoxins: NSAIDs, aminoglycosides, contrast agents (use with caution and adequate hydration)
• Blood pressure control: Target <130/80 mmHg if CKD confirmed 12
• Annual ACR monitoring: Even with normal baseline, as albuminuria can develop 10, 13
• Cardiovascular risk assessment: Elevated cystatin C independently predicts cardiovascular events and mortality 12

Key Clinical Pitfall

Do not rely on creatinine alone in patients with low muscle mass. The 2007 ESH/ESC guidelines explicitly state that "a reduction in glomerular filtration rate and an increase in cardiovascular risk may be inferred from increased serum levels of cystatin C" even when creatinine appears normal 12. Your patient exemplifies this scenario—creatinine of 0.9 mg/dL appears reassuring but masks underlying kidney dysfunction that cystatin C has detected.

The evidence strongly supports that cystatin C is superior to creatinine for detecting early CKD in patients with altered muscle mass 3, 7, 4, 9, and the combined equation provides the most accurate GFR estimation for clinical decision-making 5, 6.