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Category: Liver & Kidney  |  Also known as: CysC, Serum Cystatin C  |  Sample: Serum (fasting not required)

1. What This Test Measures

Cystatin C is a small cysteine protease inhibitor protein with a molecular weight of approximately 13 kDa. It is produced by all nucleated cells in the body at a constant rate through a housekeeping gene, making its production independent of muscle mass, dietary protein intake, age, sex, and body composition. This production constancy, combined with its handling by the kidneys, makes cystatin C a theoretically ideal endogenous marker of glomerular filtration rate (GFR).

Cystatin C's renal handling is essentially ideal for a filtration marker:

• Freely filtered by the glomeruli due to its small size and neutral charge
• Completely reabsorbed by proximal tubular cells and catabolized; not returned to the circulation
• Not secreted into the tubular lumen; unlike creatinine, which is partially secreted and therefore overestimates GFR by approximately 10 to 15%, cystatin C provides a more accurate GFR estimate

When kidney filtration declines, cystatin C accumulates in the bloodstream because less is being filtered per unit time. Because production is constant, any rise in serum cystatin C directly reflects reduced filtration capacity without the confounding effects of muscle mass that limit creatinine's reliability.

2. Why Cystatin C Outperforms Creatinine

3. Standard Lab Reference Range

Cystatin C Level	Classification
0.62 to 1.15 mg/L	Standard reference range (adult)
Above 1.15 mg/L	Elevated: impaired kidney filtration; evaluate with full kidney panel
Below 0.62 mg/L	Low: very high GFR; seen in younger adults with excellent kidney function

Cystatin C can be mildly elevated by hyperthyroidism, hypothyroidism (through decreased GFR), high-dose corticosteroids, and malignancy. These non-renal influences are smaller than the muscle mass effect on creatinine, making cystatin C still more specific for kidney function. When cystatin C is elevated without expected kidney disease, thyroid function should be checked.

4. Optimal Functional Medicine Range

Cystatin C Level	Functional Interpretation
Below 0.75 mg/L	Excellent: high filtration capacity; lowest cardiovascular and renal risk
0.75 to 0.90 mg/L	Optimal: adequate kidney function; monitor annually
0.90 to 1.00 mg/L	Borderline: early filtration decline; evaluate with SDMA; address all risk factors
1.00 to 1.15 mg/L	Low-normal (conventional) but functional decline: meaningful GFR reduction; comprehensive evaluation
Above 1.15 mg/L	Elevated: significant filtration impairment; full kidney panel; nephrology consultation if progressing

5. Cystatin C and Cardiovascular Risk

Beyond its role as a kidney filtration marker, cystatin C independently predicts cardiovascular events, heart failure, and all-cause mortality in multiple large prospective studies, even when creatinine-based eGFR is within the normal range. The mechanisms proposed include:

• Cystatin C inhibits cathepsins B, H, L, and S, protease enzymes involved in extracellular matrix remodeling in atherosclerotic plaques; altered cathepsin activity from elevated cystatin C may influence plaque vulnerability
• Elevated cystatin C more accurately captures the subclinical kidney function decline that carries cardiovascular risk; creatinine-based eGFR misses early kidney disease that is already generating cardiovascular risk through the SDMA, inflammation, and neurohormonal activation pathways of CKD
• In the Cardiovascular Health Study, cystatin C was a stronger predictor of cardiovascular mortality, heart failure, and stroke than creatinine-based eGFR across all GFR ranges

6. What Causes Elevated Cystatin C

• Reduced GFR from any cause: diabetes-related nephropathy, hypertensive nephrosclerosis, glomerulonephritis, aging, acute kidney injury; this is the most common and most clinically significant cause
• Hypothyroidism: reduces GFR through reduced cardiac output and renal blood flow; cystatin C rises in hypothyroidism independently of structural kidney disease; normalizes with thyroid treatment
• Hyperthyroidism: paradoxically also raises cystatin C through non-GFR mechanisms involving increased production; thyroid function evaluation is warranted when cystatin C rises unexpectedly
• High-dose corticosteroids: increase cystatin C production beyond what GFR alone explains; important to note when interpreting cystatin C in patients on significant steroid doses
• Malignancy: some cancers may upregulate cystatin C production; unexplained cystatin C elevation without kidney disease should include malignancy in the differential

7. How to Preserve and Improve Cystatin C-Assessed GFR

8. The Complete Kidney Panel: Pattern Interpretation

Cystatin C is most valuable when it diverges from creatinine-based eGFR, revealing what body composition is masking:

Creatinine eGFR	Cystatin C	CysC eGFR	Clinical Meaning
Above 90	Below 0.90	Above 90	Genuinely normal kidney function confirmed
65 to 75	Below 0.90	Above 85	High muscle mass artifact; creatinine overestimates degree of decline; true GFR is higher than Cr-eGFR suggests
Above 90	Above 1.00	70 to 85	Sarcopenia artifact; creatinine falsely reassures; CysC reveals true early decline
60 to 75	Above 1.10	55 to 70	Genuine early to moderate CKD; both markers agree on meaningful decline
Above 90	Above 1.15	Below 60	CysC identifies Stage 3 CKD that Cr-eGFR completely misses; critical finding in elderly or low-muscle patients

9. Related Lab Tests

10. When Testing Is Recommended

• Any patient where creatinine-based eGFR may be unreliable: elderly adults, frail or sarcopenic patients, highly muscular individuals, malnourished patients
• Creatinine-based eGFR 60 to 75 mL/min: cystatin C determines whether this reflects true kidney decline or muscle mass effect
• Diabetes and hypertension: earlier kidney decline detection enables earlier protective intervention
• Cardiovascular risk stratification: cystatin C adds independent predictive information beyond creatinine-based eGFR
• Suspected hypothyroidism or hyperthyroidism: these conditions affect cystatin C; evaluate alongside TSH
• Any longevity or biological age assessment panel: cystatin C is a primary aging biomarker
• Confirmation of kidney function when creatinine is borderline or discordant with clinical picture

11. Clinical Perspective

12. Frequently Asked Questions