Cystatin C
The cystatin C test measures serum concentration of a low-molecular-weight protease inhibitor freely filtered by the glomerulus. Unaffected by muscle mass or tubular secretion, it estimates GFR more accurately than creatinine in older adults, children, and patients with altered body composition, detects early filtration loss, and is incorporated into combined eGFR equations to refine CKD staging…
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Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the Cystatin C test?
The Cystatin C test measures the serum concentration of a small protease inhibitor whose level is determined almost exclusively by glomerular filtration. It is ordered to estimate kidney function more accurately than creatinine alone, particularly when muscle mass, age, or illness renders creatinine unreliable, and to detect early or mild loss of filtration that creatinine-based estimates may miss. Cystatin C addresses the well-recognised limitations of creatinine: it is not influenced by muscle mass, diet, or tubular secretion, so it performs better in older adults, children, patients with cirrhosis or malnutrition, and those at the extremes of body composition. The combined creatinine-cystatin C eGFR equation is now recognised by KDIGO as offering the most accurate filtration estimate across populations, and cystatin C is increasingly used to confirm CKD staging, guide medication dosing, and refine cardiovascular and progression risk when a creatinine-based eGFR sits in an uncertain range.
What does the Cystatin C test measure?
Cystatin C is a low-molecular-weight cysteine protease inhibitor produced at a constant rate by all nucleated cells. It is freely filtered at the glomerulus, then reabsorbed and catabolised by proximal tubular cells, so its serum concentration reflects glomerular filtration rate. Unlike creatinine, cystatin C is not secreted by the tubules and is minimally influenced by muscle mass, making it a particularly useful filtration marker in populations with reduced or variable muscle — children, older adults, individuals with chronic illness or limb amputation, and those with extremes of body composition.
Cystatin C is the preferred filtration marker when creatinine-based estimates are unreliable or when early or mild renal dysfunction must be detected with greater sensitivity. It is incorporated into the CKD-EPI cystatin C and combined creatinine-cystatin C equations, which improve GFR estimation across diverse populations. Practitioners use cystatin C to confirm borderline creatinine-based eGFR, to assess kidney function in patients with cirrhosis (where creatinine underestimates impairment), and to refine risk stratification for cardiovascular and progression outcomes.
Why is the Cystatin C test ordered?
- Confirmation or refinement of kidney function in patients with borderline creatinine-based eGFR (particularly 45–75 mL/min/1.73 m²) to clarify CKD staging
- Assessment of GFR in older adults, where age-related sarcopenia causes creatinine to overestimate true filtration
- Evaluation of kidney function in patients with cirrhosis, chronic illness, malnutrition, limb amputation, or neuromuscular disease, where creatinine is unreliable
- Early detection of kidney dysfunction in individuals at high cardiovascular or diabetic nephropathy risk, where cystatin C identifies impairment before creatinine rises
- Dosing guidance for renally cleared medications when precise GFR estimation is critical and creatinine-based equations are suspect
- Paediatric nephrology assessment, where muscle mass varies widely and cystatin C provides a more stable filtration marker
- Risk stratification for cardiovascular events and CKD progression, as cystatin C independently predicts outcomes beyond creatinine-based eGFR
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy at the processing laboratory's draw site (LabCorp, Access Medical Laboratories, or Boston Heart Diagnostics, depending on routing); no fasting or special preparation required. Unlike creatinine, cystatin C is unaffected by dietary protein or recent meat intake.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Cystatin C is stable refrigerated for several days and frozen for months. Note the performing laboratory and assay platform for serial comparability, as reference intervals and eGFR equations are platform-specific.
What can affect Cystatin C results?
Hyperthyroidism can raise cystatin C independently of GFR, while hypothyroidism may lower it; thyroid status should be considered when interpreting results in patients with known or suspected thyroid disease.
High-dose corticosteroids increase cystatin C production, leading to higher serum levels that overestimate true renal impairment; the effect is dose-dependent.
Some studies report modestly higher cystatin C in obesity, though the effect is smaller than the muscle-mass confounding seen with creatinine; the combined equation mitigates this.
Acute inflammation, sepsis, and some malignancies can transiently raise cystatin C through increased production, so interpret with caution in acutely unwell patients.
Cystatin C assays are standardised to the ERM-DA471/IFCC reference material, but residual between-method differences exist; serial measurements should use the same assay platform where possible.
What do Cystatin C results mean?
Elevated
Reduced glomerular filtration: acute kidney injury, chronic kidney disease of any cause (diabetic nephropathy, glomerulonephritis, hypertensive nephrosclerosis, polycystic kidney disease), and any condition impairing filtration. Cystatin C rises earlier than creatinine in mild dysfunction, so an elevated value with normal creatinine suggests early impairment. Non-renal causes — hyperthyroidism, high-dose corticosteroids, inflammation — should be excluded.
Normal
A normal cystatin C supports preserved glomerular filtration, but a single normal value does not exclude all kidney disease — tubular, interstitial, or structural abnormalities may exist without filtration loss. Interpret alongside urinalysis, albuminuria, and imaging as indicated.
Reduced
Low cystatin C is uncommon and not usually clinically significant; it may be seen in hypothyroidism. Interpret in the context of thyroid function and clinical presentation.
Note: Cystatin C is most valuable when combined with creatinine in the CKD-EPI creatinine-cystatin C equation, which improves accuracy and reduces misclassification. It is not a direct GFR measurement; measured GFR by inulin or iothalamate clearance remains the gold standard when absolute precision is required. Thyroid status, corticosteroid use, and acute illness should be noted, as each can alter cystatin C independently of filtration.
Frequently asked questions
Cystatin C is not influenced by muscle mass, age-related sarcopenia, or dietary protein, so it provides a more accurate estimate of kidney function in older adults, children, patients with cirrhosis or malnutrition, and those at extremes of body composition where creatinine is unreliable.
How to access the Cystatin C test
The Cystatin C reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. (2024)
- Inker LA, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med. (2021)
- Shlipak MG, et al. Cystatin C versus creatinine in determining risk based on kidney function. N Engl J Med. (2013)
- Stevens LA, et al. Evaluating the performance of equations for estimating glomerular filtration rate. J Nephrol. (2008)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.