Reflects Kidney Filtration, GFR Assessment & Renal Risk

Written by Medically Reviewed by Dr. Mitch Ghen | Sep 9, 2026, 1:00:21 PM

What is Cystatin C ?

Cystatin C is a low-molecular-weight protein produced at a near-constant rate by nearly all nucleated cells. It is freely filtered by the glomeruli and then almost completely reabsorbed and metabolized by the proximal tubules. Because its blood concentration is closely related to glomerular filtration rate (GFR), Cystatin C provides a useful marker for assessing kidney filtration.

Unlike creatinine, Cystatin C is substantially less influenced by muscle mass, making it particularly useful when creatinine-based estimates may not accurately reflect kidney function. This includes older or frail adults, individuals with sarcopenia or malnutrition, amputees, bodybuilders, patients at extremes of body size, and those with unusual diets or medications that affect creatinine secretion.

Cystatin C vs. Cystatin C with eGFR: Cystatin C reports the measured serum concentration of the protein. Cystatin C with eGFR pairs that measurement with a calculated estimated glomerular filtration rate, translating the Cystatin C result into a clinically familiar measure of kidney filtration. When appropriate, equations incorporating both Cystatin C and creatinine can further improve GFR estimation.

Why Test for for Cystatin C ?

Cystatin C testing helps clinicians assess and monitor kidney filtration, particularly when creatinine may provide an incomplete or misleading estimate. Clinical applications include:

• Monitoring kidney function and changes in GFR
• Confirming CKD diagnosis and supporting more accurate staging
• Clarifying kidney function when creatinine-based eGFR is unreliable
• Assessing patients with frailty, sarcopenia, malnutrition, amputation, bodybuilding, extreme obesity, or unusual diets
• Supporting medication dosing decisions, particularly when accurate kidney function estimates are needed
• Supporting kidney transplant evaluation and monitoring
• Improving risk stratification for kidney failure, cardiovascular events, heart failure, and mortality
• Providing additional assessment when medications affect creatinine tubular secretion

Clinical Significance of Reverse T3 Levels

Low Cystatin C Levels: Cystatin C levels below approximately 0.6 mg/L generally correspond with higher kidney filtration. An eGFR ≥90 mL/min/1.73 m² (G1) is considered normal or high, while 60–89 mL/min/1.73 m² (G2) represents mildly decreased filtration. An eGFR ≥60 does not establish chronic kidney disease without other evidence of kidney damage.
High Cystatin C Levels: Cystatin C levels above approximately 1.0 mg/L generally correspond with reduced kidney filtration. Declining eGFR is categorized as 45–59 (G3a), 30–44 (G3b), 15–29 (G4), and <15 mL/min/1.73 m² (G5). Persistently reduced eGFR may support CKD diagnosis and staging when interpreted with other kidney markers and clinical history.

Actionable Insights

Cystatin C is particularly valuable when creatinine may misclassify kidney function because of differences in muscle mass, nutrition, body composition, diet, or medication use. More accurate assessment of filtration can improve CKD staging, patient counseling, medication safety, risk assessment, and decisions about earlier intervention.

Cystatin C with eGFR provides greater clinical context than the Cystatin C concentration alone, because the measured result is converted into an estimate of kidney filtration. When kidney function estimates affect diagnosis, staging, medication dosing, or treatment decisions, clinicians may also consider a combined creatinine-Cystatin C eGFR equation for greater accuracy.

For practices monitoring kidney function, faster in-house Cystatin C testing compared with send-out testing can also support quicker clinical decision-making and follow-up.

Serum vs. Urine Testing: Serum Cystatin C is used for estimating GFR and assessing glomerular filtration. Urinary Cystatin C has different applications and may provide information about proximal tubular dysfunction; it should not be used interchangeably with serum Cystatin C for routine GFR assessment.

Optimal Ranges

Cystatin C	eGFR
~0.6–1.0 mg/L	≥90 mL/min/1.73 m²

Note: Reference ranges vary by laboratory. In functional medicine, some providers may consider a narrower interpretive window to help guide clinical context. These values are not standardized and should be interpreted in conjunction with the ordering provider’s judgment.

“Cystatin C is not production-constant. Thyroid status, high-dose corticosteroids, inflammation, obesity, smoking, and some high-turnover states move the number independently of GFR.”

Dr. Mitch Ghen - Director of Physician Consultations, Access Labs

Disclaimer: Content on the Access Labs blog is for informational purposes only and reflects the views of individual contributors, not necessarily those of Access Labs. We do not endorse specific treatments, products, or protocols. This content is not a substitute for professional medical advice. Always consult a qualified healthcare provider regarding any medical concerns.

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