Cystatin C with eGFR — Kidney Function Marker

What is Cystatin C?

Cystatin C is a small protein produced by nearly all nucleated cells at a relatively constant rate. It is filtered by the kidneys and is used as a marker of glomerular filtration rate, or GFR. When kidney filtration decreases, cystatin C levels tend to rise.

Unlike creatinine, cystatin C is less affected by muscle mass, body size, diet, protein intake, and some of the factors that can make creatinine-based kidney estimates misleading. For this reason, cystatin C can provide a more sensitive or more accurate estimate of kidney function in selected patients.

Why Test Cystatin C?

Cystatin C is useful when a more refined kidney-function assessment is needed beyond routine creatinine and eGFR. It may be especially helpful when creatinine may underestimate or overestimate kidney function, such as in older adults, people with low muscle mass, highly muscular individuals, malnourished patients, chronic illness, weight loss, or unusual protein intake.

Testing may be considered for:

• Confirming kidney function when creatinine-based eGFR is borderline or questionable
• Earlier detection of reduced kidney filtration
• Monitoring kidney status in patients using multiple medications or supplements
• Evaluating kidney reserve before more intensive detoxification, metabolic, or supplement protocols
• Assessing cardiorenal and metabolic risk more accurately
• Clarifying kidney function when creatinine appears normal but clinical concern remains

Functional Medicine Relevance

Cystatin C can be a valuable addition to a functional medicine assessment because kidney filtration affects detoxification capacity, mineral handling, blood pressure regulation, metabolic health, cardiovascular risk, medication clearance, and tolerance of higher-dose nutritional protocols.

In patients using higher-dose minerals, amino acids, detoxification support, methylation support, or oxidative therapies, cystatin C may help confirm that kidney filtration is adequate and that creatinine alone is not giving a falsely reassuring impression.

Related Testing to Consider

Cystatin C is often most useful when interpreted alongside:

• CMP with creatinine and eGFR
• Urinalysis
• Urine albumin/creatinine ratio
• BUN/creatinine ratio
• Electrolytes and CO2/bicarbonate
• Blood pressure and metabolic markers
• Fasting glucose, insulin, A1c, lipids, hs-CRP, and uric acid when assessing metabolic/cardiorenal risk

Summary

Cystatin C with eGFR is a practical kidney-function test that may provide a more reliable estimate of filtration than creatinine alone in selected patients. It is especially useful when kidney function needs to be confirmed, when creatinine may be misleading, or when a patient is undergoing a functional medicine program involving detoxification, methylation support, mineral therapy, metabolic correction, or higher-dose supplementation.