Serum Cystatin C & eGFR-cys
Cystatin C is a filtration marker produced by all cells in the body at a steady rate. Unlike creatinine, it is not affected by muscle mass, diet, or age, making it the premier confirmatory test for kidney function.
Reference ranges can vary slightly between clinical laboratories. Single readings should always be evaluated alongside your overall health history and repeat testing.
What Does Serum Cystatin C & eGFR-cys Measure?
Cystatin C is a low-molecular-weight protein produced continuously by all nucleated cells in the body. Because its production rate is remarkably steady and unaffected by skeletal muscle bulk, physical activity, or dietary animal meat, blood levels reflect kidney filtration efficiency with exceptional precision. Healthy kidneys freely filter cystatin C out of the blood and break it down inside proximal tubules.
When kidney filtration declines, blood levels of cystatin C rise. In individuals where creatinine is unreliable (such as bodybuilders, very athletic individuals, frail elderly people, amputees, or strict vegetarians), creatinine can produce a falsely high or falsely low eGFR. The 2024 KDIGO guidelines officially recommend checking cystatin C to verify eGFR whenever creatinine-based results are borderline or uncertain.
Reference Ranges & Interpretation
| Patient Group / Category | Value Range | Status | Clinical Meaning |
|---|---|---|---|
| Standard Healthy Adult | 0.62 - 1.15 mg/L | Normal | Optimal glomerular filtration. Corresponds to eGFR-cys of 90 or higher. |
| Mild to Moderate Elevation | 1.16 - 1.80 mg/L | Borderline High | Indicates mild to moderate filtration decline, approximately equivalent to CKD Stage 3. |
| Significant Elevation | Greater than 1.80 mg/L | High | Reflects substantial loss of filtration capacity (Stage 4 or 5 CKD). |
What Causes High Cystatin C Levels?
An elevated result does not always mean permanent kidney disease. Clinicians evaluate both kidney-specific conditions and non-kidney factors:
Chronic Kidney Disease (CKD)
Loss of functioning glomeruli directly slows cystatin C clearance, driving serum concentrations upward.
Acute Kidney Injury (AKI)
Cystatin C levels rise 24 to 48 hours earlier than serum creatinine during acute drops in kidney filtration.
Uncontrolled Hyperthyroidism
Excess thyroid hormone speeds up cellular metabolism and increases cystatin C production.
High-Dose Corticosteroid Therapy
Steroid medications can stimulate transcription of the cystatin C gene, creating higher blood levels independent of kidney function.
Severe Active Systemic Inflammation
Massive inflammatory cytokine release can slightly increase cellular turnover and cystatin C generation.
What Causes Low Cystatin C Levels?
Hypothyroidism (Underactive Thyroid)
Low thyroid hormone levels slow cellular protein production, lowering serum cystatin C concentrations.
Normal High Kidney Clearance
Healthy filtration barriers eliminate cystatin C rapidly from circulation.
What This Means for Kidney Health
When doctors calculate eGFR using both creatinine and cystatin C together (the 2021 CKD-EPI creatinine-cystatin C combined equation), the accuracy is significantly higher than using either test alone. If your creatinine-based eGFR indicates Stage 3 CKD but your cystatin C-based eGFR is normal, you may not actually have kidney disease, saving you from unnecessary medical worry or medication restrictions.
Questions to Ask Your Doctor
Bring these practical, clinically focused questions to your next appointment to discuss your Cystatin C results:
- Discussion Point: Can we calculate my combined eGFR using both creatinine and cystatin C to verify my kidney filtration?
- Discussion Point: Could my muscle mass, physical training, or dietary meat intake be skewing my routine creatinine reading?
- Discussion Point: Does my cystatin C reading confirm the CKD stage indicated by my creatinine?
- Discussion Point: Are thyroid conditions or steroid medications affecting my cystatin C results?
Frequently Asked Questions
Why isn't cystatin C checked on every routine blood test instead of creatinine?
Cystatin C is a more expensive test and requires specialized laboratory equipment compared to standard automated creatinine chemistry. For routine wellness exams, creatinine remains the initial screening tool, while cystatin C is reserved for confirmatory testing and nephrology evaluations.
Do I need to fast before a cystatin C blood test?
No fasting is required for cystatin C. In addition, unlike creatinine, you do not need to avoid eating meat or avoid exercising the day before the test, because dietary protein and muscle breakdown do not affect cystatin C levels.