Unit Converter
Cystatin C
(Low–Molecular-Weight Protein – Early Marker of Kidney Function & GFR)
Synonyms
- Cystatin C
- CysC
- γ-trace protein
- Cysteine protease inhibitor C
- Serum cystatin C
Units of Measurement
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
1 mg/L = 0.1 mg/dL
1 µg/mL = 1 mg/L
mg/dL = mg% = mg/100 mL
Description
Cystatin C is a 13 kDa low–molecular-weight protein produced steadily by all nucleated cells.
It is freely filtered by the glomeruli and then completely reabsorbed and catabolized by the proximal tubules.
Unlike creatinine:
- Production is independent of muscle mass
- Minimally affected by age, diet, sex, or inflammation
- Rises earlier in kidney dysfunction
- Provides a more accurate estimation of GFR, especially in elderly, children, or low-muscle-mass patients
Cystatin C is now incorporated into CKD-EPI 2021 eGFR equations.
Physiological Role
- Inhibits cysteine proteases
- Regulates extracellular matrix turnover
- Biomarker of non-GFR determinants (inflammation, CV risk)
Cystatin C has no major metabolic function; its clinical value is as a filtration marker.
Clinical Significance
High Cystatin C (Most Important)
Indicates reduced GFR, often earlier than creatinine rises.
1. Acute Kidney Injury (AKI)
- Detects AKI 24–48 hours earlier than creatinine
- Useful in ICU & sepsis patients
- Not affected by muscle mass
2. Chronic Kidney Disease (CKD)
- More sensitive for early CKD stages (1–3)
- Improves accuracy of eGFR in:
- Elderly
- Children
- Frail patients
- Low muscle mass
- Liver disease
- Elderly
3. Cardiovascular Risk Marker
Elevated cystatin C is associated with:
- Atherosclerosis
- Heart failure
- CV mortality independent of GFR
4. Thyroid Disorders
Hyperthyroidism → ↓ CysC
Hypothyroidism → ↑ CysC (independent of kidney function)
Low Cystatin C
Usually benign.
Seen in:
- Hyperthyroidism
- High-dose steroids
- Pregnancy
Low values NOT clinically problematic.
Reference Intervals
(Tietz 8E + IFCC + Mayo + ARUP)
Adults (General)
- 0.53 – 0.95 mg/L
Children
- Slightly lower in young children
- Stable after age 1 year
Elderly
- Mildly higher due to aging kidney and metabolic factors
Critical Cutoffs
No absolute critical value, but:
- Persistently >1.0–1.2 mg/L suggests reduced GFR
Cystatin C & GFR
Cystatin C improves estimation of GFR using:
- CKD-EPI Creatinine-Cystatin C equation (2021)
- CKD-EPI Cystatin C–only equation
These give a more accurate eGFR than creatinine alone.
Unit Meaning
| Unit | Meaning |
| mg/L | milligram per liter |
| mg/dL | milligram per deciliter |
| mg/100 mL | same as mg/dL |
| mg% | mg per 100 mL (mg/dL) |
| µg/mL | microgram per milliliter |
Diagnostic Uses
1. Early AKI Detection
Rises earlier than creatinine in:
- Sepsis
- Surgery
- Trauma
- Contrast nephropathy
2. CKD Diagnosis & Staging
Superior accuracy in:
- Elderly
- Sarcopenia
- Muscle wasting
- Liver disease
- Obesity
- Malnutrition
3. Drug Dosing
Accurate eGFR essential for:
- DOACs
- Chemotherapy agents
- Antibiotics
- Antivirals
4. Cardiovascular Risk Stratification
Higher cystatin C → higher mortality risk independent of kidney function.
5. Transplant Nephrology
Early detection of allograft injury.
Analytical Notes
- Serum or plasma
- Not affected by high bilirubin or hemolysis
- Immunonephelometric / immunoturbidimetric assays common
- Standardization by IFCC reference material ERM-DA471/IFCC
- Highly stable protein
Clinical Pearls
- A normal creatinine with elevated cystatin C may unmask hidden CKD.
- Thyroid disorders significantly influence cystatin C-always interpret with thyroid status.
- Ideal for frail patients where creatinine is falsely low.
- Adding cystatin C to eGFR equations improves prediction of renal events and CV mortality.
Interesting Fact
Cystatin C’s clinical importance has risen dramatically in the last decade—so much so that CKD-EPI now recommends combined creatinine + cystatin C equations as the most accurate measure of GFR.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Renal Markers.
- KDIGO Guidelines - AKI & CKD.
- IFCC ERM-DA471/IFCC Standardization Document.
- Mayo Clinic Laboratories - Cystatin C.
- ARUP Consult - GFR Assessment.
- MedlinePlus / NIH - Cystatin C Test.
- NEJM / Kidney Medicine reviews on Cystatin C.