Cystatin C Laboratory Testing: Reference Ranges and Interpretation
Cystatin C normal reference range in adults is approximately 0.50-1.00 mg/L, with values varying slightly by age and sex, and should be used in conjunction with creatinine-based eGFR equations to improve kidney function assessment accuracy, particularly when eGFR from creatinine is 45-59 mL/min/1.73 m² without albuminuria. 1, 2
Normal Reference Ranges
Adult Population
- General adult reference range: 0.50-1.00 mg/L 3, 4, 5
- The most commonly cited reference interval across multiple studies is 0.51-0.98 mg/L for all adults regardless of gender 5
- Alternative ranges reported include:
Age-Related Variations
Cystatin C levels increase with age, particularly after 60 years, requiring age-adjusted interpretation 4, 7:
- Ages 20-59 years: 0.74 ± 0.10 mg/L (males), 0.65 ± 0.085 mg/L (females) 4
- Ages ≥60 years: 0.83 ± 0.103 mg/L (both sexes) 4
- Ages 4-19 years (pediatric): 0.75 ± 0.089 mg/L 4
Sex Differences
Females tend to have slightly lower cystatin C levels than males in adults under 60 years 3, 4:
- Mean female levels: 0.65-0.778 mg/L 3, 4
- Mean male levels: 0.726-0.87 mg/L 3, 6
- However, some studies show no significant gender difference, allowing use of a single reference range 5, 6
Clinical Interpretation for Kidney Function Assessment
When to Use Cystatin C
KDIGO guidelines recommend measuring cystatin C as a confirmatory test when eGFR from creatinine is 45-59 mL/min/1.73 m² without albuminuria 1:
- This strategy helps identify patients who may be misclassified by creatinine-based equations alone 1
- If eGFR from cystatin C is >60 mL/min/1.73 m², the patient may not have CKD and has very low risk for CKD complications 1
Advantages Over Creatinine
Cystatin C is less dependent on muscle mass than creatinine, making it particularly useful in 8, 1:
- Older adults with reduced muscle mass
- Patients with extremes of body composition
- Situations where creatinine-based estimates are unreliable
The CKD-EPI creatinine-cystatin C combined equation (eGFRcr-cys) is more accurate than creatinine-based equations alone in older people 8:
- Creatinine-based equations can misclassify kidney disease by one stage in >30% of older participants 8
- Combined equations provide more precise GFR estimates 1
Reporting and Calculation
When cystatin C is measured, both eGFRcys (cystatin C alone) and eGFRcr-cys (combined creatinine-cystatin C) should be reported 1:
- Use validated equations with calibration traceable to international standard reference materials 2
- Report eGFR rounded to the nearest whole number in mL/min/1.73 m² 2
- Flag eGFR values <60 mL/min/1.73 m² as low 2
Laboratory Standards
Critical quality requirements for accurate cystatin C measurement 2:
- Assay precision: coefficient of variation <2.0% for cystatin C
- Desirable bias: <3.2% compared with reference methodology
- Measure creatinine on the same sample when cystatin C is ordered
- Separate serum/plasma from red blood cells within 12 hours of venipuncture
Important Caveats
Limitations in Specific Populations
In kidney transplant recipients, cystatin C-based strategies show no added value over creatinine alone for CKD staging 9:
- Similar misclassification rates occur with both approaches (21-30%) 9
- This applies even in the clinically relevant range of 30-45 mL/min/1.73 m² 9
Factors Affecting Cystatin C Levels
Non-renal factors that may influence cystatin C 4:
- Methylprednisolone increases cystatin C levels
- Cyclosporin A decreases cystatin C levels
- Moderate correlation with body mass index (though not clinically significant) 3, 4
Sample Stability
For optimal assay performance, samples should be kept at 4°C for maximum 1 week 10:
- Freeze-thaw cycles may impact stability in some samples
- Storage at 30°C can affect results