What is the reference range for Cystatin C (Cysteine Proteinase Inhibitor C)?

Reference Range for Cystatin C

The reference range for serum cystatin C in adults is 0.51-0.98 mg/L (or 0.53-0.95 mg/L depending on the laboratory), with values typically reported to the nearest hundredth (0.01 mg/L). 1

Standard Reference Intervals

For all adults (combined gender):

• The 95% reference interval is 0.51-0.98 mg/L based on a large validation study of 309 healthy blood donors 1
• An alternative reference range of 0.53-0.95 mg/L has been established, capturing 95% of healthy subjects 2
• The mean value in healthy adults is approximately 0.75 mg/L 2

Age-Related Variations

Cystatin C levels increase slightly with age:

• Adults under 50 years: 0.53-0.92 mg/L 1
• Adults over 50 years: 0.58-1.02 mg/L 1
• Ages 21-30 years: mean of 0.738 mg/L 2
• Ages >50 years: mean of 0.807 mg/L 2

The production rate of cystatin C remains constant between ages 1-50 years, making it a reliable marker across most of adulthood 3

Gender Considerations

A single reference range can be used for both genders without adjustment:

• Women: 0.49-0.94 mg/L (or 68-98 μmol/L creatinine equivalent) 1
• Men: 0.56-0.98 mg/L (or 78-123 μmol/L creatinine equivalent) 1

While some studies show slightly higher mean values in females (0.778 mg/L) compared to males (0.726 mg/L), the difference is small enough that a unified reference range is clinically acceptable for adults under 50 years 2

Clinical Interpretation Thresholds

Key decision points for kidney function assessment:

• Values >0.8 mg/L suggest the 2012 CKD-EPI equation uses this as a reference point in GFR estimation formulas 4
• Cystatin C begins to rise above the normal range when creatinine clearance falls below 94.4 mL/min (135.9 L/day), which is earlier than serum creatinine elevation 5
• When eGFRcys is <60 mL/min/1.73m², this indicates potential chronic kidney disease requiring further evaluation 6

Important Clinical Caveats

Factors that do NOT significantly affect cystatin C:

• Gender (minimal clinical impact) 3, 1
• Muscle mass 3
• Nutritional status (unlike creatinine) 3

Factors that MAY affect cystatin C levels:

• Body mass index shows weak correlation (r=0.155) 2
• Thyroid dysfunction can affect levels independently of renal function 7
• Smoking, obesity, inflammation, and certain malignancies may influence results 7

Laboratory Reporting Standards

Clinical laboratories should:

• Use assays with calibration traceable to international standard reference material 7
• Report cystatin C concentration rounded to the nearest 0.01 mg/L (hundredth of a whole number) 7
• Measure using particle-enhanced nephelometric immunoassay (PENIA) or equivalent standardized methods 8

Urinary Cystatin C Reference Range

For urinary cystatin C measurement:

• Upper reference value: 0.28 mg/L, independent of age and gender 8
• Urinary excretion increases significantly when creatinine clearance falls below 20.8 mL/min (30 L/day) 5