Cystatin C
The kidney function test that creatinine misses — muscle-mass independent
Cystatin C is produced at a constant rate by every nucleated cell in the body, independent of muscle mass, age, sex, or diet. This makes it a more accurate GFR estimate than creatinine, especially in elderly patients, sarcopenic individuals, and anyone where creatinine gives a falsely reassuring kidney function result.
Cystatin C is the ideal GFR biomarker: produced at a constant rate by all nucleated cells, freely filtered (not secreted) by the glomerulus, and completely reabsorbed and degraded in the proximal tubule — leaving nothing in the urine. Serum cystatin C reflects GFR almost exclusively, unlike creatinine which is additionally secreted by tubules and greatly influenced by muscle mass. Single creatinine-based eGFR misclassifies CKD stage in up to 30% of patients — cystatin C resolves this.
Reference Ranges
How clinicians interpret Cystatin C results — from optimal to concerning.
⚠ Reference ranges vary by laboratory and assay. Always interpret your result in context of your laboratory's own reference intervals and your clinical presentation.
What raises Cystatin C
Less filtration → cystatin C accumulates. Sensitive even in early CKD where creatinine-eGFR appears normal.
Thyroid hormones increase cystatin C production — can falsely elevate cystatin C by 10–20% without true GFR reduction.
Glucocorticoids increase cystatin C production — steroid therapy can falsely elevate cystatin C 10–20%.
Adipose tissue produces cystatin C — elevated BMI causes mild cystatin C elevation independent of GFR.
What lowers Cystatin C
Reduced thyroid hormone decreases cystatin C production — can cause falsely low cystatin C masking reduced GFR.
Effective CKD management or AKI recovery — cystatin C falls proportionally to GFR improvement.
Conditions this biomarker signals
When Cystatin C is outside normal range, these are the most clinically significant possibilities.
Creatinine-eGFR overestimates GFR in low muscle mass — cystatin C unmasks true CKD stage. Critical for drug dosing.
Cystatin C rises earlier than creatinine in AKI — detects kidney decline before creatinine becomes clearly abnormal.
More accurate GFR estimation before surgery — critical for contrast agent safety, NSAID use, and anaesthetic dosing.
Which tests measure this biomarker
Cystatin C may be included in or ordered alongside these panels.
Not in standard metabolic panels. Request when creatinine-eGFR may be inaccurate — elderly, muscle-wasted, or obese patients.
Most accurate eGFR estimation — recommended by KDIGO 2021 for confirmatory CKD staging.
Why cystatin C measures GFR more accurately than creatinine
Creatinine's core limitation as a GFR marker is its dependence on muscle mass — a muscular 25-year-old and a frail 85-year-old may have identical serum creatinine despite very different GFRs. Cystatin C avoids this because every nucleated cell produces it at a constant rate independent of muscle mass, sex, or dietary protein. In the kidney, cystatin C is completely filtered (no tubular secretion), then fully reabsorbed and catabolised in the proximal tubule — nothing enters the urine. Serum accumulation therefore reflects filtration rate almost exclusively. The 2021 KDIGO guidelines now recommend cystatin C confirmation of CKD stage in all new diagnoses.
Creatinine misclassifies CKD stage in 30% of patients
Studies comparing creatinine-eGFR to iothalamate clearance (gold standard GFR) show misclassification in up to 30% of patients — particularly sarcopenic, obese, or high-muscle-mass individuals. The 2021 KDIGO guidelines recommend cystatin C confirmation of all new CKD diagnoses, and the combined CKD-EPI creatinine-cystatin C equation is more accurate than either alone.
Why size matters: 13 kDa is the ideal glomerular filter size
Cystatin C (13 kDa, pI 9.3) is freely filtered by the glomerular capillary wall due to its small size. Its positive charge at physiological pH facilitates binding to the glomerular basement membrane filtration slit — ensuring complete filtration independent of flow rate. By contrast, albumin (67 kDa, negatively charged) is largely excluded by the glomerular barrier — a property used in UACR to detect barrier damage.
Thyroid status affects cystatin C without affecting creatinine
Hyperthyroidism increases cystatin C production (thyroid hormones upregulate all protein synthesis) without affecting GFR — causing false elevation. Hypothyroidism decreases production — causing false lowering that can mask reduced GFR. Always consider thyroid status when interpreting cystatin C, and measure TSH if results seem discordant.
Clinical use — when and why this is ordered
How clinicians use Cystatin C in practice — the real-world scenarios where it changes decisions.
CKD staging in elderly / muscle-wasted patients
Creatinine-eGFR can overestimate GFR by an entire CKD stage in sarcopenic patients — cystatin C gives the true picture for drug dosing decisions.
Contrast nephropathy risk assessment
Accurate pre-procedural GFR estimation for CT contrast, gadolinium MRI, and cardiac catheterisation — determines contrast volume limits and hydration protocols.
Early AKI detection in ICU
Cystatin C rises 12–24h earlier than creatinine in AKI — enabling earlier intervention, fluid resuscitation, and nephrotoxic drug adjustment.
Cardiovascular risk (independent predictor)
Elevated cystatin C is an independent predictor of cardiovascular mortality in CKD patients — beyond what eGFR or creatinine predict.