Cystatin C
Modern alternative to serum creatinine for estimating kidney filtration rate. Produced at a constant rate by all nucleated cells and cleared only by the kidneys — independent of muscle mass, so more accurate in elderly, frail, paediatric, amputee, bodybuilder and very-thin populations where creatinine misleads.
What it measures
Cystatin C is a low-molecular-weight protein (13 kDa) produced at a constant rate by all nucleated cells. It is freely filtered at the glomerulus and almost completely reabsorbed and catabolised by proximal tubular cells, so the serum level reflects glomerular filtration rate (GFR) without being skewed by skeletal muscle mass. Modern eGFR equations (CKD-EPI cystatin C, CKD-EPI combined creatinine + cystatin C) give more accurate GFR estimates than creatinine alone — particularly important when the patient's muscle mass is unusual.
What a high value can mean
- Chronic kidney disease — every CKD stage from G1 onward.
- Acute kidney injury — rises faster than creatinine after a renal insult.
- Untreated hyperthyroidism, high-dose corticosteroids — small non-renal upward influence.
- Older age — physiological GFR decline of ~1 mL/min/1.73 m² per year after 40.
What a low value can mean
- Generally favourable; suggests preserved kidney function.
- Hypothyroidism — small downward influence (rarely clinically relevant).
When to discuss with a doctor
Order cystatin C when serum creatinine may mislead — elderly + frail patients, body-builders, paediatric, post-amputation, very low muscle mass (e.g. cachexia), or when an unexpected creatinine result needs confirmation before a drug dose decision (chemo, contrast, vancomycin). Also useful pre-transplant donor screening. Mediora.AI computes cystatin-C-based eGFR when both creatinine and cystatin C are present; CKD staging and management is nephrology.