Cystatin-C
Cystatin-C
What it is
Cystatin-C is a small protein produced at a constant rate by all nucleated cells throughout the body. It is freely filtered by the kidney's glomeruli and almost entirely reabsorbed and degraded in the renal tubules — meaning virtually none returns to the blood under normal circumstances. Its concentration in blood therefore reflects how efficiently the kidneys are filtering: when filtration declines, cystatin-C accumulates. This makes it a direct, continuous readout of glomerular filtration rate (GFR) — the key measure of kidney function. The critical advantage over creatinine — the standard kidney marker in most clinical panels — is that cystatin-C production is not significantly influenced by muscle mass, age, sex, diet, or physical activity. Creatinine is a byproduct of muscle metabolism. Muscular individuals have higher creatinine independent of kidney function; elderly or frail individuals have lower creatinine despite reduced filtration because they have less muscle to generate it. A young athlete and an elderly woman can have identical creatinine levels but vastly different kidney function. Cystatin-C is blind to these confounders.
Why we measure it
Standard clinical panels routinely use creatinine to estimate GFR, but the confounding by muscle mass means that early kidney function decline is frequently missed — particularly in lean, elderly, or female patients. A landmark pooled analysis by Stevens et al. in the American Journal of Kidney Diseases (2008), combining data from 3,418 individuals with chronic kidney disease, demonstrated that cystatin-C-based GFR estimates are substantially more accurate than creatinine-based estimates, particularly in detecting CKD stages 2 and 3 where standard creatinine often appears normal. Beyond its role as a kidney marker, cystatin-C independently predicts cardiovascular mortality. A study by Shlipak et al. in the New England Journal of Medicine (2005) examined 4,637 adults over 65 and found that elevated cystatin-C predicted death from cardiovascular causes and heart failure more strongly than creatinine-based GFR — even among individuals whose creatinine was in the normal range. This suggests cystatin-C captures subclinical kidney dysfunction that is clinically invisible by standard testing but is already contributing to cardiovascular risk. The 2021 CKD-EPI equation — the most widely adopted GFR estimation formula — now incorporates both cystatin-C and creatinine together, with the combined equation demonstrating superior accuracy to either marker alone.
Why every 3 months
Kidney function tends to change slowly under stable conditions — gradual decline or gradual improvement in response to metabolic changes, blood pressure control, or hydration. Quarterly testing provides adequate resolution to track directional trends without overtesting. It also allows any acute perturbation — a period of dehydration, a medication change, an intercurrent illness — to be identified and placed in context against the previous stable baseline. Annual testing is too infrequent to catch early, reversible declines in filtration before they progress.
What movement means
GFR estimated from cystatin-C is staged according to the Kidney Disease: Improving Global Outcomes (KDIGO) 2012 Clinical Practice Guideline for Chronic Kidney Disease. These stages are defined by KDIGO — not by Nexuses. A single GFR reading outside the normal range is not diagnostic of chronic kidney disease; KDIGO requires the finding to be present for more than three months. The trend across quarterly draws — whether filtration is stable, declining, or recovering — is more informative than any single reading.
Normal or high
eGFR ≥ 90 mL/min/1.73m²
Normal or high kidney filtration. KDIGO G1 stage.
KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease
Mildly decreased
eGFR 60 – 89
Mildly decreased filtration. KDIGO G2. May be normal for age in older adults. Trend direction is the key signal.
KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease
Mildly to moderately decreased
eGFR 45 – 59
KDIGO G3a. Associated with increased cardiovascular risk and early markers of kidney disease complications.
KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease
Moderately to severely decreased
eGFR 30 – 44
KDIGO G3b. Associated with significantly increased cardiovascular and CKD progression risk.
KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease
In the protocol
References
- 1.
Stevens LA, Coresh J, Schmid CH, et al.. “Estimating GFR Using Serum Cystatin C Alone and in Combination with Serum Creatinine: A Pooled Analysis of 3,418 Individuals with CKD.” American Journal of Kidney Diseases. 2008.
- 2.
Shlipak MG, Sarnak MJ, Katz R, et al.. “Cystatin C and the Risk of Death and Cardiovascular Events among Elderly Persons.” New England Journal of Medicine. 2005.
- 3.
Inker LA, Schmid CH, Tighiouart H, et al.. “Estimating Glomerular Filtration Rate from Serum Creatinine and Cystatin C.” New England Journal of Medicine. 2012.
- 4.
Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. “KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.” Kidney International Supplements. 2013.
- 5.
Inker LA, Eneanya ND, Coresh J, et al.. “New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race.” New England Journal of Medicine. 2021.