This calculator estimates eGFR from serum cystatin C (mg/L) using the CKD-EPI cystatin C 2012 equation, in mL/min/1.73m². Cystatin C is filtered by the glomerulus much like creatinine but is largely independent of muscle mass.
What Is Cystatin C?
Cystatin C is a small protein produced at a near-constant rate by virtually all nucleated cells in the body. It is filtered freely at the glomerulus and then reabsorbed and broken down in the tubule, so its blood level rises as filtration falls — the same principle that makes serum creatinine useful. Its key advantage is that production does not depend on muscle mass, diet, or sex to the degree creatinine does. That independence is why a cystatin C estimate can confirm or refine a creatinine-based result when muscle mass is unusual.
How the Cystatin C Equation Works
This calculator uses the CKD-EPI cystatin C 2012 equation, which takes serum cystatin C in mg/L together with age and sex, and reports eGFR in mL/min/1.73m². Like the creatinine equation, it uses a spline form that splits the cystatin C value at a threshold and applies an age term so the estimate falls gradually over the lifespan. Notably, it needs no race coefficient and a much smaller sex adjustment than the creatinine equation, because the marker itself is less affected by those factors.
Worked Example
For a 70-year-old patient with a serum cystatin C of 1.4 mg/L, the CKD-EPI cystatin C 2012 equation returns an eGFR of roughly 48 mL/min/1.73m² — a value in the G3a band. If a creatinine-based eGFR in the same patient read closer to 60 because of low muscle mass, the lower cystatin C value would flag that the creatinine estimate is optimistic and prompt a combined estimate.
When Cystatin C Is Preferred
Cystatin C is a confirmatory marker rather than the routine first-line test. Reach for it when a creatinine-based eGFR is likely to mislead:
| Situation | Why creatinine misleads | Cystatin C advantage |
|---|---|---|
| Very low muscle mass | creatinine production is low → eGFR falsely high | muscle-independent |
| Amputation or paralysis | reduced muscle skews creatinine | unaffected by muscle |
| Very high muscle mass | high creatinine → eGFR falsely low | not driven by muscle |
| Confirming a borderline eGFR | a single marker may be imprecise | combined estimate is most accurate |
KDIGO supports a combined creatinine–cystatin C estimate as the most accurate creatinine- marker option and recommends it to confirm a creatinine-only result when staging or a treatment decision depends on the value.
Limitations and Edge Cases
Cystatin C is more muscle-independent than creatinine but is not free of non-renal influences:
- Thyroid disease: hyperthyroidism raises cystatin C and hypothyroidism lowers it, independent of filtration.
- High-dose corticosteroids: can raise cystatin C levels.
- Severe inflammation or some malignancies: may shift the level.
- Acute kidney injury or any non-steady state: like all marker-based equations, it assumes a stable level.
- Pregnancy: filtration rises physiologically and the equation is not validated.
How to Read a Cystatin C eGFR
A cystatin C eGFR maps to the same KDIGO GFR categories used for any estimate, so it can be staged on the same scale as a creatinine-based eGFR:
| eGFR | Stage | Interpretation |
|---|---|---|
| ≥90 | G1 | Normal filtration |
| 60–89 | G2 | Mildly reduced |
| 45–59 | G3a | Mild-to-moderate reduction |
| 30–44 | G3b | Moderate-to-severe reduction |
| 15–29 | G4 | Severely reduced |
| <15 | G5 | Kidney failure |
When a cystatin C estimate and a creatinine estimate disagree, the gap is itself informative: a markedly lower cystatin C value in a patient with low muscle mass suggests the creatinine result was optimistic, and a combined estimate usually lands between the two.
The Combined Creatinine–Cystatin C Estimate
The most accurate creatinine-marker estimate uses both creatinine and cystatin C together. By averaging two markers with different non-renal influences — muscle mass for creatinine, thyroid and inflammatory factors for cystatin C — the combined equation cancels out part of the error in each. KDIGO recommends this combined estimate to confirm a creatinine-only eGFR whenever an accurate value would change a decision, such as staging near a category boundary, transplant evaluation, or dosing a narrow-therapeutic-index drug. It is a confirmatory step, not a routine first-line test for every patient.
How Cystatin C Is Measured
Cystatin C is measured from a routine blood sample and reported in mg/L. Enter that value directly — unlike creatinine, it does not require a mg/dL versus µmol/L conversion. Because assays have become standardised over time, modern cystatin C equations such as the CKD-EPI cystatin C 2012 form rely on calibrated results; an older or non-standardised assay can shift the estimate, so confirm the reference method when a result is unexpected.
Cystatin C vs Creatinine at a Glance
Both cystatin C and creatinine are blood markers that rise as filtration falls, and both feed CKD-EPI equations reported per 1.73m². The difference is what interferes with each. Creatinine is sensitive to muscle mass, sex, and diet, which makes a creatinine-only eGFR unreliable at the extremes of body composition. Cystatin C sidesteps muscle entirely but can be nudged by thyroid disease, high- dose steroids, and inflammation. Because their weaknesses do not overlap, the two markers complement each other, and a combined estimate is more accurate than either alone. In routine care the creatinine estimate remains the first-line test for cost and availability, with cystatin C reserved for confirmation when the creatinine result is in doubt.