CKD-EPI 2021 Creatinine-Cystatin C eGFR Calculator
CKD-EPI 2021 Creatinine-Cystatin C eGFR Calculator
The most accurate routine estimate of GFR, combining creatinine and cystatin C in the 2021 race-free equation.
CKD-EPI 2021 Creatinine-Cystatin C eGFR
Most accurate equationMale, age 62, creatinine 1.3 mg/dL, cystatin C 1.3 mg/L
Formula
- κ
- 0.7 in women, 0.9 in men
- α
- −0.219 in women, −0.144 in men
- Scr
- mg/dL
- Scys
- mg/L
- why it works
- the two markers have unrelated confounders, so averaging their information cancels much of the error in each
Worked example
Male, age 62, creatinine 1.3 mg/dL, cystatin C 1.3 mg/L
κ = 0.9, α = −0.144 · Scr/κ = 1.444 · Scys/0.8 = 1.625
135 × 1.444^−0.544 × 1.625^−0.778 × 0.9961^62 = 59 mL/min/1.73 m²
Creatinine alone gives 62, cystatin C alone 54 — the combined estimate sits between them
Accuracy compared
| Equation | P30 accuracy | Use |
|---|---|---|
| Creatinine alone | ≈ 87% | Routine screening and staging |
| Cystatin C alone | ≈ 86% | When muscle mass is atypical |
| Creatinine + cystatin C | ≈ 92% | Confirmation, and any consequential decision |
Why combining the two markers works
Creatinine and cystatin C are wrong for different reasons. Creatinine is distorted by muscle mass, diet and tubular secretion; cystatin C by corticosteroids, thyroid state, inflammation and adiposity. Because those error sources are largely unrelated, an equation that uses both cancels much of the noise in each, and the accuracy gain is real: roughly 92% of estimates fall within 30% of measured GFR, against about 87% for either marker alone.
KDIGO recommends this equation wherever a creatinine-based eGFR will drive a consequential decision — transplant listing or donor evaluation, chemotherapy dosing, entry to a clinical trial, or a result that seems inconsistent with the clinical picture. It is also the sensible default in any patient whose body habitus makes the creatinine estimate suspect.
The practical obstacles are cost and availability rather than performance. Cystatin C is several times the price of creatinine and is not on every laboratory’s routine panel, so it tends to be used as a confirmatory rather than a screening test. Where the two estimates disagree markedly, that disagreement is itself informative: it usually points to a confounder acting on one marker, and identifying which one explains more than either number alone.
Frequently asked questions
Is the combined equation more accurate?
Yes. About 92% of its estimates fall within 30% of measured GFR, against roughly 87% for creatinine or cystatin C alone, because the two markers have largely independent sources of error.
When does KDIGO recommend using it?
To confirm a creatinine-based eGFR whenever the result will drive a consequential decision — transplant listing, donor evaluation, chemotherapy dosing — or when the estimate does not fit the clinical picture.
What if creatinine and cystatin C estimates disagree?
A large discrepancy usually means a confounder is acting on one marker. Look for atypical muscle mass or a secretion-blocking drug on the creatinine side, and corticosteroids, thyroid disease or inflammation on the cystatin C side.
Why is cystatin C not measured routinely?
Cost and availability. It is several times the price of creatinine and is not on every laboratory’s standard panel, so it is generally used as a confirmatory rather than a screening test.
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References
- Inker LA et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–49.
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.
- NIDDK. Estimated glomerular filtration rate equations for adults.