CKD-EPI 2021 Creatinine-Cystatin C

Chronic Kidney Disease Epidemiology Collaboration

Creatinine–cystatin C equation for adults, without a race coefficient

Biomarkers: Creatinine + cystatin C
Validated for: 18+ years
Inputs: Age, Sex, Creatinine, Cystatin C
Reports: mL/min/1.73 m²
Published: 2021

The eGFR guide explains which equation the calculator suggests and indexed and absolute GFR.

Formula

\text{eGFR} = 135 \times \min(\frac{\text{SCr}}{\kappa}, 1)^{\alpha} \times \max(\frac{\text{SCr}}{\kappa}, 1)^{-0.544} \times \min(\frac{\text{SCys}}{0.8}, 1)^{-0.323} \times \max(\frac{\text{SCys}}{0.8}, 1)^{-0.778} \times 0.9961^{\text{age}} \times 0.963^{[F]}

As text: eGFR = 135 × min(SCr/κ,1)^α × max(SCr/κ,1)^(-0.544) × min(Scys/0.8,1)^(-0.323) × max(Scys/0.8,1)^(-0.778) × 0.9961^age × 0.963 [if female]

Constants

Symbol	Meaning	Value
`135`	Base coefficient
$\kappa$	Sex-dependent creatinine coefficient	Female: 0.7mg/dL Male: 0.9mg/dL
$\alpha$	Creatinine exponent when SCr/κ < 1	Female: −0.219 Male: −0.144
`−0.544`	Creatinine exponent when SCr/κ ≥ 1
`0.8`	Cystatin C reference value	mg/L
`−0.323`	Cystatin C exponent when SCys/0.8 < 1
`−0.778`	Cystatin C exponent when SCys/0.8 ≥ 1
`0.9961`	Age decay factor (base for age exponent)	per year
`Sex factor`	Sex-dependent multiplier	Female: 0.963 Male: 1.000
$S_{cr}$	Serum creatinine (input)	mg/dL
`SCys`	Serum cystatin C (input)	mg/L
`Age`	Patient age (input)	years

How it was developed

Refitted without a race term on the CKD-EPI 2012 development data, which had both creatinine and cystatin C: 13 studies with 5,352 adults aged 18 or older, with GFR measured by urinary or plasma clearance of exogenous filtration markers. External validation used 12 studies (4,050 participants).

Clinical notes

Developed with creatinine and cystatin C assays traceable to reference materials
KDIGO 2024 specifies creatinine from the same sample as cystatin C

Limitations

Studied in ambulatory adults without serious coexisting conditions, not acute care
Development data excluded kidney transplant recipients
May err with very low muscle mass, high inflammation, catabolic states or steroids

References

Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737-1749. doi:10.1056/NEJMoa2102953
KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117-S314. doi:10.1016/j.kint.2023.10.018