Cystatin C eGFR Calculator (CKD-EPI)

How it works

CKD-EPI 2012 cystatin C: eGFR = 133 × min(Scys/0.8, 1)^−0.499 × max(Scys/0.8, 1)^−1.328 × 0.996^age [× 0.932 if female] · CKD-EPI 2021 creatinine–cystatin C: 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]; κ = 0.7 (F) / 0.9 (M), α = −0.219 (F) / −0.144 (M); Scr in mg/dL, Scys in mg/L

Cystatin C is a small protein made at a steady rate by all nucleated cells and filtered freely by the kidneys. Unlike creatinine it hardly depends on muscle mass or diet, so it gives a better estimate of kidney function in people with very high or low muscle mass, amputations, frailty or unusual diets. KDIGO recommends measuring cystatin C to confirm CKD when the creatinine eGFR is 45–59 without other markers of kidney damage, and the combined creatinine–cystatin C equation (Inker et al., NEJM 2021, without a race term) is the most accurate of the estimating equations.

Worked example

A 50-year-old man with cystatin C 1.0 mg/L and creatinine 80 µmol/L (0.90 mg/dL): the cystatin C equation gives 133 × (1.25)^−1.328 × 0.996^50 = 81 mL/min/1.73 m², and the combined CKD-EPI 2021 creatinine–cystatin C equation gives 93 mL/min/1.73 m² — stage G1 (normal or high). Creatinine alone (CKD-EPI 2021) would give 103.

Sources: Inker LA et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med 2021;385:1737–49 · Inker LA et al. Estimating GFR from serum creatinine and cystatin C. N Engl J Med 2012;367:20–9 · National Kidney Foundation: CKD-EPI creatinine–cystatin C equation (2021)

Frequently asked questions