eGFRcr-cys = 135 x min(Scr/κ, 1)α x max(Scr/κ, 1)-1.200 x min(Scys/0.8, 1)-0.323 x max(Scys/0.8, 1)-0.778 x 0.9961Age x 0.963 [if female]

where:

Scr = serum creatinine in mg/dL
κ = 0.7 (females) or 0.9 (males)
α = -0.219 (female) or -0.144 (male)
min(Scr/κ, 1) is the minimum of Scr/κ or 1.0
max(Scr/κ, 1) is the maximum of Scr/κ or 1.0
Scys = serum cystatin C in mg/L
Age (years)

Source: National Kidney Foundation (NKF)