The formula
eGFR = 142 × min(Cr/κ, 1)^α × max(Cr/κ, 1)^−1.200 × 0.9938^age × 1.012 if female
Where it comes from
Creatinine leaves the muscle at a fairly constant rate and the kidney clears it, so its concentration in blood mirrors how well the kidney filters. But it also depends on muscle mass: the same figure means different things in a seventy-kilo person and a fifty-kilo one. That is why the equation corrects for age and sex, and why a bodybuilder and a bedbound patient can both be badly estimated, in opposite directions.
How to work it out by hand
- Divide creatinine by κ: 0.7 in women and 0.9 in men
- Apply exponent α to the smaller of that ratio and 1, and −1.200 to the larger
- Multiply by 142 and by 0.9938 raised to the age
- If female, multiply by 1.012 as well
What is worth knowing
The 2021 version is the one that dropped the race coefficient the earlier ones carried, and it is worth knowing why: that factor multiplied Black patients' results by 1.16 on the basis of muscle mass differences measured in small samples, and in practice it meant their kidney disease was diagnosed later and they joined transplant lists later. The new equation uses creatinine, age and sex only. A filtration rate below 60 only defines chronic kidney disease if it persists for three months: one blood test is not enough.