The National Kidney Foundation recommends using the chronic kidney disease epidemiology collaboration, or CKD-EPI, creatinine equation to estimate glomerular filtration rate. GFR is the best overall index of kidney function. The formula is: GFR = 141 × min (Scr /?, 1)? × max(Scr /?, 1)-1.209 × 0.993Age × 1.018 [if female] × 1.159 [if black].
The CKD-EPI creatinine equation uses a two-slope spline to model the relationship between GFR and serum creatinine and a different relationship between age, sex and race. The equation performs better and with less bias than other equations, particularly when a patient has a high GFR, which results in reduced misclassification of CKD.
Creatinine-based kidney function estimates are useful only when the renal function is stable, as serum creatinine values while kidney function is changing provide inaccurate kidney function estimates. Situations in which creatinine-based equations are not recommended include pregnant women, individuals with serious comorbid conditions and hospitalized patients, particularly those with renal failure. The creatinine-based equations should be used only for patients whose creatinine concentrations are stable. Persons with extremes in muscle mass and diet, such as individuals who are paraplegics, amputees, bodybuilders or obese, and those who have a neuromuscular disorder or muscle-wasting disease are also not good candidates for the creatinine-based equations.