The stages of chronic kidney disease (CKD) are defined according to the glomerular filtration rate (Table 1). Accurate determination of the GFR is not easy. The most precise method measures inulin clearance and isotope methods give similar results. However, both methods are cumbersome and impractical for daily use. Measurement of serum creatinine is not satisfactory because more than 25% of older patients have normal serum creatinine levels but reduced GFRs. A single determination of the GFR does not exclude acute renal insufficiency.
Renal function can also be estimated using specially derived predictive equations that use not only serum creatinine but also characteristics such as weight, height, race and gender. The equations avoid collection of urine over 24 h.
The Cockcroft–Gault equation was published in 1976 and was widely adopted for estimation of creatinine clearance from serum creatinine levels. The hospitalised patients who were used to develop the Cockcroft–Gault equation had lower muscle mass (creatinine excretion) than healthier individuals in the general population.
The modification of diet in renal disease (MDRD) equation was published in 1999 and later simplified. The equation was developed using patients who had CKD identified by elevated serum creatinine levels and who had a fourfold higher risk for progressing to end-stage renal failure. Despite the selective nature of the equation population, it has been widely advocated that eGFR be reported when<60 mL/min/1.73 m2 instead of reporting serum creatinine levels.
Unfortunately, the predictive capabilities of these formulae are suboptimal. In addition, they are not useful for patients with a GFR >60 mL/min. Different methods can also result in very different values for GFR. For example, a 43-year-old 70-kg male patient with a creatinine level of 264 μmol/L has a glomerular filtration level of 32 mL/min/1.73 m2 if it is calculated by the Cockcroft–Gault equation. The same patient will have a glomerular filtration level calculated by the MDRD equation of 33 mL/min/1.73 m2 if he is African-American and 27 mL/min/1.73 m2 if he is Caucasian.
In 2009, a third equation—chronic kidney disease epidemiology collaboration (CKD-EPI)—was developed in an effort to create a formula more accurate than the MDRD formula, especially when the actual GFR is greater than 60 mL/min/1.73 m2. The CKD-EPI equation performs better than the MDRD equation, especially at higher GFR, with less bias and greater accuracy, and should, therefore, be preferred before contrast administration.
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