The excretory function of the kidneys can change dramatically due to age or disease. It is therefore important for clinicians to be able to determine a particular patient’s renal function before dosing a renally eliminated drug.
Creatinine is a breakdown product of muscle. The rate of breakdown is usually fairly steady except in situations of extreme stress or fasting.
Serum creatinine is nearly 100% cleared via the kidneys and as such, it’s clearance can serve as a measurement of the Glomerular Filtration Rate (GFR).
Estimating creatinine clearances is typically performed in one of three ways:
- Nowadays the estimated GFR (eGFR) using Modification of Diet in Renal Disease (MDRD) is mostly used.
GFR=32788 * [SCr] -1.154 x age -0,203 x constant
constant = 1 for men, 0,742 for women; [SCr] is in µmol/L
- 24 hour urine collection: using the volume over 24 hours = VU24, concentration of the creatinine in the urine = [CrU], and the concentration of creatinine in the serum = [SCr]:
CrCl = [CrU] x VU24 / [SCr] x 24hr x 60min
- An older method for the estimation of the creatinine clearance is the Cockcroft-Gault formula: using the patients age (in years) = age, patient’s weight = Kg, concentration of creatinine in the serum = [SCr]:
CrCl = (140 – age) x Kg x constant / [SCr]
constant = 1,23 for men, 1,04 for women