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 · constant
- constant = 1 for men, 0.742 for women
- [SCr] is serum creatinine (in µmol/L)
- 24 hour urine collection using the volume over 24 hours.
CrCl = [CrU] · VU24 / [SCr] · 24hr · 60min
- Urinve volume over 24 hours = VU24
- concentration of the creatinine in the urine = [CrU]
- concentration of serum creatinine = [SCr]:
- An older method for the estimation of the creatinine clearance is the Cockcroft-Gault formula
CrCl = (140 – age) · Kg · constant / [SCr]
- using the patients age (in years) = age
- patient’s weight = Kg
- concentration of serum creatinine = [SCr]
- constant = 1.23 for men, 1.04 for women