In clinical practice the 24-hour creatinine clearance is often used to obtain an impression of renal function. However, the glomerular filtration rate is considered to be the best indicator of renal function. For practical and financial purposes, GFR is often estimated by means of serum creatinine based equations. These equations are also used in internation guidelines to define and classify chronic kidney disease. Therefore, accurate creatinine measurements are important to make reliable estimates of renal function. However, previous research has revealed a large variability in creatinine measurements using different measuring methods. In this study the investigators aim to establish the degree of variability in different methods to measure creatinine in a heterogenous group of Caucasian people with and without renal function loss and the influence of this variability on renal function estimating equations and the 24-hour creatinine clearance.