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.

Apart from the 24-hour creatinine clearance, also formulas to estimate the glomerular filtration rate (GFR) are increasingly used to get an impression from renal function in recent years. Based on these renal function measurements, clinical decisions are made as well as drug dose adjustments. The use of reliable serum creatinine measurements is therefore important to get accurate renal function estimates. However, serum creatinine is one of the most variable routine laboratory tests.

The importance of calibration to a traceable reference measurement of serum creatinine has been stressed. However, this standardization does not correct for analytical non-specificity problems, which occurs in certain techniques to measure creatinine, leading to under- or overestimation of the true creatinine concentration.

The aim of this cross-sectional observational study is to examine the degree of variability between diverse methods to measure creatinine in plasma and urine in a heterogenous group of Caucasian people with and without renal function loss and the influence hereof on the 24-hour creatinine clearance and the Modification of Diet in Renal Disease study equation and the consequences for chronic kidney disease staging.

Inclusion Criteria:

• All patients >18 years presenting at the laboratory facility for a 24-hour creatinine clearance, or
• Persons >18 years, undergoing a renal replacing therapy by means of hemo-/peritoneal dialysis that had a KT/V appointment between May 2010 - January 2011

Exclusion Criteria:

• Patients <18 years

All patients presenting at the laboratory facility for a 24-hour creatinine clearance and patients in the dialysis population of the Isala Clinics who came for their periodical KT/V control, were informed about the study and asked to participate. Moreover, 20 'healthy' volunteers (including the investigators of this study and staff working at the clinical chemistry department of the Isala clinics) participated in this study.