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. From the 24-hour urine (and for peritoneal dialysis population also from the 24-hour peritoneal dialysate fluid) 4 samples of 4 ml were collected. Moreover, extra blood samples (16 ml in total) were drawn on top of the samples that were necessary for the (routine) 24-hour creatinine clearance.