The potassium value is important to prevent cardiac arrhythmias and sudden cardiac death. In patients with renal failure, the potassium value is not stable and tends to raise. Until now there are no data available if the potassium value has a circadian rhythm and if there are individual changes from day to day.

Potassium and cystatin c is measured every 2 hours during 24 hours, twice. There are two groups of patients: Patients with eGFR < 60 ml/min and patients with eGFR > 60 ml/min

Inclusion Criteria:

• at least 18 years old
• normal renal function and impaired renal function (GFR < 60 ml/min) respectively

Exclusion Criteria:

• pregnancy
• anaemia (Hb<10 mg/dl)

healthy persons compared to patients with renal failure