AC/DC Study: Acidification Test in Patients With Chronic Kidney Disease and Healthy Controls

Patients with CKD: * Male or female adults (≥18 years) * Chronic kidney disease stage 4 (eGFR: 15-30 ml/min/1.73 m2, calculated using the CKD-EPI equation) Healthy controls: * Healthy male or female adults (≥ 18 years) * Normal kidney function (eGFR \> 90 ml/min/1.73 m2, calculated using the CKD-EPI equation) Exclusion Criteria: * CKD patients * Plasma bicarbonate level \< 20.0 mmol/l * Serum potassium \>5.5 mmol/l * Sodium bicarbonate use in the month preceding the test * Heart failure (NYHA III or IV) * Liver cirrhosis (Child Pugh B or C) * Blood pressure \>140/90 mmHg despite the use of 3 different anti-hypertensive drugs * Kidney transplantation * Use of calcineurin inhibitors (these immunosuppressive drugs affect acid-base balance) * Known urea cycle disorder * Alcoholism or drug use * Pregnancy * Current use of antibiotics, NSAIDS or alkalizing drugs (sodium-bicarbonate, citric acid, potassium citrate, acetazolamide) * Inability to adhere to the study protocol (due to language barrier or intellectual disability) Healthy controls: * eGFR \< 90 ml/min/1.73 m2 (calculated using the CKD-EPI equation) * Plasma bicarbonate \< 20 mmol/l * History of, or drugs for, diabetes mellitus * History of chronic diarrheal disease * Ileostomy/colostomy * Known urea cycle disorder * Alcoholism or drug use * Pregnancy * Current use of antibiotics, antihypertensive drugs, NSAIDS or alkalizing drugs (sodium-bicarbonate, citric acid, potassium citrate, acetazolamide) * Inability to adhere to the study protocol (due to language barrier or intellectual disability)