The goal of this observational study is to examine the interaction between the diuretic furosemide and certain toxins called protein-bound uremic toxins (PBUTs) in patients with chronic kidney disease (CKD). The main question it aims to answer is: what is the effect of furosemide on plasma levels of PBUTs in patients with CKD? Besides, the investigators will also look at the effect of furosemide on the excretion of PBUTs via the urine. Participants will be included in the study once they will be prescribed furosemide as part of routine patient care. Before they start with the furosemide treatment, patients will undergo the following: * Blood pressure measurement * Blood sample withdrawal * Urine sample collection * 12-hour urine collection Then, one to four weeks after starting with furosemide treatment, patients will undergo the following: * Blood pressure measurement * Blood sample prior to furosemide intake (Tmin) * Blood sample withdrawal 90 minutes after furosemide intake (Tmax: time at which the highest furosemide plasma level is expected) * Urine sample collection 60-120 minutes after furosemide intake * 12-hour urine collection The investigators expect furosemide to increase plasma levels of PBUTs by decreasing the renal excretion of the toxins.
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Plasma levels of the PBUT indoxyl sulfate
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT p-cresyl sulfate
Timeframe: This will be assessed three time during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT indole-3-acetic acid
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT kynurenic acid
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT L-kynurenine
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT hippuric acid
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).
Plasma levels of the PBUT p-cresyl glucuronide
Timeframe: This will be assessed three times during the study participation period (between 1-4 weeks).