• Fernando Caravaca-Fontán, Julián Valladares, Rosa Díaz-Campillejo, Sergio Barroso, Enrique Luna, and Francisco Caravaca.
• Servicio Nefrología, Hospital Infanta Cristina, Badajoz, España. Electronic address: fcaravacaf@gmail.com.
• Nefrologia (Engl Ed). 2020 Mar 1; 40 (2): 152-159.

IntroductionHyperkalemia (HK) is a common electrolyte disorder in chronic kidney disease (CKD), mainly in the advanced stages. A positive potassium balance due to reduced renal excretory capacity is likely the main pathogenic mechanism of HK. Research into the relative role of each pathogenic element in the development of HK in CKD may help to implement more suitable therapies.ObjectiveTo investigate renal potassium handling in advanced CKD patients, and to determine the differences between patients with or without HK.Material And MethodsCross-sectional observational study in adult patients with stage 4-5 CKD pre-dialysis. Selection criteria included clinically stable patients and the ability to collect a 24hour urine sample correctly. Blood and urinary biochemical parameters were analysed including sodium and potassium (K). Fractional excretion of K (FEK) and K load relative to glomerular filtration (Ku/GFR) were calculated. HK was defined as a serum K concentration ≥5.5mmol/l.ResultsThe study group consisted of 212 patients (mean age 65±14 years, 92 females) with a mean GFR of 15.0±4.2ml/min/1.73m2. 63 patients (30%) had HK. Patients with HK had lower mean bicarbonate levels with respect to patients with normal K levels (NK) (20.3±3.1 vs. 22.8±3.2 mEq/l, P<.0001), but no differences were noted in total urinary sodium and K excretion. While mean FEK values were lower in patients with HK (32.1±12.1% vs. 36.4±14.3%, P=.038), Ku/GFR values were significantly greater with respect to the NK subgroup (4.2±1.5 vs. 3.7±1.4 mmol/ml/min, P=0,049). FEK showed a strong linear correlation with Ku/GFR (R2=0.74), and partial linear regressions demonstrated that at a similar Ku/GFR level, the FEK of patients with HK was lower than that of NK patients. By multivariate linear and logistic regression analyses, both FEK and Ku/GFR were shown to be the main determinants of K serum levels and HK.ConclusionsAlthough the K load relative to glomerular filtration (Ku/GFR) is an important determinant of HK in advanced CKD, the most noteworthy characteristic associated with HK in these patients was the limitation of compensatory urinary K excretion, as indicated by lower FEK.Copyright © 2019 Sociedad Española de Nefrología. Published by Elsevier España, S.L.U. All rights reserved.

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