Determinants of the serum potassium concentration in chronic kidney disease

Background: If C-cr is creatinine clearance, a surrogate for glomerular filtration rate (GFR), the serum potassium concentration (K-s) is the sum of E-K/C-cr and TRK/C-cr, which are amounts of potassium excreted and (net) reabsorbed per volume of filtrate (K-s = E-K/C-cr + TRK/C-cr). We investigated changes in E-K/C-cr, TRK/C-cr, and Ks through the stages of chronic kidney disease (CKD). Materials and methods: We performed a retrospective study of 452 patients with CKD stages G1 - 5. Simultaneous measurements of serum and urine potassium and creatinine concentrations (K-s, K-u, crs, and cru) were used to calculate 1,007 individual values of E-K/C-cr and TRK/C-cr as K(u)xcr(s)/cru and K-s - E-K/C-cr, respectively. Mean values of E-K/C-cr and TRK/C-cr were determined in CKD stages G1 - 5. Within each stage, means of the ratios were also ascertained in subsets with hyperkalemia (K-s > 5.1 mmol/L), normokalemia (K-s 3.8 - 5.1 mmol/L), and hypokalemia (K-s < 3.8 mmol/L). Results: In comparison to values in CKD stages G1 - 2, E-K/C-cr rose and TRK/Ccr fell in each higher stage. Decrements in TRK/C-cr equaled increments in E-K/C-cr in G3a and G3b, and Ks remained stable. In G4 - 5, the ascent of E-K/C-cr exceeded the decline in TRK/C-cr, and K-s rose accordingly. Within each CKD stage, E-K/C-cr was remarkably similar in the three kalemic subsets; consequently, differences in TRK/C-cr were the sole source of differences in Ks. Conclusion: E-K/C-cr rises and TRK/C-cr falls through the stages of CKD. Ks remains stable in stages G3a - 3b in association with equal and opposite changes in E-K/C-cr and TRK/C-cr. In stages G4 - 5, Ks increases progressively because E-K/C-cr rises more than TRK/C-cr falls. Within each CKD stage, differences in TRK/C-cr account entirely for differences in Ks among hyper-, normo-, and hypokalemic subsets. Causes of variability of TRK/C(cr )require additional investigation.