Determinants of the serum potassium concentration in chronic kidney disease.

Background: If C is creatinine clearance, a surrogate for glomerular filtration rate (GFR), the serum potassium concentration (K) is the sum of E/C and TR/C, which are amounts of potassium excreted and (net) reabsorbed per volume of filtrate (K = E/C + TR/C). We investigated changes in E/C, TR/C, and K 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, K, cr, and cr) were used to calculate 1,007 individual values of E/C and TR/C as K×cr/cr and K - E/C, respectively. Mean values of E/C and TR/C were determined in CKD stages G1 - 5. Within each stage, means of the ratios were also ascertained in subsets with hyperkalemia (K > 5.1 mmol/L), normokalemia (K 3.8 - 5.1 mmol/L), and hypokalemia (K < 3.8 mmol/L).

Results: In comparison to values in CKD stages G1 - 2, E/C rose and TR/C fell in each higher stage. Decrements in TR/C equaled increments in E/C in G3a and G3b, and K remained stable. In G4 - 5, the ascent of E/C exceeded the decline in TR/C, and K rose accordingly. Within each CKD stage, E/C was remarkably similar in the three kalemic subsets; consequently, differences in TR/C were the sole source of differences in K.

Conclusion: E/C rises and TR/C falls through the stages of CKD. K remains stable in stages G3a - 3b in association with equal and opposite changes in E/C and TR/C. In stages G4 - 5, K increases progressively because E/C rises more than TR/C falls. Within each CKD stage, differences in TR/C account entirely for differences in K among hyper-, normo-, and hypokalemic subsets. Causes of variability of TR/C require additional investigation.