Determinants of the serum potassium concentration in chronic kidney disease

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 TR_K/C_cr, which are amounts of potassium excreted and (net) reabsorbed per volume of filtrate (K_s = E_K/C_cr + TR_K/C_cr). We investigated changes in E_K/C_cr, TR_K/C_cr, and K_s through the stages of chronic kidney disease (CKD).

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, cr_s, and cr_u) were used to calculate 1,007 individual values of E_K/C_cr and TR_K/C_cr as K_u×cr_s/cr_u and K_s - E_K/C_cr, respectively. Mean values of E_K/C_cr and TR_K/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).

In comparison to values in CKD stages G1 - 2, E_K/C_cr rose and TR_K/C_cr fell in each higher stage. Decrements in TR_K/C_cr equaled increments in E_K/C_cr in G3a and G3b, and K_s remained stable. In G4 - 5, the ascent of E_K/C_cr exceeded the decline in TR_K/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 TR_K/C_cr were the sole source of differences in K_s.

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