Determinants of the serum phosphate concentration in chronic kidney disease

If C is creatinine clearance and E and TR are rates of phosphate excretion and reabsorption, the serum phosphate concentration (P ) is the sum of E /C and TR /C , i.e., the amounts of phosphate excreted and reabsorbed per volume of filtrate. At equilibrium, influx of phosphate into plasma determines E , and E /C quantifies the contribution of phosphate influx to P . We used data obtained at 688 clinic visits of 387 patients to analyze the evolution of P in chronic kidney disease (CKD) stages G1 - 5 (dialysis excluded). E /C was calculated as (P ×cr )/cr and TR /C as P -E /C (where u is urine, s is serum, and cr is creatinine). Means of these parameters were plotted against CKD stages, and correlations among variables were determined with regression analyses. In comparison to values in CKD stages G1 - 2, E /C rose and TR /C fell by the same amount in CKD G3a and G3b, and P did not change. In stages G4 and G5, E /C increased sharply, TR /C fell minimally, and P rose significantly. At estimated glomerular filtration rate (eGFR) ≥45 mL/min/1.73m , TR /C was the principal determinant of P at eGFR < 45 mL/min/1.73m , contributions of E /C and TR /C to P were comparable. Taken together, our results show that in CKD stages G4 and G5, the effect of phosphate reabsorption on P changes negligibly while that of phosphate influx increases dramatically. Because the tubular response to rising E /C is limited, maintenance of stable P in advanced CKD requires extreme reduction of phosphate influx into plasma. TR /C may define the lowest attainable P .