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.