Determinants of the serum phosphate concentration in chronic kidney disease

If C-cr is creatinine clearance and EP and TRP are rates of phosphate ex-cretion and reabsorption, the serum phosphate concentration (P-s) is the sum of E-P/C-cr and TRP/C-cr, i.e., the amounts of phosphate excreted and reabsorbed per volume of fil-trate. At equilibrium, influx of phosphate into plasma determines EP, and E-P/C-cr quantifies the contribution of phosphate influx to P-s. We used data obtained at 688 clinic visits of 387 patients to analyze the evolution of P-s in chronic kidney disease (CKD) stages G1 - 5 (dialysis excluded). E-P/C-cr was calculated as (Puxcrs)/cru and TRP/Ccr as P-s-E-P/C-cr (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-P/C-cr rose and TRP/Ccr fell by the same amount in CKD G3a and G3b, and P-s did not change. In stages G4 and G5, E-P/C-cr increased sharply, TRP/Ccr fell minimally, and P-s rose significantly. At estimated glomerular filtration rate (eGFR) >= 45 mL/min/1.73m(2), TRP/Ccr was the principal determinant of P-s; at eGFR < 45 mL/min/1.73m(2), contributions of E-P/C-cr and TRP/Ccr to P-s were comparable. Taken together, our results show that in CKD stages G4 and G5, the effect of phosphate reabsorption on P-s changes negligibly while that of phosphate influx increases dramati-cally. Because the tubular response to rising E-P/C-cr is limited, maintenance of stable P-s in advanced CKD requires extreme reduction of phosphate influx into plasma. TRP/Ccr may define the lowest attainable P-s.