Parameters of phosphorus homeostasis at normal and reduced GFR: empiric observations

Influx (IP) determines urinary excretion of phosphorus (EP). Contributions of IP and reabsorption (TRP) to serum phosphorus ([P]s) can be depicted by normalization to creatinine clearance (EP/Ccr and TRP/Ccr) or by calculation of fractional excretion and reabsorption (FEP and FTRP). We analyzed these parameters at normal and reduced GFR. We studied 29 patients with chronic kidney disease (CKD) and 28 controls. From [cr] and [P] in serumand urine we calculated [P]u/[cr]u, EP/Ccr, TRP/Ccr, (TRP/Ccr)/(EP/Ccr), FEP, and FTRP. We compared means between groups and examined pertinent linear regressions. [P]s was not different in CKD and controls. [Cr]s, EP/Ccr, and FEP were higher and TRP/Ccr, (TRP/Ccr)/(EP/Ccr), and FTRP were lower in CKD. [P]u/[cr]u, a surrogate for IP, was similar in both groups. In CKD, [P]s correlated with EP/Ccr and TRP/Ccr; EP/Ccr with [P]u/[cr]u and [cr]s; and FEP with EP/Ccr, TRP/Ccr, [P]u/[cr]u, and [cr]s. In controls, [P]s correlated with TRP/Ccr; EP/Ccr with [P]u/[cr]u; and FEP with EP/Ccr and [P]u/[cr]u. In both groups, FEP was a precise inverse function of (TRP/Ccr)/(EP/Ccr). Despite wide variation in TRP/Ccr, FEP was < 20% in 26/28 controls and > 20% in 27/29 patients with CKD. GFR affected determinants of [P]s, EP/Ccr, and FEP. FTRP was often dissociated from TRP/Ccr at normal or reduced GFR.