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Evidence that TmP/GFR can be estimated with the Walton-Bijvoet nomogram in chronic kidney disease
Background:
The Walton-Bijvoet nomogram incorporates the assumption that GFR = creatinine clearance (Ccr). It determines phosphate tubular maximum per volume of filtrate (TmP/GFR) from serum phosphorus ([P]s) and fractional excretion of P (FEP), and equates TmP/GFR with actual reabsorption per volume of filtrate (TRP/Ccr) at FEP ≥ 20%. It has not been validated in chronic kidney disease (CKD).
Methods:
We studied 28 controls and 30 patients with stages 3 - 4 CKD. From samples of serum (s) and urine (u), we calculated P excretion per volume of filtrate (EP/Ccr) as [P]u[cr]s/[cr]u, TRP/Ccr as [P]s - EP/Ccr, and FEP as [P]u[cr]s/[P]s [cr]u or 1/{1 + (TRP/Ccr)/(EP/Ccr)}. Because a plot of TRP/Ccr against EP/Ccr resembled a hyperbola, we performed a linear regression of TRP/Ccr on 1/(EP/Ccr). From the resulting equation, we derived a hyperbolic formula relating TRP/Ccr to EP/Ccr; from that formula, we calculated TRP/Ccr and FEP as functions of EP/Ccr.
Results:
The regression of TRP/Ccr on 1/(EP/Ccr) was significant. The horizontal limb of the derived hyperbola commenced at FEP ≍ 20% and depicted stable P reabsorption at FEP > 20%.
Conclusions:
TRP/Ccr was a hyperbolic function of EP/Ccr over a wide range of GFR. Like the Walton-Bijvoet nomogram, this function projected a near-constant TRP/Ccr - i.e., a TmP/GFR - at FEP > 20%. The nomogram depicts TmP/GFR accurately in CKD. .