Assessment of maximal tubular phosphate reabsorption: comparison of direct measurement with the nomogram of Bijvoet

It is well established that plasma phosphate (Pp) is largely determined by the renal phosphate threshold, which is best described by the maximal rate of tubular phosphate reabsorption divided by the glomerular filtration rate (Tm_p/GFR). For its clinical assessment either direct phosphate loading with simultaneous measurement of GFR is performed, or the nomogram described by Walton and Bijvoet is used. In order to test the validity of the two methods, we compared in 20 infants and 31 children the fasting values of phosphate reabsorption endogenous phosphate reabsorption/inulin clearance (T_p/C_in) and T_p] with those obtained after phosphate loading maximal phosphate reabsorption (Tm_p) and Tm_p/C_in], and both with those derived from the nomogram. In addition the fasting T_p/C_in of 50 infants and 143 children could be compared with the nomogram. The results demonstrate that the directly measured T_p/C_in was the same as the directly measured Tm_p/C_in and that the measured Tm_p/C_in was correctly estimated by the nomogram. However, the comparison of fasting T_p/C_in with nomogram-derived values showed a systematic error, by which the latter values were higher than those measured. The discrepancy was due to the splay of the phosphate titration curve, which was found by Bijvoet when the ratio of phosphate clearance (C_p) corrected for GFR (C_p/GFR) fell below 0.2. The incorporation of this splay in the nomogram could not be confirmed by data measured in our children. It is concluded that fasting T_p is already maximal and that, therefore, no phosphate loading is necessary to estimate Tm_p. Furthermore, there is no evidence of a major splay, which makes the nomogram incompatible below a C_p/GFR ratio of 0.2. For clinical assessment we recommend use of the formula Tm_p/GFR=P_p–(U_p×P_crea/U_crea) where P_p, U_p, P_crea and U_crea refer to the plasma and urinary concentration of phosphate and creatinine respectively. This formula can be applied easily without the need to collect timed urinary specimens and is independent of the phosphate load.