Calculation of renal tubular reabsorption of phosphate: the algorithm performs better than the nomogram

The ratio of the renal tubular maximum reabsorption rate of phosphate to the glomerular ®ltration rate (TmP/GFR) can be used to indicate the need for phosphate replacement (and the extent of its intracellular repletion), to monitor recovery of renal tubular function after damage and to help with the diagnosis of some rare diseases, e.g. X-linked hypophosphataemic rickets, hereditary hypophosphataemic rickets with hypercalciuria, and oncogenic osteomalacia. Calculation of the ratio requires measurement of the concentrations of plasma and urine phosphate (Pp and Up) and creatinine (Pcr and Ucr) in specimens taken in a fasting steady state. The TmP/GFR is calculated with the nomogram derived by Walton and Bijvoet or with the algorithm derived by Kenny and Glen from the data of Bijvoet, Morgan and Fourman. Both methods require the calculation of the fractional tubular reabsorption of phosphate (TRP):