ASSESSMENT AND INTERPRETATION OF THE TUBULAR THRESHOLD FOR PHOSPHATE IN INFANTS AND CHILDREN

Studies in the last decade demonstrated that in children tubular maxim um phosphate reabsorption per glomerular filtration rate (TmP/GFR) is identical to TP/GFR; TP indicating tubular phosphate reabsorption unde r basal conditions, without phosphate load. TP/GFR is calculated from the formula TP/GFR = S(P) - U(P) x S(Cr): U(Cr), based on simultaneous urine and blood creatinine and phosphate concentrations, and is appli cable in both the fasting and non-fasting child. These studies also de monstrated that the use of Walton and Bijvoet nomogram in children may result in overestimation of TmP/GFR compared with TP/GFR calculated f rom the above formula. When using the formula, one should bear in mind that creatinine is used to express GFR and as a result a significant deviation from true GFR may occur in patients with renal failure. Ther efore when employing TP/GFR for the investigation of the renal handlin g of phosphate in children, three factors should be taken into conside ration: (1) the formula in reality expresses TP/C(Cr); (2) only data o btained by exactly the same methodology can be used as reference value s; data obtained from studies in which the nomogram was utilized or in which methods other than C(Cr) were used to measure GFR should not be used for reference; (3) in patients with renal failure, TP/C(Cr) will significantly overestimate TP/C(inulin).