Direct Measurement of TP/GFR: A Simple and Reliable Parameter of Renal Phosphate Handling

Abstract

As the Walton-Bijvoet nomogram for estimating renal phosphate (P) threshold (TmP/GFR) is not applicable to children of all ages, we sought an alternative method for measuring renal handling of P. Recognizing that the nomogram represents an indirect correlation between TmP/GFR and TP/GFR under fasting conditions, we examined this directly in 26 children. An excellent correlation was found, expressed as TmP/GFR = (fasting TP/GFR × 1.1) -0.3 (r = 0.95). The regression line in adults, expressed as TmP/GFR = (fasting TP/GFR × 1.4) – 0.9 (calculated from published studies) is markedly different at the higher values typical for children. Since no advantage could be seen in the use of a mathematically derived TmP, we investigated the direct use of measured TP/GFR (tubular P reabsorption per 100 ml glomerular filtrate) as a measure of renal P handling in clinical practice. No differences were found between morning fasting and nonfasting values. Measurements in 151 healthy subjects aged 3 days to 53 years established normal values in relation to age. The use of this parameter in patients is shown to accurately reflect defects and changes in renal P handling. We believe it to be the preferred parameter because it represents a directly measured physiologic function applicable to all age-groups.

Renal Reabsorption of Phosphate in Children with Sickle Cell Anemia

“…However, subsequent studies [7,8] showed that in children TmP/GFR values following a phosphate load are very close to values of TP/GFR (TP = tubular phosphate reabsorption under fasting or nonfasting conditions without phosphate load); therefore, we used the following formula to calculate tubular phosphate reabsorption per deciliter glomerular filtrate [5]:…”

“…Random urine specimens and blood samples were taken at the same time for creatinine and phosphate level estimation then the renal phosphate reabsorption was calculated using the formula [5]:…”

Serum Fibroblast Growth Factor 23 Is a Useful Marker to Distinguish Vitamin D-Deficient Rickets from Hypophosphatemic Rickets

“…Laboratory measurements included serum levels of calcium (reference range: 8.4-10.0 mg/dl), phosphate (4.2-6.2 mg/dl for the age of 1 year), AP (353-1,009 U/l for the age of 1 year), PTH (10-60 pg/ml), 1,25(OH) 2 D (20-60 pg/ml), 25(OH)D (the lower limit, 20 ng/ml [13]), and FGF23 (10-50 pg/ml for adults [21]), as well as TmP/GFR (2.7-6.3 mg/dl for the ages 1-24 months [22]) and urine calcium/creatinine ratio (U-Ca/Cr). TmP/GFR was calculated from the formula: TmP/GFR = serum phosphate - urine phosphate × serum creatinine/urine creatinine [23].…”

Phosphorus Intake in Preterm Babies and Variation of Tubular Reabsorption for Phosphate per Liter Glomerular Filtrate

“…Urine and serum electrolyte concentrations were measured by a Hitachi Analyser using ion-specific electrodes. The urinary excretion of calcium was expressed in mmol/kg/24 h. TP/GFR was calculated by the formula [ 12]:…”

“…Therefore, Stark et al [12] described a direct measurement of tubular reabsorption for phosphate per liter glomerular filtrate (TP/GFR) as a reliable pa rameter of renal phosphate handling for all age groups and found a very good correlation with TmPCVGFR. In the present study, we demonstrate that low TP/GFR for age in pre term babies can be a reflection of poor phos phorus intake and that in spite of immaturity, the kidney of preterm babies shows a very rapid ability to adapt itself to an increase in phosphorus diet content.…”