title: 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 X 1.1) -0.3 (r = 0.95). The regression line in adults, expressed as TmP/GFR = (fasting TP/GFR X 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.
mesh_terms: D000293:Adolescent; D000367:Age Factors; D002648:Child; D002675:Child, Preschool; D005919:Glomerular Filtration Rate; D006801:Humans; D007668:Kidney / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D012044:Regression Analysis
chemical_list: D010710:Phosphates