ESPE Abstracts (2015) 84 P-2-214

TmP/GFR is a Useful Marker in Making a Clinical Diagnosis of X-Linked Hypophosphataemic Rickets Caused by the PHEX Gene Mutation

Ryojun Takedaa, Kentaro Miyaia, Masaki Takagia, Masahiro Gotoa, Daisuke Ariyasub, Masako Izawac, Junko Igakid, Eri Suzukie, Yoshie Nakamuraf & Yukihiro Hasegawaa


aDivision of Endocrinology and Metabolism, Tokyo Metropolitan Children’s Medical Center, Tokyo, Japan; bDivision of Developmental Genetics, Institute of Resource Development and Analysis, Kumamoto University, Kumamoto, Japan; cDepartment of Pediatric Endocrinology and Metabolism, Aichi Children’s Health and Medical Center, Nagoya, Japan; dDivision of Endocrinology and Metabolism, Gunma Children’s Medical Center, Gunma, Japan; eDepartment of Pediatrics, National Hospital Organization Tokyo Medical Center, Tokyo, Japan; fDivision of Genetic Research, Tokyo Metropolitan Children’s Medical Center, Tokyo, Japan


Background: The clinical diagnosis of x-linked hypophosphatemic (XLH) rickets is based on a number of biochemical observations. These include a reduction in the percentage of tubular reabsorption of phosphate (%TRP), and in the maximal tubular phosphate reabsorption capacity corrected for glomerular filtration rate (TmP/GFR). However, it is important to maintain sufficient renal blood flow in order to accurately calculate TmP/GFR.

Objective: The aims of this study were to compare the normal reference values of %TRP and TmP/GFR with the values detected in XLH patients with and without pre-examination water loading, and to compare the time-dependent alterations in %TRP and TmP/GFR values in XLH patients who performed water loading either from the beginning of the test or the day before testing.

Results: i) In the analysis of %TRP and TmP/GFR, calculated in 286 samples from 26 XLH patients without water loading, 64 samples for %TRP and four samples for TmP/GFR showed values within the reference ranges. ii) A time-dependent decrease in %TRP and TmP/GFR was observed in cases of XLH patients who performed water loading from the beginning of the test. However, time-dependent alterations in %TRP and TmP/GFR were not identified in patients who performed water loading from the day before testing. iii) In the analysis of %TRP and TmP/GFR, calculated in 48 samples from 22 XLH patients after water loading, 25 samples for %TRP showed values within the reference range, and all samples for TmP/GFR showed values lower than the reference value.

Conclusion: TmP/GFR is a more useful marker than %TRP in making a clinical diagnosis of XLH caused by the PHEX gene mutation. Furthermore, it is important to maintain sufficient renal blood flow in order to accurately calculate TmP/GFR.