Tubular phosphate handling: references from child to adulthood in the era of standardized serum creatinine.

Background: The assessment of phosphate homeostasis in clinical practice relies not only on circulating phosphate levels but also on phosphate tubular reabsorption, ideally assessed using the tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR). TmP/GFR reference values were established before the onset of isotope-dilution mass spectrometry-standardized (IDMS) creatinine assays and thus need to be updated. Our objective is to provide reference values for TmP/GFR from childhood to adulthood, using the gold-standard of GFR assessment and IDMS-standardized creatinine values.

Methods: We retrospectively analysed all the inulin and iohexol clearances [measured glomerular filtration rate (mGFR)] performed in children and in adults screened for a living-donation in our unit since the beginning of IDMS-creatinine assays. TmP/GFR was calculated on a fasting sample, using the conventional formula without correction for tubular reabsorption of phosphate (TRP) in subjects below 19 years of age.

Results: A total of 2051 subjects (1711 children, 340 adults), aged from 1.9 to 73.4 years with normal GFR, normal phosphate and normal calcium levels, were included for TmP/GFR analysis. As expected, there was a progressive decrease along puberty in both genders of plasma phosphate and TmP/GFR, the decrease occurring earlier in girls. After the age of 19 years, there was a stabilization of plasma phosphate and TmP/GFR levels until the age of 55 years, phosphate levels and TmP/GFR being slightly lower in men than in women.

Conclusion: We present the largest cohort describing TmP/GFR reference values in the era of IDMS-standardized creatinine assays. We believe that these data will help physicians to better diagnose and manage patients with abnormal phosphate metabolism in daily clinical routine.