Assessment of Renal Phosphate Wasting

Enter fasting, second-morning-void serum and urine values (mg/dL) to calculate FePhos, TRP, and TmP/GFR.

This calculator is intended for educational purposes only and does not constitute medical advice. UC San Diego Health assumes no liability for clinical management decisions made using this tool. Results should be interpreted by a qualified provider in the context of the patient's overall clinical picture.

Fractional Excretion of Phosphate (FePhos) and Tubular Reabsorption of Phosphate (TRP)

Renal handling of serum phosphate is complex and differences in phosphate excretion can be indicative of multiple derangements including differences in blood phosphate levels, differences in kidney function as measured by GFR, and differences in kidney handling of phosphate specifically. In persons with low phosphate levels a key question that arises, is whether this is abnormalities of serum phosphate are due to abnormal renal handling of phosphate. Nephrologists often utilize fractional excretions to assess handling of various elements and molecules, like fractional excretion of sodium or urea. FePhos has been utilized as a tool for assessment of renal phosphate wasting disorders and is calculated as urine phosphate x serum creatinine divided by serum phosphate x urine creatinine. TRP=1-FePhos. While both related values are informative, the relationship of renal phosphate handling with TRP becomes non-linear at high TRP (and low FePhos) levels, and thus  nomograms have been developed for the estimation of renal phosphate handling across all levels.

TmP/GFR: The ratio of maximum tubular reabsorption of phosphate (TmP) to glomerular filtration rate (GFR).

TmP/GFR represents the theoretical serum phosphate level at which the kidney will maximally reabsorb phosphate. This calculation incorporates overall kidney function, so that persons with low GFR are not misdiagnosed as having low or normal phosphate excretion in cases where renal phosphate wasting is present. Additionally, it builds in the non-linear relationship of renal phosphate handling with TRP at high TRP values. Thus, TmP/GFR is considered the gold standard in assesment of renal phosphate handling in patients with abnormalities of serum phosphate.

Collection

As serum phosphate has a circadian pattern, measurement of TmP/GFR is standardized to the time of day, specifically it should be a second morning void specimen.  Thus, patients should be counseled that on the morning of collection, they should void normally and this sample should not be collected. They should then remain fasting and go to lab. At that time, they should have serum phosphate and creatinine levels as well as a urine sample collected for phosphate and creatinine. These fasting, second morning void values can be input into the calculator below to provide FePhos, TRP, and TmP/GFR.

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