How is renal tubular reabsorption of phosphate (TRP) calculated during the workup of hypophosphatemic rickets?

Updated: May 02, 2018
  • Author: James CM Chan, MD; Chief Editor: Sasigarn A Bowden, MD  more...
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The renal tubular reabsorption of phosphate (TRP) is calculated with the following formula:

1 - [Phosphate Clearance (CPi) / Creatinine Clearance (Ccr)] X 100

The following formula calculates CPi:

[Urine Phosphate (mg/dL) X Volume (mL/min)] / Plasma Phosphate (mg/dL)

By substituting creatinine values for phosphate in the same formula, Ccr can also be calculated. A single early morning urine sample can be used, because CPi divided by Ccr causes units of urine volume to cancel each other.

The TRP in X-linked hypophosphatemia is 60%; normal TRP exceeds 90% at the same reduced plasma phosphate concentration.

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