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Calculates the fractional excretion of phosphate (FEP) to localize the cause of hypophosphatemia: a high FEP indicates renal phosphate wasting, whereas a low FEP suggests redistribution or extrarenal loss.
FEP (%) = (Urine Phosphate × Plasma Creatinine) ÷ (Serum Phosphate × Urine Creatinine) × 100
~10–20% in healthy adults; interpretation is most useful in hypophosphatemia
FEP estimates proximal tubular phosphate handling. It is most valuable in the evaluation of hypophosphatemia to separate renal phosphate wasting from redistribution or gastrointestinal losses.
Specialty
Internal MedicineCategory
NephrologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
~10–20% in healthy adults; interpretation is most useful in hypophosphatemia
| Classification | Range |
|---|---|
Non-renal cause of hypophosphatemia during hypophosphatemia | <5% |
Renal phosphate wasting during hypophosphatemia | 5–20% |
Markedly elevated (pronounced renal wasting) e.g., Fanconi syndrome, tumor-induced osteomalacia | >20% |
Quantifies the fractional excretion of phosphate (FEP) to localize the cause of hypophosphatemia: a high FEP indicates renal phosphate wasting, whereas a low FEP favors redistribution or gastrointestinal losses.
During hypophosphatemia, a FEP above 5% indicates inappropriate renal phosphate wasting (e.g., FGF23 excess, Fanconi syndrome, tumor-induced osteomalacia). A FEP below 5% favors redistribution or gastrointestinal loss. FEP rises physiologically as GFR falls and is not useful when serum phosphate is elevated.
FEP separates renal phosphate wasting from redistribution or gastrointestinal causes of hypophosphatemia, guiding evaluation of FGF23-mediated disorders and proximal tubular disease.
A 60-year-old woman with hypophosphatemia has a urine phosphate of 50 mg/dL, serum phosphate 3.0 mg/dL, urine creatinine 80 mg/dL, and plasma creatinine 1.2 mg/dL.
Inputs:
FEP = (50 × 1.2) ÷ (3.0 × 80) × 100 = 25%. This markedly elevated value indicates renal phosphate wasting.
In a hypophosphatemic patient, FEP helps distinguish renal phosphate wasting (FEP > 5%) from causes such as redistribution into cells or gastrointestinal losses.
As GFR falls, fractional phosphate excretion rises physiologically due to FGF23/PTH-driven phosphaturia, so a high FEP is expected and not diagnostic.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. FEP must be interpreted with the clinical context and the prevailing serum phosphate.