💧 Fractional Excretion of Phosphate FEPO₄ / TRP
The fractional excretion of phosphate (FEPO₄) separates renal phosphate wasting from extrarenal causes of hypophosphatemia.
In hypophosphatemia, FEPO₄ >5% (TRP <80–85%) points to renal wasting such as hyperparathyroidism, FGF23 excess, or Fanconi syndrome (original synthesis · not guideline verbatim).
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When to use
Enter urine and serum phosphate plus urine and serum creatinine; the tool returns FEPO₄ and tubular reabsorption of phosphate (TRP).
How it works
FEPO₄ (%) = (urine P × serum Cr) / (serum P × urine Cr) × 100; TRP = 100 − FEPO₄.
Key points
- In hypophosphatemia, FEPO₄ >5% (TRP <80–85%) points to renal wasting such as hyperparathyroidism, FGF23 excess, or Fanconi syndrome (original synthesis · not guideline verbatim).
- FEPO₄ <5% indicates an appropriate renal response and an extrarenal cause (poor intake, intracellular shift, GI loss).
- Interpretation must be paired with the serum phosphate, since a high FEPO₄ is only abnormal when phosphate is low.
References
- Walton RJ, Bijvoet OL. Nomogram for derivation of renal threshold phosphate concentration. Lancet. 1975.
- Imel EA, Econs MJ. Approach to the hypophosphatemic patient. J Clin Endocrinol Metab. 2012.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Urine phosphate | 15 mmol/L |
|---|---|
| Serum phosphate | 0.8 mmol/L |
| Urine creatinine | 8000 μmol/L |
| Serum creatinine | 80 μmol/L |
→FEPO₄18.8%
- TRP (tubular reabsorption)：81.3%
- Formula：(urine P × serum Cr) / (serum P × urine Cr) × 100
Frequently asked questions
- What is Fractional Excretion of Phosphate FEPO₄ / TRP?
- The fractional excretion of phosphate (FEPO₄) separates renal phosphate wasting from extrarenal causes of hypophosphatemia.
- How is Fractional Excretion of Phosphate FEPO₄ / TRP calculated? What is the core formula?
- FEPO₄ (%) = (urine P × serum Cr) / (serum P × urine Cr) × 100; TRP = 100 − FEPO₄.
- When is Fractional Excretion of Phosphate FEPO₄ / TRP used?
- Enter urine and serum phosphate plus urine and serum creatinine; the tool returns FEPO₄ and tubular reabsorption of phosphate (TRP).
- What are the key clinical points for Fractional Excretion of Phosphate FEPO₄ / TRP?
- In hypophosphatemia, FEPO₄ >5% (TRP <80–85%) points to renal wasting such as hyperparathyroidism, FGF23 excess, or Fanconi syndrome (original synthesis · not guideline verbatim). FEPO₄ <5% indicates an appropriate renal response and an extrarenal cause (poor intake, intracellular shift, GI loss). Interpretation must be paired with the serum phosphate, since a high FEPO₄ is only abnormal when phosphate is low.
- What are the limits and cautions when using Fractional Excretion of Phosphate FEPO₄ / TRP?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Fractional Excretion of Phosphate FEPO₄ / TRP calculated in practice? Can you show a worked example?
- Inputs: Urine phosphate 15 mmol/L, Serum phosphate 0.8 mmol/L, Urine creatinine 8000 μmol/L, Serum creatinine 80 μmol/L → Result: FEPO₄ 18.8 %（TRP (tubular reabsorption): 81.3%, Formula: (urine P × serum Cr) / (serum P × urine Cr) × 100）