💧 Fractional Excretion of Phosphate FEPO₄ / TRP
The fractional excretion of phosphate (FEPO₄) separates renal phosphate wasting from extrarenal causes of hypophosphatemia.
Fractional Excretion of Phosphate FEPO₄ / TRP
Urine phosphate (mmol/L)
Serum phosphate (mmol/L)
Urine creatinine (μmol/L)
Serum creatinine (μmol/L)
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.
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.