Fractional Excretion of Phosphate (FEPO4) Calculator
Fractional Excretion of Phosphate (FEPO4) Calculator
Identify renal phosphate wasting as the cause of hypophosphataemia, where the expected response is near-complete tubular conservation.
Fractional Excretion of Phosphate (FEPO4)
PO4 clearance ratioUrine phosphate 60 mg/dL, serum phosphate 2.1 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
Formula
- urine phosphate, serum phosphate
- mg/dL
- urine creatinine, serum creatinine
- mg/dL
- interpretation
- the number is only diagnostic when read against the serum phosphate — a normal-range result is abnormal if phosphate is low
Worked example
Urine phosphate 60 mg/dL, serum phosphate 2.1 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
60 × 2.4 = 144
2.1 × 90 = 189
144 ÷ 189 × 100 = 76.2%
With hypophosphataemia (2.1 mg/dL) this is markedly inappropriate → renal phosphate wasting
Interpretation
| FEPO4 | Pattern | Notes |
|---|---|---|
| < 5% | Appropriate conservation | Expected when serum phosphate is low |
| 5 – 20% | Normal range | Inappropriate if phosphate is also low |
| > 20% | Renal phosphate wasting | Investigate FGF-23, PTH, and recent drug exposures |
Reading FEPO4 against serum phosphate
Renal phosphate reabsorption happens mostly in the proximal tubule and is tightly regulated by parathyroid hormone and FGF-23. In a phosphate-replete person that regulation allows a fair amount of phosphate to appear in the urine, and a FEPO4 anywhere from about 5% to 20% is unremarkable. The physiology changes once serum phosphate falls: the appropriate response is to reabsorb phosphate almost completely, driving FEPO4 down towards zero.
That is what makes the test useful in a hypophosphataemic patient. If the kidney is doing its job, FEPO4 should be low despite the low serum phosphate. When it is not — when a meaningful fraction of filtered phosphate is still escaping into the urine — the kidney itself is the source of the phosphate loss, not increased losses elsewhere or reduced intake.
The differential for inappropriate renal phosphate wasting centres on FGF-23 excess, whether from a phosphaturic mesenchymal tumour in tumour-induced osteomalacia, from a genetic cause such as X-linked hypophosphataemia, or as an adverse effect of intravenous ferric carboxymaltose, which transiently raises intact FGF-23. Primary or secondary hyperparathyroidism and Fanconi syndrome are the other major causes and should be excluded alongside it.
Frequently asked questions
What FEPO4 indicates renal phosphate wasting?
Above roughly 5% in a patient with hypophosphataemia is inappropriate; above 20% is clearly diagnostic of a renal phosphate leak rather than reduced intake or extrarenal loss.
Why does FEPO4 need to be read alongside serum phosphate?
Because 5–20% is the normal range for a phosphate-replete patient. The same result is abnormal — evidence of inappropriate wasting — if serum phosphate is low at the time it was measured.
What causes renal phosphate wasting?
FGF-23 excess (tumour-induced osteomalacia, X-linked hypophosphataemia), primary or secondary hyperparathyroidism, Fanconi syndrome, and a transient rise in FGF-23 after intravenous ferric carboxymaltose.
Can ferric carboxymaltose cause hypophosphataemia?
Yes. It raises intact FGF-23 and can produce clinically significant hypophosphataemia with an inappropriately high FEPO4, usually within weeks of infusion and typically reversible.
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References
- Payne RB. Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation. Ann Clin Biochem. 1998;35(2):201–6.
- Imel EA, Econs MJ. Approach to the hypophosphatemic patient. J Clin Endocrinol Metab. 2012;97(3):696–706.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.