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.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/