Fractional Excretion of Uric Acid (FEUA) Calculator

Help separate SIADH from volume depletion in hyponatraemia — a raised FEUA persists in SIADH even after the sodium is corrected.

Last updated 31 Aug 2026

Fractional Excretion of Uric Acid (FEUA)

Urate clearance ratio

Urine uric acid (mg/dL)

Serum uric acid (mg/dL)

Urine creatinine (mg/dL)

Serum creatinine (mg/dL)

35.6%Example

Urine uric acid 40 mg/dL, serum uric acid 3.0 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL

Formula

FEUA = (urine uric acid × serum creatinine) ÷ (serum uric acid × urine creatinine) × 100

urine uric acid, serum uric acid: mg/dL
urine creatinine, serum creatinine: mg/dL
SIADH: unlike urea and sodium, a raised FEUA in SIADH typically persists even after the serum sodium has been corrected

Worked example

Urine uric acid 40 mg/dL, serum uric acid 3.0 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
40 × 2.4 = 96
3.0 × 90 = 270
96 ÷ 270 × 100 = 35.6%
Above 10%, with a low serum urate typical of SIADH, favours SIADH over volume depletion

Interpretation in hyponatraemia

Serum uric acid itself is often low in SIADH from volume-expansion effects on the proximal tubule; the fractional excretion adds a second, more specific marker.
FEUA	Pattern	Favours
< 4%	Low	Volume depletion
4 – 10%	Normal	Euvolaemia
> 10%	High	SIADH

FEUA in the hyponatraemia work-up

Distinguishing SIADH from hypovolaemic hyponatraemia matters because the treatments are opposite — fluid restriction in one, fluid and sodium replacement in the other — yet both can present with a euvolaemic-looking patient and a low serum sodium. Serum and fractional urate handling offer a clue that is harder to obtain from volume status alone.

In SIADH, mild volume expansion from water retention increases proximal tubular urate excretion, lowering serum uric acid and raising the fractional excretion of urate. In hypovolaemic hyponatraemia the opposite physiology applies: the kidney conserves urate along with sodium and water, so FEUA stays low. A FEUA above about 10% in a hyponatraemic patient therefore favours SIADH.

The feature that makes FEUA more useful than serum urate alone, or than FENa, is persistence. Once hyponatraemia in SIADH is corrected — with fluid restriction, salt tablets or a vaptan — serum sodium normalises but the fractional excretion of urate typically remains elevated, unlike FENa, which normalises along with the sodium. That persistence can help confirm SIADH retrospectively, or distinguish it from cerebral salt wasting, in which both sodium loss and a raised FEUA persist despite volume depletion, arguing for careful clinical correlation rather than reliance on any single index.

Frequently asked questions

What FEUA suggests SIADH?

Above roughly 10% in a hyponatraemic patient supports SIADH over volume depletion, particularly when serum uric acid is also low.

Why is serum uric acid low in SIADH?

Mild volume expansion from water retention increases proximal tubular uric acid excretion, lowering the serum level and raising the fractional excretion at the same time.

Does FEUA stay high after hyponatraemia is corrected?

Yes, and this is what distinguishes it from FENa. In SIADH the fractional excretion of urate typically remains elevated even after serum sodium has normalised with treatment.

Can FEUA distinguish SIADH from cerebral salt wasting?

Not reliably on its own — FEUA can be raised in both. Volume status, clinical context (often neurosurgical for cerebral salt wasting) and the response to fluid administration remain essential to separate them.

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References

Decaux G, Schlesser M, Coffernils M et al. Uric acid, anion gap and urea concentration in the diagnostic approach to hyponatremia. Clin Nephrol. 1994;42(2):102–8.
Maesaka JK, Imbriano LJ, Miyawaki N. High prevalence of renal salt wasting without cerebral disease as cause of hyponatremia in general medical wards. Am J Med Sci. 2018;356(1):15–22.