Urine-to-Plasma Electrolyte Ratio Calculator
Urine-to-Plasma Electrolyte Ratio Calculator
Add urine sodium and potassium, divide by plasma sodium, and get the Furst ratio — the one bedside number that says whether fluid restriction can raise a hyponatraemic patient’s sodium at all.
Urine-to-Plasma Electrolyte Ratio
(UNa + UK) ÷ PNaUrine sodium 70 mmol/L, urine potassium 30 mmol/L, plasma sodium 125 mmol/L
Formula
< 0.5 → restriction to about 1 L/day · 0.5–1.0 → about 500 mL/day · ≥ 1.0 → restriction alone will not work
- urine Na + urine K
- the osmotically effective cations in urine, which set how much of the urine volume is electrolyte-free water. Omitting potassium is the commonest mistake: in a potassium-rich urine it can be a third of the total and its absence makes the ratio look favourably low
- plasma Na
- the reference against which urine electrolyte concentration is judged. Urine more concentrated in electrolytes than plasma means free water is being retained
- ratio of 1.0
- the point of zero electrolyte-free water clearance. Above it the kidney is net-retaining free water and the plasma sodium falls with every litre passed, however little the patient drinks
- relation to free water clearance
- electrolyte-free water clearance is urine volume × (1 − ratio). The ratio is the sign-carrying part of that calculation, and it needs no urine volume, which is why it is the bedside version
- what it does not do
- diagnose the cause. It assumes the hyponatraemia has been worked up — volume state, urine osmolality, urine sodium, thyroid and cortisol, drugs — and answers only whether restriction can succeed
Worked example
Urine sodium 70 mmol/L, urine potassium 30 mmol/L, plasma sodium 125 mmol/L
70 + 30 = 100 mmol/L of urine electrolytes
100 ÷ 125 = 0.80
Between 0.5 and 1.0 → only a modest fraction of the urine volume is free water
Restriction has to be tight, conventionally about 500 mL a day, and the sodium rechecked at 24 to 48 hours
Had the potassium been omitted, the ratio would read 70 ÷ 125 = 0.56 — still in the same band here, but the error moves a ratio of 1.04 to 0.80 and flips the advice
Ratio, free water, and what to do
| Ratio | Electrolyte-free water clearance | Conventional restriction | Comment |
|---|---|---|---|
| < 0.5 | Substantially positive | About 1 L/day | Restriction should work. Also ask whether this is really antidiuretic hormone excess |
| 0.5 – 1.0 | Positive but small | About 500 mL/day | Achievable on paper, hard in practice. Recheck the sodium at 24–48 hours |
| ≥ 1.0 | Zero or negative | No volume will succeed | Every litre passed lowers the plasma sodium. Needs urea, a vaptan, or hypertonic saline in acute symptomatic disease |
Predictors of fluid restriction failure — and how well they perform
| Predictor | Threshold | Status |
|---|---|---|
| Urine-to-plasma electrolyte ratio (Furst) | > 1.0 | In the European hyponatraemia guideline. In a recent prospective multicentre study it did not predict treatment response |
| Urine osmolality | > 500 mOsm/kg | In the European guideline. Also failed to predict response in the same study |
| 24-hour urine volume | < 1.5 L | Widely quoted alongside the other two |
| Rise in plasma sodium on restriction | < 2 mmol/L in 24–48 h on ≤ 1 L/day | The most reliable indicator, because it is a measured response rather than a prediction |
Whether restriction can work, before you ask a patient to try it
Fluid restriction is the first-line treatment for chronic hyponatraemia from the syndrome of inappropriate antidiuresis, and it fails often enough that predicting failure has real value: a patient asked to live on 500 mL a day for a week for no gain has lost a week, and so has the clinician. The urine-to-plasma electrolyte ratio, described by Furst and colleagues, is the simplest useful predictor. Add the urine sodium and potassium, divide by the plasma sodium, and you have the concentration of osmotically effective cations in urine expressed relative to plasma.
The physiology is worth holding on to, because it makes the thresholds obvious rather than memorisable. Think of any urine as a mixture of an isotonic electrolyte solution and pure water. If the urine electrolytes are much less concentrated than plasma, most of the volume is free water and passing it raises the plasma sodium. As the ratio approaches 1.0, the free water fraction approaches nothing. At a ratio of 1.0 the urine is, in electrolyte terms, isotonic to plasma and no free water is being cleared; above 1.0 the kidney is net-retaining free water, and the plasma sodium falls with every litre passed no matter how little the patient drinks. Electrolyte-free water clearance is exactly urine volume multiplied by one minus this ratio — the ratio is the part that carries the sign, and it needs no urine volume to compute.
So a ratio below 0.5 suggests a restriction of about a litre a day should be enough; 0.5 to 1.0 calls for around 500 mL a day; and at or above 1.0 no achievable restriction will correct the sodium, and the options are treating the cause, urea, a vasopressin receptor antagonist, or hypertonic saline if the hyponatraemia is acute and symptomatic. A ratio above 1.0 is one of the predictors of fluid restriction failure listed in the European hyponatraemia guideline, alongside a urine osmolality above 500 mOsm/kg and a 24-hour urine volume below 1.5 litres.
Two cautions. First, include the potassium. A urine sodium of 70 with a potassium of 40 gives a ratio of 0.88 at a plasma sodium of 125; the sodium alone gives 0.56, which reads as a comfortable restriction when it is not. Second, the predictors are less good than their ubiquity suggests: a recent prospective multicentre study found that neither the Furst ratio nor a urine osmolality above 500 mOsm/kg predicted who responded to restriction. Use the ratio to set the initial target and to flag the patients unlikely to succeed, then judge by what the sodium actually does over 24 to 48 hours. This calculator supports a clinician’s assessment and does not replace it.
Frequently asked questions
What is the Furst ratio and what does it predict?
It is the sum of urine sodium and urine potassium divided by the plasma sodium. It predicts whether fluid restriction can raise the plasma sodium: below 0.5 a restriction of about 1 litre a day usually suffices, 0.5 to 1.0 needs around 500 mL a day, and at or above 1.0 no achievable restriction will work because the kidney is not excreting free water.
Why does a ratio above 1 mean fluid restriction will fail?
Because the urine electrolyte concentration then equals or exceeds the plasma sodium, so the urine contains no electrolyte-free water. Every litre passed removes more electrolyte than water and lowers the plasma sodium further. Restricting intake cannot create free water excretion that the kidney is not performing.
Do I have to include urine potassium?
Yes. Potassium is osmotically effective in urine and can be a substantial fraction of the total cation concentration. Omitting it makes the ratio falsely low and the prognosis for restriction falsely good — a urine sodium of 70 with potassium of 40 at a plasma sodium of 125 gives 0.88, while the sodium alone gives 0.56.
How does the ratio relate to electrolyte-free water clearance?
Electrolyte-free water clearance equals the urine volume multiplied by one minus the ratio. The ratio is therefore the sign-carrying part of the same calculation, and it can be worked out from a spot sample without measuring urine volume, which is why it is the bedside version.
How reliable is the ratio at predicting fluid restriction failure?
Less reliable than its place in the guidelines implies. A ratio above 1.0 and a urine osmolality above 500 mOsm/kg are both listed as predictors of failure in the European hyponatraemia guideline, but a recent prospective multicentre study found neither predicted treatment response. The plasma sodium response over 24 to 48 hours remains the most dependable indicator.
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References
- Furst H, Hallows KR, Post J, Chen S, et al. The urine/plasma electrolyte ratio: a predictive guide to water restriction. Am J Med Sci. 2000;319(4):240–244.
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47.
- Lawless SJ, Thompson C, Garrahy A. The management of acute and chronic hyponatraemia. Ther Adv Endocrinol Metab. 2022;13:20420188221097343.
- Cuesta M, Garrahy A, Slattery D, et al. Predictors of failure to respond to fluid restriction in SIAD in clinical practice; time to re-evaluate clinical guidelines? QJM. 2017;110(8):489–492.
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.