Furst Ratio (urine/plasma electrolytes, SIADH fluid restriction)
Furst formula, the urine-to-plasma electrolyte ratio (Furst 2000): (urine Na + urine K) / serum Na, all mmol/L.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Urine sodium
- 60 mmol/L
- Urine potassium
- 40 mmol/L
- Serum sodium
- 125 mmol/L
Result: Furst U/P electrolyte ratio 0.80 - Published starting point: fluid restriction of 500 mL/day.
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Under 0.5
- Published starting point: fluid restriction of 1000 mL/day.
- 0.5 to 1.0
- Published starting point: fluid restriction of 500 mL/day.
- Above 1.0
- No electrolyte-free water is being excreted. Restriction alone is unlikely to help however tight; each void lowers the sodium further.
- Scope
- Guides HOW to restrict. Not a diagnosis of the cause, not a correction-rate plan, and not a reason to delay treating a symptomatic patient.
What you enter
- Urine sodium
- Urine potassium
- Serum sodium
How this is calculated
- Formula
- (urine Na + urine K) / serum Na, all in mmol/L, from a spot urine.
Furst H, Hallows KR, Post J, et al. The urine/plasma electrolyte ratio: a predictive guide to water restriction. Am J Med Sci. 2000;319(4):240-244. Bands per Grant P, et al. Eur J Clin Invest. 2015;45(8):888-894. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.