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Calculates urinary electrolyte-free water clearance (EFWC) from urine flow and urinary/plasma sodium and potassium. It is used to distinguish renal from extrarenal water losses and to guide fluid therapy in dysnatremia.
EFWC = V × (1 − (Urine Na + Urine K) ÷ Plasma Na)
Near zero in steady state; markedly positive with renal water losses (e.g., osmotic diuresis, diabetes insipidus)
Electrolyte-free water clearance was introduced by Goldberg and popularized by Rose. Unlike CH₂O, it excludes urea, making it more accurate in osmotic diuresis and when predicting the direction of serum sodium change.
Specialty
Internal MedicineCategory
NephrologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Near zero in steady state; markedly positive with renal water losses (e.g., osmotic diuresis, diabetes insipidus)
| Classification | Range |
|---|---|
Renal water loss (raises serum Na) e.g., osmotic diuresis, furosemide, DI | positive (large) |
Extrarenal / insensible loss e.g., GI or insensible losses | near zero or negative |
Calculates urinary electrolyte-free water clearance (EFWC) from urine flow and urinary/plasma sodium and potassium to distinguish renal from extrarenal water losses and guide fluid therapy in dysnatremia.
An EFWC above 0 indicates ongoing renal electrolyte-free water loss (osmotic diuresis, diuretics, diabetes insipidus, renal failure) that tends to raise serum sodium. Values near zero or negative point to gastrointestinal or insensible losses as the source of hypernatremia. The value estimates the free water that must be replaced to prevent further sodium rise.
By excluding urea, EFWC reflects the tonicity-relevant water loss and is more accurate than CH₂O in osmotic diuresis, guiding whether hypernatremia is renal in origin and how much free water is being lost.
A 70-year-old man with hypernatremia during an osmotic diuresis has a urine flow of 1.5 mL/min, urine sodium 80 mmol/L, urine potassium 40 mmol/L, and plasma sodium 140 mmol/L.
Inputs:
EFWC = 1.5 × (1 − (80 + 40)/140) ≈ 0.21 mL/min. This positive value indicates ongoing renal electrolyte-free water loss.
CH₂O uses total urine osmolality (including urea); EFWC uses only urine sodium + potassium relative to plasma sodium, so it reflects tonicity and is more accurate in osmotic diuresis.
It means the kidneys are excreting substantial electrolyte-free water — the cause of the hypernatremia is renal, and replacement therapy must match this ongoing loss.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Fluid therapy in dysnatremia must account for the full clinical picture and serial laboratory monitoring.