- Use as an inpatient monitoring tool in dysnatremia — to see whether the kidney is currently adding or removing electrolyte-free water.
- Enter the urine volume over a defined interval and the collection duration, plus urine Na, urine K, and plasma Na.
- The tool reports electrolyte-free water clearance in mL/hour and L/day and states the direction.
- Read it as a trend, not a prediction of the next serum sodium — unmeasured osmoles, changing volume, and ongoing therapy all limit it.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Electrolyte-free water clearance (EFWC) quantifies whether the urine is a route of free-water loss (positive EFWC — tends to raise serum sodium) or free-water retention/generation (negative EFWC — tends to lower serum sodium, as in the syndrome of inappropriate antidiuresis). It refines simple urine osmolality when managing hypo- or hypernatremia on the ward.
Appropriate setting
Inpatient monitoring of dysnatremia where you have a timed urine collection and want to understand the direction the kidney is pushing the serum sodium under the current regimen.
When not to rely on it
Not a stand-alone predictor of the serum sodium trajectory. Rapid changes, inaccurate urine volume, unmeasured osmoles (glucose, urea, mannitol), and ongoing fluids all degrade it. Do not use it to set a correction rate by itself.
Pearls & Pitfalls
Positive EFWC = losing free water
When (urine Na + urine K) is less than plasma Na, EFWC is positive: the urine is electrolyte-poor relative to plasma, so the kidney is excreting free water and serum sodium tends to rise.
Negative EFWC = retaining free water
When (urine Na + urine K) exceeds plasma Na, EFWC is negative: the urine is electrolyte-rich, the kidney is generating free water, and serum sodium tends to fall — the classic SIAD pattern that defeats isotonic saline.
Critical pitfalls
(1) It is a trend, not a serum-sodium calculator — pair it with serial sodium measurement. (2) Accurate timed urine volume is essential. (3) In SIAD, a negative EFWC explains why saline can worsen hyponatremia. (4) Never let it override the safe correction-rate limits for symptomatic dysnatremia.
Why Use It
Serum sodium is set by the balance of water and electrolyte, and urine osmolality alone cannot tell you which way the kidney is moving that balance when urea contributes to the osmolality. Electrolyte-free water clearance strips urea out of the picture and isolates the electrolyte-relevant water handling, so it directly answers the bedside question in dysnatremia: right now, is the urine helping or hurting my target?
Electrolyte-Free Water Clearance
Enter a timed urine collection with urine and plasma sodium. Interpret as a monitored trend.
⚕ EFWC = V × [1 − (U_Na + U_K) ÷ P_Na]. Positive = excreting free water (Na tends to rise); negative = generating free water (Na tends to fall). A monitored trend, not a serum-sodium predictor.
Next Steps
- Negative EFWC in hyponatremia: the urine is electrolyte-rich (SIAD physiology) — isotonic saline may worsen it; consider fluid restriction and the cause of antidiuresis. Confirm with serial serum sodium.
- Positive EFWC in hyponatremia: free-water excretion is occurring — reassess intake and the correction plan.
- In hypernatremia: a strongly positive EFWC quantifies ongoing renal free-water loss driving the sodium up.
- Always honour the safe correction-rate limits for symptomatic dysnatremia; this tool monitors direction, it does not set the rate.
Evidence & References
Formula
| Quantity | Equation |
|---|---|
| Urine flow (V) | Urine volume ÷ interval |
| Electrolyte-free water clearance | V × [ 1 − (U_Na + U_K) ÷ P_Na ] |
| Positive EFWC | Urine hypotonic to plasma electrolytes → free water excreted (Na tends to rise) |
| Negative EFWC | Urine hypertonic to plasma electrolytes → free water generated (Na tends to fall) |
