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Electrolyte-Free Water Clearance

Electrolyte-free water clearance from urine output and urine/plasma electrolytes.

For healthcare professionalsElectrolytes & laboratoryv1.0.1

For healthcare professionals / clinical use. Not a self-diagnosis tool.

Your measurements

L/day
mmol/L
mmol/L
mmol/L

Calculated on your device. Nothing is sent to a server.

Your result will appear here

Enter your measurements, then press Calculate.

Methodology

Electrolyte-free water clearance

EFWC = V × (1 − (UNa + UK) ÷ PNa)

A positive value means the kidneys are excreting electrolyte-free water; a negative value means they are retaining it. This is the urinary part only; with other intake or losses it does not predict the change in plasma sodium (Nguyen & Kurtz 2006).

Who it is for
Adults with dysnatraemia.
Version
1.0.1

How this calculator works

Where the formula comes from

  • The classic electrolyte-free water clearance formula is V × (1 − (urine Na + urine K) ÷ plasma Na); it assumes urine is isonatric when urine Na + K equals plasma Na.[1]

Versions and alternatives

  • Nguyen and Kurtz (2005) derived a modified formula from the Edelman equation, V × [1 − 1.03 × (urine Na + K) ÷ (plasma Na + 23.8)], with a further glucose term for hyperglycaemia.[1]
  • Electrolyte-free water balance counts the non-isotonic parts of all inputs and outputs and is proportional to the rate of change in plasma sodium.[2]

Where accuracy falls (published evidence)

  • Urinary electrolyte-free water clearance alone cannot predict the direction of change in plasma sodium when other sodium, potassium and water inputs or non-renal outputs are present.[3]
  • The classic formulas require modification in hyperglycaemia; Nguyen and Kurtz (2005) derived a modified equation that includes a glucose term.[1]

Sources

  1. Nguyen MK, Kurtz I. Derivation of a new formula for calculating urinary electrolyte-free water clearance based on the Edelman equation. Am J Physiol Renal Physiol. 2005;288(1):F1–F7. doi:10.1152/ajprenal.00259.2004
  2. Shah SR, Bhave G. Using electrolyte free water balance to rationalize and treat dysnatremias. Front Med (Lausanne). 2018;5:103. doi:10.3389/fmed.2018.00103
  3. Nguyen MK, Kurtz I. Whole-body electrolyte-free water clearance: derivation and clinical utility in analyzing the pathogenesis of the dysnatremias. Clin Exp Nephrol. 2006;10(1):19–24. doi:10.1007/s10157-005-0395-2

Compiled from the published sources above. Every statement was agreed independently by three AI research tools (Claude, ChatGPT, DeepSeek) and its source verified in PubMed or Crossref by the Hakeemly editorial team. Not reviewed by a doctor unless stated. Agreed on

Worked example: how to calculate free water clearance

Inputs

Urine output
2 L/day
Urine sodium
60 mmol/L
Urine potassium
40 mmol/L
Plasma sodium
125 mmol/L

Result

Electrolyte-free water clearance
0.4 L/day

Calculation:

2 × (1 − 100/125) = 0.4

An illustration with sample values, worked out with the same method the tool above uses.

Limitations

  • Ignores insensible and gastrointestinal losses and fluid intake.
  • The classic formula needs modification in hyperglycaemia (Nguyen & Kurtz 2005).

References

  1. Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. New York: McGraw-Hill; 2001.
  2. Nguyen MK, Kurtz I. Derivation of a new formula for calculating urinary electrolyte-free water clearance based on the Edelman equation. Am J Physiol Renal Physiol. 2005;288(1):F1–F7. doi:10.1152/ajprenal.00259.2004
  3. Nguyen MK, Kurtz I. Whole-body electrolyte-free water clearance: derivation and clinical utility in analyzing the pathogenesis of the dysnatremias. Clin Exp Nephrol. 2006;10(1):19–24. doi:10.1007/s10157-005-0395-2

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