Free Water Clearance (CH₂O)

What This Calculator Measures

Calculates renal free water clearance (CH₂O) from urine flow, urine osmolality, and plasma osmolality to characterize renal water handling in polyuria, hyponatremia, and hypernatremia.

How to Use

1. Enter urine flow, urine osmolality, and plasma osmolality from the same time point.
2. Any consistent urine flow unit works; the result shares that unit (mL/min is shown).
3. Review the sign of CH₂O: positive means free water excretion, negative means free water conservation.

Interpretation Guide

A positive CH₂O (dilute urine, Uosm < Posm) is seen in diabetes insipidus, primary polydipsia, and after water loading. A negative CH₂O (concentrated urine, Uosm > Posm) is seen in SIADH, volume depletion, and dehydration. Because urea contributes to osmolality but not tonicity, CH₂O can mislead in high-urea osmotic diuresis — use electrolyte-free water clearance there.

Clinical Significance

Free water clearance is a classic renal physiology metric that quantifies whether the kidney is excreting or conserving water, supporting the evaluation of polyuria and dysnatremia.

When to Use

• Evaluation of polyuria (diabetes insipidus vs polydipsia)
• Dysnatremia assessment and water balance
• Post-water-load physiology evaluation

When Not to Use

• In high-urea (osmotic) diuresis, where EFWC is preferable
• As a substitute for serum sodium and urine osmolality in isolation
• Without concurrent plasma and urine osmolality measurement

Limitations

• Urea contributes to urine osmolality but not tonicity, limiting CH₂O in osmotic diuresis.
• It is a point estimate that does not account for intake or non-renal losses.
• Requires accurate simultaneous osmolality measurements.

Worked Example

A 30-year-old man with suspected diabetes insipidus has a urine flow of 1.5 mL/min, urine osmolality 80 mOsm/kg, and plasma osmolality 290 mOsm/kg.

CH₂O = 1.5 × (1 − 80/290) ≈ 1.09 mL/min. This positive value indicates dilute urine with free water excretion.

Evidence

Source

Original derivation (classic renal physiology)

Reference

Rose BD. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001.

Reviewed By

MedCalcHub Clinical Team

Version

1.0

Updated

2026-08

1. Rose BD. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001.
2. Goldberg M. Hyponatremia. Med Clin North Am. 1981;65(2):251-269.

References

1. Rose BD. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001. (Textbook)
2. Goldberg M. Hyponatremia. Med Clin North Am. 1981;65(2):251-269. (Expert Review)

Frequently Asked Questions