💧 Free Water Clearance (CH₂O)
UseWhen to use: hyponatremia / polyuria teaching with paired Uosm, Posm, and urine volume.
StrataPositive CH₂O = free water excretion; negative = concentration / free-water retention cue.
LimitsWhen not to use: incomplete timed urine. · Pitfall: confusing with EFWC. · Not a diagnosis alone.
SourceStandard free water clearance definition
This-shift next steps
After scoring: open the scene pack, peer scores, and the matching pathway. Local reference — no HIS write.
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Methodology · worked example, guidelines & full notes
V − Cosm.
When to use
When to use: hyponatremia / polyuria teaching with paired Uosm, Posm, and urine volume.
How it works
CH₂O = V − Cosm; Cosm = (Uosm×V)/Posm.
Key points
- V − Cosm.
- ≠ EFWC.
- Next: electrolytes / ADH context.
References
- Standard free water clearance definition
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Urine osmolality | 400 mOsm/kg |
|---|---|
| Plasma osmolality | 290 mOsm/kg |
| Urine flow rate | 1.5 mL/min or L/day |
| Flow unit | mL/min |
→CH₂O-0.57mL/min
- Cosm：2.07 mL/min
- V used：1.50 mL/min
- Cue：Negative CH₂O — free water reabsorption / concentrating cue
| Urine osmolality | 400 mOsm/kg |
|---|---|
| Plasma osmolality | 290 mOsm/kg |
| Urine flow rate | 1.5 mL/min or L/day |
| Flow unit | L/day |
→CH₂O-0.40mL/min
- Cosm：1.44 mL/min
- V used：1.04 mL/min (from L/day)
- Cue：Negative CH₂O — free water reabsorption / concentrating cue
Verification & sources · governance
FAQ
Frequently asked questions
- Does Free Water Clearance (CH₂O) upload patient data? Where is it calculated?
- Calculation runs locally in the browser; inputs and results are not uploaded. For licensed clinicians only; interpret with history, investigations and local protocols — not a diagnosis or prescription.
- How is Free Water Clearance (CH₂O) calculated in practice? Can you show a worked example?
- Inputs: Urine osmolality 400 mOsm/kg, Plasma osmolality 290 mOsm/kg, Urine flow rate 1.5 mL/min or L/day, Flow unit mL/min → Result: CH₂O -0.57 mL/min（Cosm: 2.07 mL/min, V used: 1.50 mL/min, Cue: Negative CH₂O — free water reabsorption / concentrating cue） Inputs: Urine osmolality 400 mOsm/kg, Plasma osmolality 290 mOsm/kg, Urine flow rate 1.5 mL/min or L/day, Flow unit L/day → Result: CH₂O -0.40 mL/min（Cosm: 1.44 mL/min, V used: 1.04 mL/min (from L/day), Cue: Negative CH₂O — free water reabsorption / concentrating cue）