💧 Free Water Deficit (Hypernatraemia)
Estimate the free water deficit in hypernatraemia to plan fluid replacement.
Clinical takeaway
Add ongoing and insensible losses to the calculated deficit.
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When to use
Guide water replacement in hypernatraemia.
How it works
Deficit = TBW × (Na/140 − 1), where TBW = body-water factor × weight (0.6 male, 0.5 female/elderly).
Key points
- Add ongoing and insensible losses to the calculated deficit.
- Lower sodium by ≤ 10–12 mmol/L per 24 h in chronic cases.
- Over-rapid correction risks cerebral oedema.
References
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.
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.
| Weight | 70 kg |
|---|---|
| Current sodium | 160 mmol/L |
| Total body water factor | Adult male (0.6) |
→Free water deficit6.0L
- Total body water：42.0 L
- Formula：TBW × (Na/140 − 1)
- Correction rate：Lower Na by ≤ 10–12 mmol/L per 24 h (≤ 0.5/h chronic) to avoid cerebral oedema.
| Weight | 70 kg |
|---|---|
| Current sodium | 160 mmol/L |
| Total body water factor | Elderly female (0.45) |
→Free water deficit4.5L
- Total body water：31.5 L
- Formula：TBW × (Na/140 − 1)
- Correction rate：Lower Na by ≤ 10–12 mmol/L per 24 h (≤ 0.5/h chronic) to avoid cerebral oedema.
Frequently asked questions
- What is Free Water Deficit (Hypernatraemia)?
- Estimate the free water deficit in hypernatraemia to plan fluid replacement.
- How is Free Water Deficit (Hypernatraemia) calculated? What is the core formula?
- Deficit = TBW × (Na/140 − 1), where TBW = body-water factor × weight (0.6 male, 0.5 female/elderly).
- When is Free Water Deficit (Hypernatraemia) used?
- Guide water replacement in hypernatraemia.
- What are the key clinical points for Free Water Deficit (Hypernatraemia)?
- Add ongoing and insensible losses to the calculated deficit. Lower sodium by ≤ 10–12 mmol/L per 24 h in chronic cases. Over-rapid correction risks cerebral oedema.
- What are the limits and cautions when using Free Water Deficit (Hypernatraemia)?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Free Water Deficit (Hypernatraemia) calculated in practice? Can you show a worked example?
- Inputs: Weight 70 kg, Current sodium 160 mmol/L, Total body water factor Adult male (0.6) → Result: Free water deficit 6.0 L（Total body water: 42.0 L, Formula: TBW × (Na/140 − 1), Correction rate: Lower Na by ≤ 10–12 mmol/L per 24 h (≤ 0.5/h chronic) to avoid cerebral oedema.） Inputs: Weight 70 kg, Current sodium 160 mmol/L, Total body water factor Elderly female (0.45) → Result: Free water deficit 4.5 L（Total body water: 31.5 L, Formula: TBW × (Na/140 − 1), Correction rate: Lower Na by ≤ 10–12 mmol/L per 24 h (≤ 0.5/h chronic) to avoid cerebral oedema.）