Hyponatraemia (Low Sodium) Diagnostic Interpreter
Hyponatraemia (Low Sodium) Diagnostic Interpreter
Work out why a sodium is low. Follow the 2014 European hyponatraemia guideline step by step: correct for glucose, rule out pseudohyponatraemia and hypertonic causes, then read the urine osmolality, the urine sodium and diuretic use, and only then the volume state. The page gives the likely cause — SIAD, low effective volume, oedema, diuretics, adrenal or thyroid, primary polydipsia or low solute intake — the biochemical severity, and what symptoms change.
Why is the sodium low?
Sodium, glucose, urine tests, volume → cause and severityA 71-year-old woman with pneumonia is confused and nauseated. Sodium 124 mmol/L, glucose 6.0 mmol/L, serum osmolality 258 mOsm/kg, urine osmolality 460 mOsm/kg, urine sodium 54 mmol/L. No diuretics, no kidney disease, clinically euvolaemic.
The steps, in the guideline’s order
2. Not hypotonic? Glucose, mannitol, glycine, pseudohyponatraemia — stop here
3. Urine osmolality ≤ 100 mOsm/kg → relative excess water intake
4. Otherwise urine sodium ≤ 30 mmol/L → low effective arterial volume (then ECF expanded or not)
5. Urine sodium above 30 → diuretics or kidney disease? If neither: ECF reduced → vomiting, adrenal, salt wasting; ECF normal → SIAD, secondary adrenal insufficiency, hypothyroidism
Severity: mild 130–135, moderate 125–129, profound below 125 mmol/L; symptoms override the number
- 2.4 per 5.5 mmol/L
- Hillier’s correction for glucose, which the European guideline uses; Katz’s older factor is 1.6 per 100 mg/dL
- hypotonic
- measured serum osmolality below 275 mOsm/kg; at 275 or above, work out whether urea or alcohol is responsible
- SIAD
- syndrome of inappropriate antidiuresis — a diagnosis of exclusion
Worked example
A 71-year-old woman with pneumonia is confused and nauseated. Sodium 124 mmol/L, glucose 6.0 mmol/L, serum osmolality 258 mOsm/kg, urine osmolality 460 mOsm/kg, urine sodium 54 mmol/L. No diuretics, no kidney disease, clinically euvolaemic.
Corrected sodium = 124 + 2.4 × (6.0 − 5.5) ÷ 5.5 = 124 + 0.22 = 124.2 mmol/L — glucose explains almost nothing
Osmolality 258 is below 275 → hypotonic; no mannitol, no lipaemia
Urine osmolality 460 is above 100 → vasopressin is acting
Urine sodium 54 is above 30, no diuretic, no kidney disease, extracellular volume normal → SIAD pattern
Check cortisol and TSH before calling it SIAD. Biochemically profound (below 125); symptoms moderately severe
Where the European and US guidance differ on diagnosis
| Question | European guideline 2014 | US expert panel 2013 |
|---|---|---|
| First step after confirming hypotonicity | Urine osmolality, then urine sodium; volume status last | Clinical volume status first: hypovolaemic, euvolaemic or hypervolaemic |
| Why | Clinical volume assessment has sensitivity 0.5–0.8 and specificity 0.3–0.5 | Volume status plus urine electrolytes judged enough to categorise |
| Urine sodium split | 30 mmol/L | A range: below 20 to 30 mmol/L in hypovolaemia; above 20 to 30 in SIADH |
| Urine osmolality | 100 mOsm/kg or less → excess water intake | Above 100 mOsm/kg defines inappropriate antidiuresis |
| Vasopressin measurement for SIAD | Advised against | Not required |
Two ways to grade hyponatraemia (European guideline)
| By sodium | By symptoms |
|---|---|
| Mild: 130–135 mmol/L | Moderately severe: nausea without vomiting, confusion, headache |
| Moderate: 125–129 mmol/L | Severe: vomiting, cardiorespiratory distress, abnormal and deep somnolence, seizures, coma (GCS 8 or less) |
| Profound: below 125 mmol/L | Acute if documented for less than 48 hours; chronic if 48 hours or more, or unknown |
Why symptoms matter more than the number
Hyponatraemia (spelt hyponatremia in the US) means a serum sodium below 135 mmol/L — a low sodium. It is the commonest electrolyte disorder in hospital and one of the calls a laboratory gets most often. Asking why the sodium is low is a separate question from asking how low it is, and the 2014 European guideline from the endocrine, intensive care and renal societies, led by Spasovski, answers it in a fixed order.
First, prove the plasma is hypotonic. Glucose pulls water out of cells and dilutes the sodium, so the guideline corrects for it before anything else (use the sodium correction for hyperglycaemia calculator for both published factors). Mannitol and absorbed irrigation fluid do the same without hypotonicity, and very high lipids or paraprotein make the main analyser under-read sodium (pseudohyponatraemia). A measured osmolality below 275 mOsm/kg settles the question. Then read the urine: osmolality at or below 100 mOsm/kg means the kidney is diluting as hard as it can and the patient is taking in more water than their solute intake lets them excrete. Above 100, the urine sodium splits low effective arterial volume (30 mmol/L or less) from everything else. Only then are diuretics, kidney disease and the clinical volume state used. Putting volume last is deliberate: bedside volume assessment performs poorly, and the guideline found that clinicians who read the urine first diagnosed better. The US expert panel of 2013 (Verbalis) starts with volume status instead, and uses a 20 to 30 mmol/L range where the European guideline uses 30.
Severity is graded two ways, and the guideline is clear which one drives treatment. A sodium that fell over hours can cause seizures at 128 mmol/L; one that fell over weeks can be tolerated at 115. The number does not tell you how swollen the brain is — symptoms do. Severe symptoms mean prompt hypertonic saline whatever the cause, while the diagnosis runs in parallel. And every chronic hyponatraemia carries the opposite risk too: correcting too fast causes osmotic demyelination, so the guideline caps the rise at 10 mmol/L in the first 24 hours and 8 mmol/L in each day after (the Adrogué-Madias calculator helps plan fluids).
When polyuria rather than a low sodium is the problem, the water deprivation test interpreter and the copeptin interpreter are the relevant pages. This interpreter supports, and does not replace, clinical judgement.
Frequently asked questions
What is the first test to order for a low sodium?
After confirming the sodium, check glucose and a measured serum osmolality to prove the plasma is hypotonic. Then send a spot urine osmolality and urine sodium from the same time. The European guideline reads urine osmolality first.
What urine sodium points to SIAD?
Above 30 mmol/L on a normal salt and water intake, with urine osmolality above 100 mOsm/kg, normal extracellular volume, no recent diuretic, and no adrenal, thyroid, pituitary or kidney failure. The US expert panel uses a range of 20 to 30 mmol/L rather than a single figure.
Why is severity by symptoms more important than by sodium level?
Because symptoms reflect brain swelling, which depends on how fast the sodium fell as much as how far. A rapid fall to 128 can cause seizures while a slow fall to 115 may cause few symptoms. The European guideline chooses treatment by symptoms and chronicity.
What does a urine osmolality below 100 mean?
Maximally dilute urine, so vasopressin is suppressed. The guideline accepts relative excess water intake: primary polydipsia, or too little solute in the diet to excrete the water drunk, as in beer potomania or a tea-and-toast diet.
How fast can a low sodium be corrected?
The European guideline limits the rise to 10 mmol/L in the first 24 hours and 8 mmol/L in every 24 hours after that, to avoid osmotic demyelination. The US expert panel sets its limits differently.
Related calculators
References
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47. Joint guideline of the European Society of Endocrinology, the European Society of Intensive Care Medicine and European Renal Best Practice; also in Nephrol Dial Transplant. 2014;29(Suppl 2):i1–i39.
- Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1–S42.
- Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399–403.
- Spasovski G. Hyponatraemia — treatment standard 2024. Nephrol Dial Transplant. 2024;39(10):1583–1592.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/