Clinical overview
Hyponatraemia is a serum sodium concentration below the local reference interval. The first clinical tasks are to identify severe neurological symptoms, establish whether the result represents hypotonic hyponatraemia and determine whether the process is acute, chronic or of uncertain duration.
Why this matters in practice
Hyponatraemia: Diagnostic Assessment and Safe Correction should be interpreted as part of a structured clinical assessment. The numerical result, symptom burden, rate of change, comorbidity, medicines and response to treatment all influence risk. A single threshold can support a decision, but it should not override clear evidence of physiological deterioration.
Physiology and pathophysiology
Most clinically important hyponatraemia reflects impaired excretion of electrolyte-free water, usually because antidiuretic hormone is active. Causes include hypovolaemia, heart failure, cirrhosis, kidney disease, adrenal insufficiency, medicines and the syndrome of inappropriate antidiuresis. Low solute intake and excessive water intake can also contribute.
Measurement and pre-analytical checks
Confirm patient identity, specimen timing, units and the laboratory method. Review haemolysis, contamination, transport delay, sampling from an infusion line and other pre-analytical factors when the result is unexpected. Compare with previous measurements from the same laboratory where possible, because reference intervals and assay behaviour can differ.
Structured clinical assessment
Assess airway, breathing and circulation, neurological status and symptom onset. Confirm serum osmolality and glucose. Review volume status, urine osmolality, urine sodium, renal function, thyroid function, morning cortisol where appropriate, medicines and recent fluid intake. SIADH is a diagnosis made after excluding important mimics.
Interpretation in context
Interpret the value alongside observations, examination, fluid status, renal and liver function, acid-base data, medicines and relevant imaging or microbiology. Trend is often more informative than a single measurement. A modest abnormality that is changing rapidly may be more important than a stable, longstanding result outside the reference interval.
Special populations
Reference intervals and diagnostic performance may differ in pregnancy, children, older adults, critical illness, advanced kidney or liver disease and people with unusual body composition. Use population-specific guidance and specialist advice where the ordinary interpretation is unreliable. Do not transfer adult thresholds directly to paediatric or obstetric care.
Common pitfalls
Clinical volume assessment is imperfect, diuretics can distort urine sodium and a normal-looking patient may still have adrenal insufficiency. The major treatment hazard is overly rapid correction, especially in chronic hyponatraemia, malnutrition, alcohol dependence, liver disease or hypokalaemia.
Red flags and escalation
Seizures, reduced consciousness, severe vomiting, cardiorespiratory distress or rapidly developing symptoms require immediate protocol-led treatment, frequent sodium checks and specialist input. Any unexpected rapid rise in sodium should prompt action to prevent osmotic demyelination.
Monitoring and follow-up
Set the repeat-testing interval according to clinical risk and the expected speed of change. Document the value, interpretation, action and follow-up plan. After treatment, confirm both biochemical and clinical response and watch for rebound, overcorrection or treatment-related complications.
Communication and handover
Communicate the actual result, units, relevant reference interval, trend and working interpretation. State what has been done, what remains uncertain and which finding should trigger escalation. Avoid vague shorthand when a result has direct implications for medicines, monitoring or level of care.
Frequently asked questions
Can one result establish the diagnosis?
No. Most abnormalities require confirmation, context and a review of trend, symptoms and potential confounders.
Should treatment aim only to normalise the number?
No. Treatment should address the underlying mechanism and improve patient-centred outcomes while avoiding complications of overcorrection.
When should specialist advice be requested?
Seek advice when the abnormality is severe, rapidly changing, unexplained, treatment-resistant, associated with organ dysfunction or outside the population in which the usual pathway is validated.
Key points for practice
- Confirm the result and the clinical context before acting.
- Assess severity, symptoms and rate of change.
- Review medicines, comorbidity and reversible causes.
- Use local protocols and population-specific guidance.
- Trend the response and document a clear follow-up plan.
- Escalate physiological deterioration regardless of a reassuring isolated number.