Serum Sodium
Also known as: Na+, Serum Na, Hyponatraemia/Hypernatraemia test
Sodium is the primary electrolyte that determines blood osmolality and fluid distribution. Both low and high sodium levels can cause severe neurological symptoms and require careful correction.
Venous blood (serum separator tube)
1–2 hours
Not required
Metabolic Panel
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Normal | 136 – 145 | mmol/L |
| Mild hyponatraemia | 130 – 136 | mmol/L |
| Moderate hyponatraemia | 120 – 130 | mmol/L |
| Severe hyponatraemia | ≤ 120 | mmol/L |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
Sodium is the dominant cation in extracellular fluid and the primary determinant of plasma osmolality — which dictates how water distributes between cells and the bloodstream. Low sodium causes water to shift into brain cells (cerebral oedema); high sodium draws water out of cells. The kidneys regulate sodium — and therefore water — via ADH (antidiuretic hormone) and aldosterone.
When Is This Test Ordered?
- Part of every metabolic panel and electrolyte assessment
- Monitoring diuretic therapy
- Heart failure, cirrhosis, or nephrotic syndrome management
- SIADH (syndrome of inappropriate ADH) investigation
- Diabetes insipidus evaluation
- Dehydration assessment
When Results Are High
Hypernatraemia (above 145 mmol/L) almost always means water deficit relative to sodium — either insufficient intake (in the elderly who lose thirst sensation) or excessive loss (diabetes insipidus, profuse sweating). Correction must be slow to avoid cerebral oedema from rapid re-expansion.
When Results Are Low
Hyponatraemia (below 136 mmol/L) is the most common electrolyte abnormality in hospitalised patients. Causes range from SIADH (excess ADH from medications, lung disease, brain pathology), to heart failure (dilutional), to diuretics, to hypothyroidism. Rapid correction risks osmotic demyelination syndrome (central pontine myelinolysis).
Factors That Can Affect Your Result
- Hyperlipidaemia or hyperproteinaemia can give falsely low sodium on older analysers (pseudohyponatraemia)
- Hyperglycaemia draws water into plasma, diluting sodium — correct by adding 1.6 mmol/L per 100 mg/dL glucose above 100
- Marked protein or fat in blood (multiple myeloma, lipaemia) affect indirect ISE methods
Tracking This Test Over Time
Chronic mild hyponatraemia (128–134 mmol/L) — even when asymptomatic — is associated with falls, fractures, cognitive impairment, and worse cardiovascular outcomes. It is not benign and deserves investigation and management, not watchful waiting.
Track your Sodium in LoomsRelated Tests
Potassium
Potassium is the most important intracellular electrolyte and plays a critical role in heart rhythm and muscle function. Even small deviations outside the reference range can cause life-threatening cardiac arrhythmias.
Creatinine / eGFR
Creatinine is a waste product filtered by the kidneys; eGFR (estimated glomerular filtration rate) converts it into a percentage of normal kidney function. Together they are the primary tools for detecting and staging chronic kidney disease.
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.