Chemistry / Electrolytes
SodiumNa⁺
Principal extracellular cation; a marker of water balance, not salt intake.
Sample
Serum or plasma
Clinical reference range
135 to 145 mmol/L
Adult
Conv: 135–145 mmol/LFunc: 138–142 mmol/L
Biology
What it measures
Tightly regulated by ADH-mediated free-water handling, RAAS, and thirst. Serum [Na⁺] reflects the ratio of body sodium to body water, not absolute sodium content.
Clinical use
Why we order it
Diagnose dys-osmolar states; the single most important electrolyte in acute care.
Lens · Clinical
Interpretation by lens
Clinical interpretation
Symptomatic acute hyponatremia (<48 h, seizures/coma) warrants 100 mL boluses of 3% saline regardless of starting Na.
Research note
Copeptin (a stable surrogate of ADH) is increasingly used to discriminate primary polydipsia from central diabetes insipidus.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Free-water deficit (impaired thirst, no access, diabetes insipidus)
- ↑Excessive sodium gain (rare; iatrogenic hypertonic saline, primary hyperaldosteronism mildly)
Causes of LOW
- ↓SIADH (most common in hospital)
- ↓Hypovolemic (GI loss, diuretics, adrenal insufficiency)
- ↓Hypervolemic (heart failure, cirrhosis, nephrotic syndrome)
- ↓Pseudohyponatremia (severe hyperlipidemia/hyperproteinemia with indirect ISE)
- ↓Translocational (severe hyperglycemia: correct +1.6 mmol/L per 100 mg/dL glucose >100)
Pitfalls
Pre-analytic & interpretation traps
- !Do not correct chronic hyponatremia faster than 8 mmol/L per 24 h — risk of osmotic demyelination.
- !Always pair with serum and urine osmolality and urine sodium for diagnosis.
Follow-up orders
Logical next-step labs
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