Serum Sodium (Na⁺)
| Test | Normal Range | Critical Values |
|---|---|---|
| Serum Sodium (Na⁺) | 136–145 mEq/L | <125 mEq/L (severe hyponatremia) or >155 mEq/L (severe hypernatremia) — immediate notification required |
Clinical Significance
NCLEX Pattern
NCLEX questions about sodium focus on neurological symptom recognition (confusion, seizures), correct IV fluid selection (hypotonic vs. hypertonic saline), and the principle that sodium follows water — imbalances are often managed by controlling water, not just sodium.
Sodium is the most abundant electrolyte in the extracellular fluid (ECF) and is essential for fluid balance in the intravascular and interstitial spaces. It is maintained by the sodium-potassium pump and regulated through the renin-angiotensin-aldosterone axis and ADH (antidiuretic hormone).
Hypernatremia (Na⁺ >145 mEq/L)
- Causes: Insufficient fluid intake, excessive water loss (vomiting, diarrhea, fever), high-sodium diet, diabetes insipidus
- Mechanism: Elevated sodium → osmotic movement of water out of cells → cellular dehydration (cells shrink)
- Signs and symptoms: Severe thirst, sticky/dry mucous membranes, confusion, irritability, lethargy, seizures (neurological effects due to brain cell shrinkage)
- Treatment: Increase oral water intake; rehydrate with hypotonic IV solution (e.g., 0.45% NaCl); decrease dietary sodium
Hyponatremia (Na⁺ <136 mEq/L)
- Causes: Excess water intake, excessive hypotonic IV fluid administration, SIADH, heart failure, kidney disease; marathon runners who rehydrate with water only
- Mechanism: Decreased sodium → osmotic movement of water into cells → cellular swelling
- Signs and symptoms: Headache, confusion, nausea, seizures, coma (neurological effects due to brain cell swelling)
- Treatment: Restrict fluid intake; discontinue hypotonic IV solutions; for severe cases, administer hypertonic saline (3% NaCl) slowly — rapid correction risks osmotic demyelination syndrome
- Nursing priority: Never correct hyponatremia too rapidly — gradual rise ≤10–12 mEq/L per 24 hours
Related Labs
- serum-potassium — Potassium shifts often accompany sodium imbalances; aldosterone regulates both
- arterial-blood-gas-abg — Acid-base disturbances affect electrolyte distribution
- intravenous-fluid-categories-tonicity-and-infusion-regulation — IV fluid selection (hypotonic, isotonic, hypertonic) based on sodium status
- sodium-balance-disorders — Detailed pathophysiology and nursing care for sodium imbalances
- fluid-electrolyte-regulation-by-organs — Renal and hormonal regulation of sodium and water balance