What is the initial workup for a patient presenting with hyponatremia?

Initial Workup for Hyponatremia

The initial workup for hyponatremia should include serum and urine osmolality, urine sodium concentration, urine electrolytes, serum uric acid, and clinical assessment of extracellular fluid (ECF) volume status to determine the underlying cause. 1, 2

When to Initiate Full Workup

• Begin comprehensive evaluation when serum sodium drops below 131 mmol/L, though hyponatremia is technically defined as <135 mmol/L 1, 2
• Even mild hyponatremia (130-135 mmol/L) warrants attention as it increases fall risk (21% vs 5% in normonatremic patients) and mortality (60-fold increase with sodium <130 mmol/L) 2, 3

Essential Laboratory Tests

Serum Studies

• Serum osmolality (normal: 275-290 mOsm/kg) to exclude pseudohyponatremia from hyperglycemia or hypertriglyceridemia 1, 4
• Serum glucose - adjust sodium by 1.6 mEq/L for each 100 mg/dL glucose >100 mg/dL 2
• Serum uric acid - levels <4 mg/dL have 73-100% positive predictive value for SIADH 1, 4
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1, 2
• Serum creatinine and BUN to assess renal function 1, 2

Urine Studies

• Urine osmolality - >100 mOsm/kg suggests impaired water excretion; >500 mOsm/kg strongly suggests SIADH 1, 4, 5
• Urine sodium concentration - this is the critical discriminator:
  • <30 mmol/L suggests hypovolemic hyponatremia with 71-100% positive predictive value for saline responsiveness 1, 4

  • 20-40 mmol/L with high urine osmolality suggests SIADH or cerebral salt wasting 4, 5

• Spot urine sodium/potassium ratio - ratio >1 correlates with 24-hour sodium excretion >78 mmol/day with ~90% accuracy 2

Clinical Volume Status Assessment

Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for determining volume status, so combine clinical findings with laboratory data. 1, 4

Hypovolemic Signs

• Orthostatic hypotension (>10% increase in pulse or >10% decrease in systolic BP when upright) 1
• Dry mucous membranes, decreased skin turgor 1, 4
• Flat neck veins 4

Euvolemic Signs

• No edema, no orthostatic hypotension 2
• Normal skin turgor, moist mucous membranes 2

Hypervolemic Signs

• Peripheral edema, ascites 1, 4
• Jugular venous distention 1, 4
• Pulmonary congestion 2

Tests NOT Recommended

• ADH levels - not supported by evidence and have limited diagnostic value; SIADH has been documented in patients with no detectable ADH 1, 4
• Natriuretic peptides - conflicting data and not recommended for routine evaluation 1
• Serum calcium, magnesium, or phosphate - no prospective studies support routine testing in otherwise healthy patients 1

Special Considerations for Neurosurgical Patients

In patients with CNS pathology, distinguishing SIADH from cerebral salt wasting (CSW) is critical as they require opposite treatments. 1, 2, 4

• SIADH characteristics: Euvolemic state, CVP 6-10 cm H₂O, urine sodium >20-40 mmol/L, urine osmolality >500 mOsm/kg 4, 5
• CSW characteristics: Hypovolemic state, CVP <6 cm H₂O, urine sodium >20 mmol/L despite volume depletion, clinical signs of hypovolemia 2, 4

Diagnostic Algorithm

1. Confirm true hyponatremia: Check serum osmolality to exclude pseudohyponatremia 1, 4
2. Assess volume status: Clinical examination plus urine sodium 1, 4
3. Categorize by volume status:
   • Hypovolemic (urine Na <30 mmol/L): extrarenal losses vs. (urine Na >20 mmol/L): renal losses, diuretics, CSW 1, 4
   • Euvolemic (urine Na >20-40 mmol/L, urine osm >300 mOsm/kg): SIADH after excluding hypothyroidism, adrenal insufficiency 4, 5
   • Hypervolemic (edema, ascites): heart failure, cirrhosis, renal failure 1, 2

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status without laboratory confirmation 1, 4
• Ordering ADH or natriuretic peptide levels, which delay diagnosis without adding value 1, 4
• Failing to distinguish SIADH from CSW in neurosurgical patients, leading to inappropriate fluid restriction in CSW 1, 2
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant when it carries real morbidity 2, 3