What tests are used to diagnose hyponatremia?

Diagnostic Tests for Hyponatremia

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

Initial Laboratory Evaluation

• Serum sodium level: Confirms hyponatremia when <135 mmol/L, with further investigation recommended when <131 mmol/L 2
• Serum osmolality: Differentiates between hypotonic hyponatremia (most common), isotonic pseudohyponatremia, and hypertonic hyponatremia (e.g., hyperglycemia) 1, 3
• Urine osmolality: Helps determine if water excretion is impaired (>100 mOsm/kg) or appropriate (<100 mOsm/kg) 3
• Urine sodium concentration: Critical for differentiating causes of hyponatremia 2, 4
  • <30 mmol/L suggests hypovolemic hyponatremia (71-100% positive predictive value for response to saline) 2

  • 20-40 mmol/L with euvolemia suggests SIADH 4

• Serum uric acid: Level <4 mg/dL has 73-100% positive predictive value for SIADH (though may also be seen in cerebral salt wasting) 2, 4

Volume Status Assessment

• Physical examination findings for volume status have limited accuracy (sensitivity 41.1%, specificity 80%) 2, 4
• Hypovolemia indicators: Orthostatic hypotension (increase in pulse >10% or decrease in systolic BP >10% from supine to standing), dry mucous membranes, decreased skin turgor 4
• Euvolemia indicators: No edema, no orthostatic changes, normal skin turgor 4
• Hypervolemia indicators: Edema, ascites, distended jugular veins 1

Additional Tests Based on Clinical Context

• Thyroid function tests (TSH): To rule out hypothyroidism 1, 3
• Cortisol level: To exclude adrenal insufficiency 1, 3
• Liver function tests: For suspected liver disease 1
• Renal function tests: Serum creatinine and BUN to assess kidney function 1
• Fractional excretion of sodium and urea: Low values associated with saline responsiveness 2
• Complete blood count: To assess for infection or other systemic illness 1

Specialized Tests

• Central venous pressure (CVP) measurement: Can help differentiate SIADH from cerebral salt wasting in neurosurgical patients 4
  • CSW: CVP <6 cm H₂O (hypovolemia)
  • SIADH: CVP 6-10 cm H₂O (normal volume)
• Plasma ADH levels: Limited diagnostic value in hyponatremia 2
• Natriuretic peptide levels: Data on utility is conflicting 2

Diagnostic Algorithm

1. Confirm true hyponatremia: Check serum osmolality 1, 3

   • Normal/high osmolality (≥280 mOsm/kg): Consider pseudohyponatremia or hyperglycemia
   • Low osmolality (<280 mOsm/kg): True hyponatremia

2. Assess volume status: Clinical examination plus laboratory tests 4

   • Hypovolemic: Decreased skin turgor, orthostatic hypotension
   • Euvolemic: Normal examination findings
   • Hypervolemic: Edema, ascites, distended jugular veins

3. Check urine osmolality and sodium: 4, 3

   • Urine osmolality <100 mOsm/kg: Appropriate ADH suppression (primary polydipsia)
   • Urine osmolality >100 mOsm/kg with:
     • Urine sodium <20 mmol/L: Suggests extrarenal losses
     • Urine sodium >40 mmol/L: Suggests renal losses, SIADH, or adrenal insufficiency

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status is inadequate due to poor sensitivity (41.1%) 2, 4
• Failing to distinguish between SIADH and cerebral salt wasting in neurosurgical patients can lead to inappropriate treatment 1
• Obtaining ADH and natriuretic peptide levels is not supported by evidence and should be avoided 2
• Ignoring mild hyponatremia (130-134 mmol/L) as clinically insignificant, as it may indicate worsening clinical status, especially in patients with liver disease 1
• Misdiagnosing the type of hyponatremia can lead to inappropriate treatment that worsens outcomes 1, 5

By following this systematic approach to laboratory diagnosis, clinicians can accurately determine the cause of hyponatremia and implement appropriate treatment strategies to improve morbidity, mortality, and quality of life outcomes.