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

Essential Laboratory Tests

First-Line Serum Tests

• Serum sodium to confirm hyponatremia (<135 mmol/L), with full workup indicated when sodium drops below 131 mmol/L 1, 2
• Serum osmolality to exclude pseudohyponatremia (normal: 275-290 mOsm/kg) - high or normal values may indicate laboratory error, hyperglycemia, or hypertriglyceridemia 1, 2
• Serum glucose to adjust for pseudohyponatremia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 1
• Blood urea nitrogen (BUN) and creatinine to assess renal function and volume status - elevated BUN/creatinine ratio suggests volume depletion 1, 3
• Serum uric acid - levels <4 mg/dL have 73-100% positive predictive value for SIADH 1, 2

First-Line Urine Tests

• Urine osmolality to assess water excretion capacity - >100 mOsm/kg indicates impaired water excretion, while <100 mOsm/kg suggests appropriate ADH suppression 1, 2
• Urine sodium concentration - this is the single most important urine test for determining etiology 1, 2
  • <30 mmol/L suggests hypovolemic hyponatremia with extrarenal losses (71-100% positive predictive value for saline responsiveness) 1, 2

  • 20-40 mmol/L with high urine osmolality suggests SIADH or cerebral salt wasting 1, 2

Additional Serum Tests Based on Clinical Context

• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1, 3
• Morning cortisol and ACTH if Addison's disease is suspected (particularly if elevated BUN/creatinine ratio or signs of volume depletion) 3
• Complete blood count as part of comprehensive evaluation 1
• Liver function tests if cirrhosis is suspected 1

Clinical Assessment of Volume Status

Physical Examination Findings

Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for determining volume status, so it must be combined with laboratory data. 1, 2

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1, 2
• Euvolemic signs: no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1
• Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1, 2

Diagnostic Algorithm Based on Initial Results

Step 1: Confirm True Hyponatremia

• Calculate plasma osmolality: 2 × Na (mEq/L) + BUN (mg/dL)/2.8 + glucose (mg/dL)/18 1
• If serum osmolality is normal or high, consider pseudohyponatremia 1, 2

Step 2: Classify by Volume Status

• Hypovolemic (urine Na <30 mmol/L): extrarenal losses (GI losses, burns, dehydration) 1, 2
• Hypovolemic (urine Na >20 mmol/L): renal losses (diuretics, cerebral salt wasting, adrenal insufficiency) 1, 2
• Euvolemic (urine Na >20-40 mmol/L, urine osm >300 mOsm/kg): SIADH most likely 1, 2
• Hypervolemic (edema, ascites present): heart failure, cirrhosis, advanced renal failure 1, 2

Step 3: Special Considerations in Neurosurgical Patients

• Distinguish SIADH from cerebral salt wasting (CSW) - both have elevated urine sodium but opposite treatments 1, 2
• CSW shows true hypovolemia with CVP <6 cm H₂O, while SIADH has normal volume with CVP 6-10 cm H₂O 2
• Assessment of extracellular fluid volume status is critical in this population 1

Tests NOT Recommended

Do not obtain ADH or natriuretic peptide levels - these are not supported by evidence and should not delay treatment 1, 2

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) - even mild hyponatremia increases fall risk (21% vs 5%) and mortality (60-fold increase with Na <130 mmol/L) 1, 4
• Relying solely on physical examination for volume status determination - combine with urine sodium and other laboratory parameters 1, 2
• Failing to check glucose levels - hyperglycemia causes pseudohyponatremia requiring correction 1
• Not ruling out Addison's disease when BUN/creatinine ratio is elevated - check morning cortisol and ACTH immediately 3
• Misdiagnosing volume status in neurosurgical patients - can lead to inappropriate therapy (fluid restriction in CSW worsens outcomes) 1, 2