What labs are needed for a hyponatremia (low sodium levels) workup?

Laboratory Workup for Hyponatremia

The essential initial laboratory tests for hyponatremia workup include serum sodium, serum osmolality, urine osmolality, and urine sodium concentration 1, 2. These core tests allow you to determine the underlying mechanism and guide appropriate treatment.

Initial Essential Laboratory Tests

Serum Studies:

• Serum sodium - confirms hyponatremia (<135 mEq/L) and establishes severity 1, 2
• Serum osmolality - differentiates true hyponatremia from pseudohyponatremia (normal: 275-290 mOsm/kg) 1, 3
• Serum glucose - hyperglycemia causes pseudohyponatremia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 1
• Serum uric acid - low levels (<4 mg/dL) have 73-100% positive predictive value for SIADH 1, 2

Urine Studies:

• Urine osmolality - determines if water excretion is appropriate; >100 mOsm/kg suggests impaired water excretion 1, 3
• Urine sodium concentration - differentiates renal from extrarenal causes 1, 2
  • <20-30 mEq/L suggests extrarenal losses (GI losses, dehydration) 1, 3

  • 20-40 mEq/L suggests renal losses, SIADH, or cerebral salt wasting 1, 2

Additional Laboratory Tests Based on Clinical Context

To rule out secondary causes:

• Thyroid-stimulating hormone (TSH) - excludes hypothyroidism 1, 2
• Serum creatinine and BUN - assesses renal function and helps determine volume status 1, 2
• Serum potassium - often abnormal with diuretic use or adrenal insufficiency 1
• Morning cortisol - if adrenal insufficiency suspected 3

For specific populations:

• Liver function tests and albumin - if cirrhosis suspected 1
• Complete blood count - part of comprehensive evaluation 1

What NOT to Order

Do not routinely measure ADH or natriuretic peptide levels - obtaining these hormone levels is not supported by evidence and should not delay treatment 1, 2. The diagnosis can be made clinically with the basic laboratory panel outlined above.

Diagnostic Algorithm Using Laboratory Results

Step 1: Confirm true hyponatremia

• Check serum osmolality; if normal (275-290 mOsm/kg) or high, consider pseudohyponatremia from laboratory error, hyperglycemia, or hypertriglyceridemia 1

Step 2: If serum osmolality is low (<275 mOsm/kg)

• Check urine osmolality:
  • <100 mOsm/kg = appropriate ADH suppression (primary polydipsia) 1, 3

  • 100 mOsm/kg = impaired water excretion, proceed to Step 3 1

Step 3: Check urine sodium

• <20-30 mEq/L = extrarenal losses (GI losses, third-spacing, remote diuretic use) 1, 3

• 20-40 mEq/L = renal losses, SIADH, cerebral salt wasting, or adrenal insufficiency 1, 2

Step 4: Assess volume status clinically

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor 1
• Euvolemic: no edema, normal blood pressure, normal skin turgor 1
• Hypervolemic: edema, ascites, jugular venous distention 1

Common Pitfalls to Avoid

• Physical examination alone has poor accuracy for determining volume status (sensitivity 41.1%, specificity 80%) - rely heavily on laboratory data 1, 2
• Do not delay treatment while pursuing a complete diagnostic workup if the patient is severely symptomatic 4, 5
• A spot urine sodium <30 mmol/L has 71-100% positive predictive value for response to normal saline infusion 1, 2
• Serum uric acid <4 mg/dL strongly suggests SIADH but may also occur in cerebral salt wasting 1, 2
• In neurosurgical patients, distinguish between SIADH and cerebral salt wasting using central venous pressure (CVP <6 cm H₂O suggests CSW; CVP 6-10 cm H₂O suggests SIADH) 1