What labs are included in the initial workup for a patient with hyponatremia?

Initial Laboratory Workup for Hyponatremia

The initial laboratory workup for hyponatremia should include serum and urine studies to determine volume status, osmolality, and underlying causes: complete blood count, urinalysis, serum electrolytes (including sodium, potassium, calcium, and magnesium), blood urea nitrogen, serum creatinine, glucose, serum osmolality, urine osmolality, urine sodium, liver function tests, and thyroid-stimulating hormone. 1, 2

Essential Laboratory Tests

Serum Studies:

• Serum sodium (to confirm hyponatremia)
• Complete blood count
• Serum osmolality (to differentiate true from pseudohyponatremia)
• Blood urea nitrogen and creatinine (to assess kidney function)
• Glucose (hyperglycemia can cause pseudohyponatremia)
• Serum electrolytes (potassium, calcium, magnesium)
• Liver function tests
• Thyroid-stimulating hormone (hypothyroidism can cause hyponatremia)
• Cortisol levels (if adrenal insufficiency is suspected)

Urine Studies:

• Urine osmolality (key for differentiating SIADH)
• Urine sodium concentration
• Urinalysis

Diagnostic Algorithm Based on Volume Status

Hyponatremia workup should be organized according to volume status assessment:

Step 1: Confirm true hyponatremia

• Measure serum osmolality
  • If <275 mOsm/kg: true hyponatremia
  • If normal/elevated: pseudohyponatremia (from hyperlipidemia, hyperproteinemia) or translocational hyponatremia (from hyperglycemia)

Step 2: Assess volume status clinically

• Hypovolemic: orthostatic hypotension, tachycardia, dry mucous membranes
• Euvolemic: no signs of volume depletion or overload
• Hypervolemic: edema, ascites, elevated jugular venous pressure

Step 3: Laboratory assessment based on volume status

For Hypovolemic Hyponatremia:

• Urine sodium typically <20 mEq/L (with non-renal losses)
• Urine sodium >20 mEq/L (with renal losses like diuretics)
• Elevated BUN/creatinine ratio

For Euvolemic Hyponatremia (SIADH):

• Urine osmolality >500 mOsm/kg
• Urine sodium >20-40 mEq/L
• Normal renal function
• Absence of adrenal, thyroid, or pituitary insufficiency
• Serum uric acid <4 mg/dL

For Hypervolemic Hyponatremia:

• Urine sodium typically <20 mEq/L (in heart failure, cirrhosis)
• Urine sodium >20 mEq/L (in renal failure)
• Elevated BUN and creatinine

Special Considerations

• Timing matters: Obtain labs before initiating treatment that might alter results
• For suspected SIADH, additional tests may include fractional excretion of urate 1
• If severe symptoms are present (seizures, altered mental status), obtain labs rapidly but don't delay treatment with hypertonic saline 2, 3
• Consider medication review as part of workup (diuretics, antidepressants, antipsychotics, and antiepileptics can cause hyponatremia)

Common Pitfalls to Avoid

• Failing to check serum osmolality (missing pseudohyponatremia)
• Not measuring urine studies before fluid administration (alters results)
• Overlooking adrenal insufficiency or hypothyroidism
• Misinterpreting volume status clinically
• Not considering medication-induced hyponatremia

By systematically evaluating these laboratory parameters and clinical findings, clinicians can determine the underlying cause of hyponatremia and guide appropriate treatment decisions to improve morbidity and mortality outcomes.