What is the initial step in the workup of a patient with hypernatremia?

Hypernatremia Workup

Initial Diagnostic Step

The initial step in the workup of hypernatremia is to assess the patient's volume status (hypovolemic, euvolemic, or hypervolemic) through physical examination, followed by measurement of urine osmolality and urine sodium concentration. 1, 2, 3

Systematic Diagnostic Approach

Step 1: Confirm True Hypernatremia

• Verify serum sodium >145 mmol/L and exclude pseudohypernatremia 1, 3
• Calculate glucose-corrected sodium concentration (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 4

Step 2: Determine Acuity

• Acute hypernatremia: <24-48 hours duration 1, 5
• Chronic hypernatremia: >48 hours duration 1, 5
• This distinction is critical as it determines correction rate and risk of complications 5

Step 3: Assess Volume Status (Most Critical Step)

Examine for specific clinical signs:

Hypovolemic signs (suggests water loss exceeding sodium loss): 1, 2

• Orthostatic hypotension
• Dry mucous membranes
• Decreased skin turgor
• Tachycardia

Euvolemic signs (suggests pure water loss): 1, 3

• Normal blood pressure
• No edema
• Normal jugular venous pressure

Hypervolemic signs (suggests sodium excess): 1, 3

• Edema
• Elevated jugular venous pressure
• Hypertension

Step 4: Measure Urine Osmolality and Sodium

Urine osmolality interpretation: 1, 3

• >700-800 mOsm/kg: Appropriate renal response, suggests extrarenal water losses (GI losses, insensible losses, burns)
• <300 mOsm/kg: Inappropriate dilute urine, suggests diabetes insipidus

Urine sodium interpretation: 3

• >20 mmol/L: Renal sodium losses
• <20 mmol/L: Extrarenal sodium losses

Step 5: Classify by Pathophysiology

Hypovolemic hypernatremia (most common): 1, 2

• Renal losses: osmotic diuresis, diuretics
• Extrarenal losses: GI losses (diarrhea, vomiting), burns, excessive sweating

Euvolemic hypernatremia: 1, 3

• Central diabetes insipidus: traumatic brain injury, neurosurgery, pituitary pathology
• Nephrogenic diabetes insipidus: lithium, hypokalemia, hypercalcemia, chronic kidney disease

Hypervolemic hypernatremia (rare): 1

• Hypertonic saline or sodium bicarbonate administration
• Primary hyperaldosteronism

Step 6: Additional Diagnostic Tests When Indicated

For suspected diabetes insipidus: 1, 3

• Measure ongoing urinary electrolyte-free water clearance
• Consider arginine vasopressin/copeptin levels
• Water deprivation test (if clinically stable)
• Desmopressin challenge test to distinguish central from nephrogenic DI

Common Diagnostic Pitfalls

• Failing to assess volume status accurately can lead to inappropriate fluid replacement 2, 3
• Not distinguishing acute from chronic hypernatremia risks overly rapid correction and osmotic demyelination syndrome 5
• Overlooking medication causes: lithium, diuretics, osmotic agents 1, 3
• Missing impaired thirst mechanism in elderly or neurologically impaired patients 2

Key Laboratory Panel

Initial workup should include: 4, 2

• Serum sodium, glucose, BUN, creatinine
• Serum osmolality
• Urine osmolality
• Urine sodium concentration
• Serum potassium and calcium (can cause nephrogenic DI)