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

Initial Workup of Hypernatremia

The initial step in the workup of a patient with hypernatremia should be assessment of volume status by evaluating clinical signs, measuring urine osmolality, and determining urine sodium concentration. 1

Volume Status Assessment

Volume status assessment is critical as it guides subsequent management and helps identify the underlying cause of hypernatremia. The three categories of volume status in hypernatremia are:

• Hypovolemic hypernatremia: Water loss exceeds sodium loss
• Euvolemic hypernatremia: Pure water loss or impaired water intake
• Hypervolemic hypernatremia: Sodium gain exceeds water gain

Clinical Assessment of Volume Status

• Hypovolemic signs: Orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, oliguria
• Euvolemic signs: Normal vital signs, no edema, no signs of dehydration
• Hypervolemic signs: Peripheral edema, ascites, elevated jugular venous pressure

Laboratory Evaluation

After assessing volume status, the following laboratory tests should be performed:

1. Urine osmolality and urine sodium concentration - Critical for determining the cause:

   • Hypovolemic: Variable urine osmolality, urine sodium <20 mEq/L
   • Euvolemic: Elevated urine osmolality (>500 mOsm/kg), urine sodium >20-40 mEq/L
   • Hypervolemic: Elevated urine osmolality, urine sodium <20 mEq/L 1

2. Additional laboratory tests:

   • Complete blood count
   • Serum electrolytes (including calcium and magnesium)
   • Blood urea nitrogen and serum creatinine
   • Fasting blood glucose
   • Liver function tests
   • Thyroid-stimulating hormone 2

Diagnostic Algorithm

1. Exclude pseudohypernatremia: Verify that hypernatremia is not due to laboratory error or severe hyperlipidemia/hyperproteinemia

2. Confirm glucose-corrected sodium concentrations: High glucose can cause falsely elevated sodium readings

3. Determine extracellular volume status: Clinical assessment plus laboratory values

4. Measure urine sodium levels and osmolality: Helps differentiate between renal and extrarenal causes

5. Calculate urinary electrolyte free water clearance: Helps assess ongoing water losses

6. Assess for other electrolyte disorders: Particularly potassium abnormalities 2

Common Pitfalls and Caveats

• Don't delay treatment while pursuing diagnosis: In severe symptomatic hypernatremia, treatment should be initiated promptly while diagnostic workup continues

• Avoid overly rapid correction: Correction should not exceed 8-10 mmol/L per 24 hours to prevent cerebral edema, especially in chronic hypernatremia 1, 2

• Consider special populations: Elderly patients often have impaired thirst mechanisms, and critically ill patients frequently have impaired consciousness affecting water intake 3

• Look for medication causes: Diuretics, osmotic agents, and certain antibiotics can cause or worsen hypernatremia

By following this systematic approach to the initial workup of hypernatremia, clinicians can quickly identify the underlying cause and initiate appropriate treatment to prevent complications and improve outcomes.