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

Initial Workup of Hypernatremia

The initial step in the workup of hypernatremia should be assessment of the patient's volume status through clinical examination and basic laboratory tests. 1, 2

Understanding Hypernatremia

Hypernatremia is defined as a serum sodium concentration greater than 145 mmol/L. It represents a disorder of water homeostasis and can be classified based on:

• Duration: Acute vs. chronic
• Severity: Mild, moderate, or threatening
• Volume status: Hypervolemic, hypovolemic, or euvolemic 1

Step-by-Step Diagnostic Approach

1. Volume Status Assessment (First Priority)

Determining the patient's volume status is crucial as it directs subsequent diagnostic and therapeutic approaches:

• Hypovolemic hypernatremia: Look for signs of dehydration (dry mucous membranes, tachycardia, orthostatic hypotension, decreased skin turgor)
• Euvolemic hypernatremia: Normal vital signs without edema
• Hypervolemic hypernatremia: Signs of fluid overload (edema, elevated JVP) 3, 2

2. Laboratory Evaluation

After assessing volume status, obtain the following:

• Serum electrolytes (including calcium and magnesium)
• Blood urea nitrogen and creatinine
• Urine sodium concentration
• Urine osmolality
• Urine volume measurement 2

3. Exclude Pseudohypernatremia

Rule out laboratory artifacts or conditions that may falsely elevate sodium readings, such as severe hyperlipidemia or hyperproteinemia 2

4. Determine Glucose-Corrected Sodium

Hyperglycemia can cause factitious hypernatremia. For every 100 mg/dL increase in glucose above normal, serum sodium decreases by approximately 1.6 mEq/L 2

5. Measure Urine Output and Osmolality

• Low urine osmolality (<300 mOsm/kg) with polyuria suggests diabetes insipidus
• High urine osmolality with low urine volume suggests extrarenal water losses 2

Diagnostic Algorithm Based on Volume Status

Hypovolemic Hypernatremia

• Urine Na+ <20 mEq/L: Suggests extrarenal losses (diarrhea, vomiting, excessive sweating)
• Urine Na+ >20 mEq/L: Suggests renal losses (diuretics, osmotic diuresis, intrinsic renal disease)

Euvolemic Hypernatremia

• Primary cause is water loss without adequate replacement
• Consider diabetes insipidus (central or nephrogenic)
• Assess for inadequate water intake due to altered mental status or lack of access to water

Hypervolemic Hypernatremia

• Rare condition
• Consider iatrogenic causes (excessive administration of hypertonic saline or sodium bicarbonate)
• Consider primary hyperaldosteronism for chronic cases 1, 2

Common Pitfalls to Avoid

1. Failure to correct for hyperglycemia: Always check glucose levels and calculate corrected sodium
2. Overlooking medication effects: Review all medications, particularly those that can cause nephrogenic diabetes insipidus (lithium, amphotericin B)
3. Inadequate assessment of volume status: This is the cornerstone of diagnosis and guides treatment
4. Delayed treatment: Acute hypernatremia requires prompt intervention to prevent neurological complications
5. Focusing only on sodium levels: Consider associated electrolyte abnormalities that may coexist 2

Special Considerations

• In elderly patients, impaired thirst mechanism is a common cause of hypernatremia
• In hospitalized patients, inadequate free water administration is a frequent iatrogenic cause
• Patients with altered mental status are at higher risk due to inability to express thirst or access water 4

Remember that proper assessment of volume status is the critical first step in evaluating hypernatremia, as it narrows down potential causes and guides appropriate treatment strategies.