What is the normal range for serum osmolality?

Normal Serum Osmolality Range

The normal range for serum osmolality is 275-295 mOsm/kg of water. 1

Understanding Serum Osmolality

Serum osmolality is the primary indicator of hydration status and is defined as the concentration of osmotically active components in the blood, measured as milliosmoles per kilogram of water. It represents the sum of concentrations of several key components:

• Sodium, chloride, bicarbonate, and potassium are the primary electrolytes contributing to serum osmolality 2
• Glucose and urea are the main non-electrolyte contributors 3
• The most commonly used formula for calculating serum osmolality is: Posm = 2[Na+] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 3

Clinical Significance of Abnormal Serum Osmolality

Hyperosmolality (>300 mOsm/kg)

• Indicates low-intake dehydration, a serious condition particularly in older adults 2
• Associated with increased risk of mortality and disability 2
• Requires prompt intervention to prevent poor outcomes 2
• Management includes:
  • For patients who appear well: increased oral fluid intake with preferred beverages 2
  • For patients who appear unwell: subcutaneous or intravenous fluids alongside oral intake 2

Hyposmolality (<275 mOsm/kg)

• May indicate overhydration or inappropriate ADH secretion 4
• Can lead to symptoms including weakness, confusion, headache, and nausea at lower levels 4
• Severe hyposmolality (<120 mEq/L sodium) can be life-threatening 4

Measurement Methods and Considerations

Direct Measurement vs. Calculation

• Direct measurement using freezing point depression is the gold standard method 3, 5
• Calculated osmolality can introduce systematic bias compared to direct measurement 6
• When direct measurement is unavailable, the recommended formula is:
  • Osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all in mmol/L) 2
  • An action threshold of >295 mmol/L should be used when using this calculation 2

Important Caveats

• Simple clinical signs like skin turgor, mouth dryness, or urine color should NOT be used to assess hydration status, especially in older adults 2
• Bioelectrical impedance is not reliable for hydration assessment 2
• Assessment of fluid intake is often highly inaccurate, particularly in residential care settings 2
• Interpretation of serum osmolality depends on checking that serum glucose and urea are within normal ranges 2

Clinical Applications

• Serum osmolality measurement is particularly important in neurological intensive care to manage cerebral edema 5
• In heart failure patients, optimal serum osmolality appears to be close to the upper limit of normal range (292-293 mOsm/kg) for best cardiovascular prognosis 1
• Regular monitoring of serum osmolality is recommended for older adults, especially those at risk of dehydration 2