What is serum osmolality?

Serum Osmolality: Definition and Clinical Significance

Serum osmolality is the sum of concentrations of osmotically active components in blood, primarily sodium, chloride, bicarbonate, potassium, glucose, and urea, measured in milliosmoles per kilogram of water (mOsm/kg). 1, 2

Definition and Measurement

• Serum osmolality represents the concentration of dissolved particles in blood serum that create osmotic pressure across membranes 3
• Normal serum osmolality range is 275-295 mOsm/kg of water 2, 4
• Serum osmolality can be directly measured in the laboratory or calculated using formulas 1
• The most accurate method is direct laboratory measurement of serum osmolality 1

Calculation Methods

• When direct measurement isn't available, serum osmolality can be calculated using the formula: Osmolarity = 1.86 (Na+ + K+) + 1.15 glucose + urea + 14 (all measured in mmol/L) 1, 2
• A simplified formula commonly used is: Osmolality = 2 × [Na+] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 1, 4
• When using calculated osmolality, an action threshold of >295 mmol/L should be used to screen for dehydration 1

Components Contributing to Osmolality

• Sodium and its associated anions (primarily chloride and bicarbonate) are the major determinants of serum osmolality 3, 2
• Potassium, glucose, and urea are other significant contributors 1, 3
• Under normal conditions, these components may remain within normal ranges individually, but their cumulative effect determines total osmolality 1

Clinical Significance

Hyperosmolality (>300 mOsm/kg)

• Indicates low-intake dehydration, particularly important in older adults 1
• Associated with increased risk of mortality and disability 1
• Common causes include:
  • Inadequate fluid intake 1
  • Hypernatremia 3
  • Hyperglycemia 3
  • Certain intoxications 3, 5

Hyposmolality (<275 mOsm/kg)

• May indicate overhydration or inappropriate ADH secretion 2
• Can occur with:
  • Volume depletion due to diarrhea, vomiting, or renal sodium loss 1
  • Acute diarrheal illnesses 6

Osmolal Gap

• The difference between measured osmolality and calculated osmolality 5, 4
• Elevated osmolal gap may indicate presence of unmeasured osmotically active substances, such as:
  • Toxic alcohols (methanol, ethylene glycol) 5
  • Mannitol or other hypertonic treatments 3

Important Clinical Considerations

• Serum osmolality is the primary indicator of hydration status, particularly in older adults 1
• Clinical signs like skin turgor, mouth dryness, or urine color should NOT be used to assess hydration status, especially in older adults 1
• Interpretation of serum osmolality requires checking that serum glucose and urea are within normal ranges 1, 2
• Regular monitoring of serum osmolality is recommended for older adults at risk of dehydration 1, 2

Osmolality vs. Osmolarity

• Osmolality is measured as milliosmoles per kilogram of solvent (mOsm/kg) 4, 7
• Osmolarity is measured as milliosmoles per liter of solution (mOsm/L) 4, 7
• In dilute solutions like serum, the numerical difference is minimal, but the distinction is important for precise clinical applications 7