What is calculated with serum osmolality?

Serum Osmolality: Measurement and Clinical Significance

Serum osmolality measures the concentration of osmotically active particles in blood serum and is the primary indicator of hydration status, with values >300 mOsm/kg indicating dehydration. 1, 2

What Serum Osmolality Measures

• Serum osmolality is the sum of concentrations of all osmotically active components in blood, particularly sodium and associated anions, potassium, glucose, and urea 3
• Under normal conditions, serum sodium concentration is the major determinant of serum osmolality 3
• Normal serum osmolality range is 275-295 mOsm/kg 2, 4
• Values >300 mOsm/kg indicate low-intake dehydration, while values <275 mOsm/kg suggest overhydration or inappropriate ADH secretion 2, 4

Measurement Methods

Direct Measurement vs. Calculation

• Direct measurement of serum osmolality is the gold standard for assessing hydration status, especially in older adults 1
• When direct measurement is unavailable, calculated osmolality can be used with the following formula:
  • Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) 1, 2
• An action threshold of >295 mmol/L should be used when using calculated osmolality 1, 2

Osmolal Gap

• The osmolal gap is the difference between measured osmolality and calculated osmolality 5
• An increased osmolal gap can indicate the presence of unmeasured osmotically active substances such as:
  • Toxic alcohols (methanol, ethylene glycol)
  • Mannitol or other hypertonic treatments
  • Other exogenous substances not included in the calculation 5

Clinical Significance

Dehydration Assessment

• Serum osmolality >300 mOsm/kg is the definitive indicator of low-intake dehydration 1, 2
• High-quality cohort studies have consistently found that older adults with raised serum osmolality have an increased risk of mortality and disability 1
• Simple clinical signs like skin turgor, mouth dryness, or urine color should NOT be used to assess hydration status, especially in older adults 2

Monitoring Requirements

• All older persons should be screened for low-intake dehydration when they contact the healthcare system 1
• Regular monitoring is recommended, especially for older adults at risk of dehydration 2, 4
• For patients receiving medications like desmopressin, serum osmolality should be monitored to prevent hyponatremia 6

Interpretation Considerations

• Interpretation depends on checking that serum glucose and urea are within normal ranges 1, 2
• In older adults, renal function is often poor, making renal parameters less reliable indicators of hydration status 1, 4
• Effective osmolality (tonicity) considers only solutes that cannot freely cross cell membranes and is calculated as: 2[Na⁺] + glucose/18 7

Clinical Applications

• Diagnosing and monitoring hydration status, particularly in older adults 1, 2
• Evaluating disorders related to sodium, glucose, and water balance 3
• Identifying toxic alcohol ingestions through the osmolal gap 5
• Monitoring patients on medications that affect water balance, such as desmopressin 6
• Assessing the need for fluid restriction or supplementation in various clinical scenarios 2, 4

Common Pitfalls

• Relying on clinical signs rather than laboratory measurements for hydration assessment 1, 2
• Using inappropriate formulas for calculating osmolality when direct measurement is unavailable 8, 7
• Failing to consider the osmolal gap when evaluating patients with altered mental status 5
• Not monitoring serum osmolality in high-risk patients, such as older adults or those on medications affecting water balance 1, 6