Calculated Osmolality in Clinical Practice

Assessment of Osmolality

The American College of Clinical Nutrition recommends that directly measured serum or plasma osmolality should be used to identify low-intake dehydration, with an action threshold of >300 mOsm/kg indicating dehydration requiring intervention 1, 2
Serum osmolality is the sum of concentrations of osmotically active components, especially sodium, chloride, bicarbonate, potassium, glucose, and urea 1
Directly measured serum osmolality is the reference standard for assessing hydration status in clinical practice 1, 2
When direct measurement is unavailable, calculated osmolarity using the equation: osmolarity = 1.86 (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L) with an action threshold of >295 mmol/L should be used 1, 2

Serum osmolality >300 mOsm/kg indicates low-intake dehydration requiring intervention 1, 4
Interpretation of raised serum osmolality depends on checking that serum glucose and urea are within normal range; if not, these should be normalized by appropriate treatment 2

For patients who appear well with low-intake dehydration, the American College of Clinical Nutrition recommends encouraging increased oral fluid intake using drinks preferred by the patient 4
For patients who appear unwell with low-intake dehydration, the American College of Clinical Nutrition recommends offering subcutaneous or intravenous fluids in parallel with encouraging oral fluid intake 4

Simple signs and tests commonly used to assess dehydration should NOT be used to assess hydration status in older adults 1
Bioelectrical impedance should NOT be used to assess hydration status in older adults 1
Assessment of fluid intake is often highly inaccurate in older adults in residential care settings 4
The American Geriatrics Society notes that do not confuse low-intake dehydration with volume depletion, as they require different fluid management approaches 5

In older adults, renal function is often poor, making renal parameters unreliable indicators of low-intake dehydration 1
For elderly patients, individualized fluid management is crucial, considering comorbidities and medication use 5