Does osmolality (concentration of solutes in the blood) indicate dehydration?

Serum Osmolality as an Indicator of Dehydration

Directly measured serum osmolality is the gold standard for diagnosing dehydration, with a value >300 mOsm/kg indicating low-intake dehydration requiring intervention. 1, 2

Diagnostic Accuracy of Serum Osmolality

• Serum osmolality is the sum of concentrations of osmotically active components, especially sodium, chloride, bicarbonate, potassium, glucose, and urea 1
• A threshold of >300 mOsm/kg has been rigorously established to classify a person as dehydrated 1
• This cut-off value concurs with observations from cohort studies assessing effects of raised serum osmolality in older people 1
• Plasma osmolality represents a valuable, objective surrogate marker of hypertonic dehydration 3

Alternative Methods When Direct Measurement is Unavailable

• 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, 4
• This equation has been validated and found to be an accurate objective diagnostic tool to assess dehydration with a sensitivity of 90% and specificity of 68% 5
• The equation is useful in predicting serum osmolality in people aged 65 years with and without diabetes, poor renal function, in various care settings 1

Unreliable Methods for Assessing Dehydration

• Simple signs and tests commonly used to assess dehydration should NOT be used to assess hydration status, especially in older adults 1
• These unreliable indicators include:
  • Skin turgor
  • Mouth dryness
  • Weight change
  • Urine color or specific gravity
  • Bioelectrical impedance 1, 6
• Studies have shown that these physical signs demonstrate poor sensitivity (0%-44%) for detecting dehydration 6
• Urine specific gravity, urine color, and urinary osmolality have diagnostic accuracy too low to be useful indicators of hydration status 7

Important Clinical Considerations

• 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 1
• In low-intake dehydration, it is common that despite raised serum osmolality, none of the major components (sodium, potassium, urea or glucose) is raised out of the normal range 1
• Dehydration is often under-recognized and poorly managed in hospital and community-based care settings 3
• Only about 20% of dehydrated patients (according to measured osmolality) are correctly clinically diagnosed with dehydration 5

Distinguishing Types of Dehydration

• "Dehydration" and "hypovolaemia" are incorrectly used interchangeably 3
• Low-intake dehydration (hyperosmolar) is distinct from volume depletion due to excess fluid losses 4, 7
• Extracellular water loss (volume depletion) due to diarrhea, vomiting or renal sodium loss is connected with normal or low plasma osmolality 1

In conclusion, serum osmolality is the most reliable indicator of dehydration status, particularly for detecting low-intake dehydration. Other commonly used clinical signs and urine parameters have poor diagnostic accuracy and should not be relied upon for assessing hydration status.