Is serum osmolality high or low in a dehydrated individual?

Serum Osmolality in Dehydration

In dehydration, serum osmolality is high (>300 mOsm/kg), which is the primary diagnostic indicator of low-intake dehydration. 1

Physiological Basis

When a person becomes dehydrated due to inadequate fluid intake, the fluid within and around cells becomes more concentrated, leading to:

• Increased serum osmolality (>300 mOsm/kg)
• Concentration of osmotically active components in the blood
• Triggering of protective physiological mechanisms including thirst and increased urine concentration 1

This increased osmolality occurs because the total amount of solutes in the blood remains relatively constant while the water content decreases, resulting in a more concentrated solution.

Types of Dehydration and Osmolality

It's important to distinguish between different types of dehydration:

1. Low-intake dehydration (water-loss dehydration):

   • Characterized by high serum osmolality (>300 mOsm/kg)
   • Results from inadequate fluid intake
   • Primary concern in most clinical dehydration scenarios 1

2. Extracellular volume depletion:

   • Results from conditions like diarrhea, vomiting, or renal sodium loss
   • Associated with normal or low plasma osmolality
   • Different pathophysiology than simple dehydration 1

Diagnostic Thresholds

The most rigorous assessment of dehydration uses the following thresholds:

• >300 mOsm/kg: Definitive dehydration
• 295-300 mOsm/kg: Impending dehydration
• <295 mOsm/kg: Normal hydration 1

These thresholds have been validated in studies comparing hydrated and dehydrated individuals and are associated with increased risk of mortality and disability in older adults 1.

Measurement Methods

For accurate assessment of hydration status:

1. Direct measurement of serum osmolality is the gold standard (using freezing point depression method) 1

2. Calculated osmolarity can be used when direct measurement isn't available:

   • Formula: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all measured in mmol/L)
   • Action threshold of >295 mOsm/L
   • Sensitivity of 85%, specificity of 59% 1, 2

Unreliable Assessment Methods

The following should NOT be used to assess hydration status, particularly in older adults:

• Skin turgor
• Mouth dryness
• Weight change
• Urine color or specific gravity
• Bioelectrical impedance 1

Clinical judgment alone is often unreliable, especially in older adults, with studies showing that clinicians correctly identify only about 20% of dehydrated patients based on clinical signs 2.

Physiological Effects

Dehydration with elevated serum osmolality affects multiple body systems:

• Sodium, as the primary determinant of interstitial fluid osmotic pressure, becomes more concentrated 3
• Electrolyte imbalances may develop, potentially leading to alkalosis if chloride excretion exceeds intake 3
• Increased risk of acute kidney injury in critically ill patients 4
• Potential neurological effects if significant osmolality differences develop between serum and cerebrospinal fluid 5

Key Takeaways

1. Serum osmolality is the primary indicator of hydration status
2. Dehydration is definitively diagnosed at >300 mOsm/kg
3. Direct measurement is preferred, but calculated osmolarity can be used for screening
4. Traditional clinical signs of dehydration are unreliable, especially in older adults
5. Both high and low serum osmolality are associated with increased risk of complications