Which laboratory test best indicates dehydration?

Serum Osmolality is the Best Laboratory Test for Diagnosing Dehydration

Directly measured serum or plasma osmolality is the gold standard laboratory test for diagnosing dehydration, with values >300 mOsm/kg indicating definitive dehydration. 1

Understanding Dehydration Assessment

Dehydration diagnosis requires objective laboratory measurement rather than relying on clinical signs alone. The evidence clearly indicates that serum osmolality provides the most accurate assessment of hydration status:

• Serum osmolality thresholds indicate hydration status:

  • 300 mOsm/kg: Definitive dehydration

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

• When direct measurement is unavailable, calculated osmolarity can be used with a formula of 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with an action threshold of >295 mmol/L indicating potential dehydration 1

Why Serum Osmolality is Superior

Serum osmolality has been rigorously assessed as the most accurate indicator of hydration status:

• It directly measures the concentration of osmotically active components in the blood (sodium, chloride, bicarbonate, potassium, glucose, and urea) 2
• The threshold of >300 mOsm/kg was developed by assessing hydrated versus dehydrated states in controlled studies 2
• This threshold concurs with observations from cohort studies assessing effects of raised serum osmolality in older people 2

Limitations of Other Common Dehydration Tests

Many commonly used tests for dehydration are unreliable:

• Physical examination findings (skin turgor, mouth dryness) are often insensitive and unreliable indicators of dehydration, particularly in older adults 1
• Urine tests (specific gravity, color, osmolality) should not be used as standalone tests for dehydration diagnosis 1
• Bioelectrical impedance shall NOT be used to assess hydration status in older adults as it has not been shown to be usefully diagnostic 2
• Weight change alone is not reliable for assessing dehydration 2

Types of Dehydration and Laboratory Assessment

Understanding the type of dehydration is crucial for proper interpretation of laboratory values:

• Low-intake dehydration (hyperosmolar): Characterized by elevated serum osmolality (>300 mOsm/kg) and may present with elevated serum sodium, commonly seen in older adults with inadequate fluid intake 1
• Volume depletion (from fluid losses): May present with normal or low osmolality, caused by vomiting, diarrhea, or excessive sweating 1

Additional Laboratory Tests to Consider

While serum osmolality is the gold standard, other laboratory tests can provide supplementary information:

• Blood urea nitrogen (BUN)/creatinine ratio: Elevated in dehydration, but less specific than osmolality 3
• Serum sodium: Important for determining the type of dehydration (hypernatremic vs. hyponatremic) 1
• Bicarbonate levels: Can help assess the degree of dehydration, particularly in children with gastroenteritis 4

Clinical Application

For practical application in clinical settings:

1. Measure serum osmolality as the primary diagnostic test for suspected dehydration
2. Use >300 mOsm/kg as the definitive threshold for dehydration diagnosis
3. Consider calculated osmolarity when direct measurement is unavailable
4. Interpret results in the context of the patient's clinical condition
5. Do not rely on physical examination findings or urine tests alone

Common Pitfalls to Avoid

• Relying solely on clinical signs: Physical examination findings are often unreliable, especially in older adults 2, 1
• Using urine tests as primary indicators: These are not reliable standalone tests for dehydration 1
• Confusing dehydration with volume depletion: These are distinct conditions requiring different management approaches 1
• Failing to measure serum osmolality: This is the gold standard and should be prioritized over other tests 2, 1
• Underestimating dehydration in older adults: Clinical signs are particularly unreliable in this population 2

Serum osmolality remains the most accurate and reliable laboratory test for diagnosing dehydration, providing objective measurement that guides appropriate clinical management and improves patient outcomes.