What lab results indicate dehydration in a patient?

Laboratory Indicators of Dehydration

Serum osmolality >300 mOsm/kg is the most accurate laboratory indicator of dehydration in patients. 1

Primary Laboratory Indicators

• Directly measured serum or plasma osmolality is the gold standard for identifying dehydration, with values >300 mOsm/kg indicating dehydration 1
• When direct measurement is unavailable, calculated osmolarity using the formula: Osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all in mmol/L) with a threshold of >295 mmol/L can be used 1
• Serum osmolality is the sum of concentrations of osmotically active components, especially sodium, chloride, bicarbonate, potassium, glucose, and urea 1, 2
• 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, 2

Secondary Laboratory Indicators

• Blood urea nitrogen to creatinine ratio >20 may indicate water-and-solute-loss dehydration 3
• In low-intake dehydration, despite raised serum osmolality, none of the major components (sodium, potassium, urea, glucose) may be raised outside the normal range—but general fluid concentration leads to small rises within the normal range in all these components 1
• Abnormal levels of serum sodium correlate with increased severity of dehydration 4

Unreliable Indicators

• Simple signs and tests commonly used to assess dehydration such as skin turgor, mouth dryness, weight change, urine color or specific gravity should NOT be used to assess hydration status, especially in older adults 1, 2
• Bioelectrical impedance should NOT be used to assess hydration status as it has not been shown to be usefully diagnostic 1
• Urinary measures (color, specific gravity, osmolality) have limited diagnostic value for dehydration 5, 3

Clinical Implications

• Dehydration is associated with increased morbidity and mortality, particularly in older adults 1, 6
• For patients with measured serum or plasma osmolality >300 mOsm/kg who appear unwell, subcutaneous or intravenous fluids should be offered in parallel with encouraging oral fluid intake 1
• Regular monitoring of serum osmolality is recommended for older adults, especially those at risk of dehydration 2

Special Considerations

• In older adults, renal function is often impaired, making renal parameters less reliable indicators of hydration status 1, 2
• Assessment of fluid intake is often highly inaccurate, especially in care settings, making laboratory assessment of hydration status particularly important 1
• Saliva osmolality may have some utility as a non-invasive marker for dehydration assessment, showing moderate diagnostic accuracy compared to traditional physical signs 3

Diagnostic Algorithm

1. Measure serum osmolality directly (gold standard)
2. If >300 mOsm/kg: diagnose dehydration 1
3. If direct measurement unavailable, calculate osmolarity using the formula and use >295 mmol/L as threshold 1
4. Check that serum glucose and urea are within normal ranges to properly interpret osmolality results 1
5. Consider BUN/creatinine ratio as a secondary indicator 3
6. Do not rely on physical signs or urinary indices for diagnosis 1