What bloodwork results indicate dehydration?

Laboratory Assessment of Dehydration Status

Serum/plasma osmolality is the most reliable laboratory indicator of hydration status, with values >300 mOsm/kg definitively indicating dehydration. 1, 2

Primary Laboratory Indicators

• Directly measured serum or plasma osmolality is the gold standard for identifying dehydration, with a threshold of >300 mOsm/kg indicating dehydration 2, 1
• When direct osmolality measurement is unavailable, calculated serum osmolarity can be used with the formula: osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all measured in mmol/L), with an action threshold of >295 mmol/L 2
• Serum sodium, potassium, glucose, and urea are the key components that contribute to osmolality and should be measured together 1
• Blood urea nitrogen (BUN) and creatinine levels are typically elevated in dehydration due to decreased renal perfusion 2, 3
• BUN-to-creatinine ratio >20:1 often suggests dehydration 3

Interpretation of Laboratory Values

• In low-intake dehydration, serum osmolality is elevated (>300 mOsm/kg) even when individual components (sodium, potassium, urea, glucose) may each remain within normal ranges 2
• When interpreting elevated serum osmolality, check that serum glucose and urea are within normal ranges; if not, these should be normalized by appropriate treatment 2, 1
• Corrected serum sodium for hyperglycemia should be calculated (add 1.6 mEq to sodium value for each 100 mg/dL glucose >100 mg/dL) to accurately assess true sodium status 2
• Hemoconcentration due to dehydration may cause elevated hemoglobin and hematocrit values 3

Types of Dehydration and Laboratory Findings

• Hyperosmolar dehydration (water loss dehydration): characterized by elevated serum osmolality (>300 mOsm/kg) and often hypernatremia (Na >145 mEq/L) 4, 5
• Iso-osmolar dehydration (salt and water loss): characterized by normal serum osmolality with signs of volume depletion 5, 6
• Hypo-osmolar dehydration: characterized by low serum osmolality (<280 mOsm/kg) and hyponatremia (Na <135 mEq/L), typically seen when water intake exceeds sodium intake during rehydration 4, 1

Ineffective Laboratory Tests

• Urinary indices (color, specific gravity, osmolality) should NOT be used to assess hydration status in older adults as they have been shown to be unreliable 2, 1
• Bioelectrical impedance analysis (BIA) should NOT be used to assess hydration status as it has not been shown to be consistently diagnostic 2
• Simple clinical signs and tests such as skin turgor, mouth dryness, and weight change should NOT be used as sole indicators of dehydration status, especially in older adults 2, 4

Monitoring During Rehydration

• Electrolytes (sodium, potassium, chloride, bicarbonate) should be checked every 2-4 hours during initial treatment of severe dehydration 2
• Blood glucose should be monitored every 1-2 hours until stable in cases of hyperglycemia-related dehydration 2, 1
• Calculate effective serum osmolality regularly to guide fluid management and prevent complications like cerebral edema 1, 2

Common Pitfalls in Laboratory Assessment

• Failing to correct serum sodium for hyperglycemia in diabetic patients can mask true sodium status 2, 1
• Misinterpreting elevated BUN as renal failure rather than dehydration 3
• Using urinary parameters as primary indicators of hydration status, which are unreliable especially in older adults 2, 1
• Relying on a single laboratory value rather than evaluating the complete picture including osmolality, electrolytes, and renal function tests 4, 1
• Confusing volume depletion (hypovolemia) with dehydration, which are distinct physiological states with different laboratory findings 4, 6