What lab values indicate hypovolemia or dehydration?

Laboratory Values Indicating Hypovolemia or Dehydration

Directly measured serum or plasma osmolality is the primary laboratory indicator for assessing hydration status, with values >300 mOsm/kg indicating dehydration. 1

Distinguishing Between Hypovolemia and Dehydration

These terms are often confused but represent different physiological states:

• Dehydration: Loss of water from the intracellular compartment, typically due to hypernatremia 2
• Hypovolemia: Depletion of water from the extracellular space, which can occur with hypernatremia, eunatremia, or hyponatremia 2

Key Laboratory Values for Dehydration (Low-Intake Dehydration)

• Serum/plasma osmolality >300 mOsm/kg: The gold standard for diagnosing dehydration 1
• Calculated osmolarity >295 mmol/L: When direct measurement is unavailable, using the formula: osmolarity = 1.86 (Na⁺ + K⁺) + 1.15 glucose + urea + 14 (all measured in mmol/L) 1, 3
• Small rises in sodium, potassium, urea, and glucose: Often all remain within normal ranges, but show concentration effects 1

Key Laboratory Values for Hypovolemia

• Blood Urea Nitrogen (BUN): Elevated due to reduced glomerular filtration rate 4
• BUN/Creatinine ratio >20:1: Suggests prerenal azotemia from volume depletion 5
• Hematocrit and total serum protein: Elevated due to hemoconcentration, though these may underestimate the actual deficit 4
• Urine specific gravity >1.020: Indicates concentrated urine from volume depletion 5
• Urine sodium <20 mEq/L: Suggests appropriate renal response to volume depletion 5
• Urine osmolality >500 mOsm/kg: Indicates concentrated urine from volume conservation 5

Laboratory Findings in Specific Types of Hypovolemia

Hypovolemic Hyponatremia

• Serum sodium <134 mEq/L
• Elevated hematocrit and protein levels
• Low urine sodium (<20 mEq/L)
• High urine osmolality 1, 5

Hypovolemic Hypernatremia

• Serum sodium >145 mEq/L
• Elevated serum osmolality
• Concentrated urine (high specific gravity) 5

Hypovolemic Eunatremia

• Normal serum sodium (135-145 mEq/L)
• Proportional losses of water and sodium
• Signs of volume depletion in other laboratory parameters 2

Important Caveats

• Simple clinical signs like skin turgor, mouth dryness, weight change, urine color or specific gravity should NOT be used to assess hydration status, especially in older adults 1, 3
• Interpretation of serum osmolality requires checking that serum glucose and urea are within normal ranges 1
• Postural vital signs (increase in heart rate ≥30 beats/min upon standing) have high specificity but low sensitivity for moderate blood loss 6
• Laboratory assessment is essential when diagnostic certainty is required, as clinical signs alone have limited reliability 6
• Hemoglobin concentration dynamics in response to fluid loading can help distinguish between baseline normohydration and dehydration 7

Clinical Algorithm for Laboratory Assessment

1. First-line test: Measure serum/plasma osmolality directly (>300 mOsm/kg indicates dehydration) 1
2. If direct measurement unavailable: Calculate osmolarity using the formula (>295 mmol/L indicates dehydration) 1
3. Additional tests to characterize the type of fluid disturbance:
   • Serum electrolytes (sodium, potassium, chloride, bicarbonate)
   • BUN and creatinine
   • Complete blood count (for hematocrit)
   • Urine studies (sodium, osmolality, specific gravity) 5, 4
4. Interpret results together to distinguish between:
   • Hypovolemia with normal osmolality
   • Hypovolemia with abnormal osmolality
   • Dehydration without hypovolemia 2, 5