Which blood tests (serum sodium, serum osmolality, blood urea nitrogen-to-creatinine ratio, hematocrit) indicate dehydration?

Blood Tests Indicating Dehydration

Serum osmolality >300 mOsm/kg is the single most reliable blood test for diagnosing dehydration, with directly measured values strongly preferred over calculated estimates. 1

Primary Diagnostic Tests

Serum Osmolality (Gold Standard)

• Directly measured serum or plasma osmolality >300 mOsm/kg indicates low-intake dehydration and carries a Grade B recommendation with 94% consensus from the European Society for Clinical Nutrition and Metabolism (ESPEN). 1
• This threshold is associated with increased mortality risk and doubled risk of 4-year disability, making it clinically significant beyond just laboratory abnormality. 1
• Normal range is 275-295 mOsm/kg; values >300 mOsm/kg require immediate intervention. 1, 2

Calculated Osmolarity (When Direct Measurement Unavailable)

• Use the formula: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with an action threshold of >295 mmol/L. 1
• This carries a Grade B recommendation with 94% agreement from ESPEN as a second-line option. 1
• Alternative simplified formula for rapid mental calculation: Osmolality = 2[Na⁺] + glucose/18 + BUN/2.8 + 9 (with glucose in mg/dL and BUN in mg/dL). 3

Supporting Laboratory Tests

Blood Urea Nitrogen (BUN) and BUN/Creatinine Ratio

• BUN/Creatinine ratio ≥15 suggests dehydration and is associated with poor clinical outcomes in acute conditions. 4
• BUN is a strong predictor of serum osmolality (beta = 0.315, p<0.000) and should be included in assessment. 5
• Elevated BUN with normal creatinine indicates prerenal azotemia from volume depletion. 6

Serum Sodium (with Correction for Hyperglycemia)

• Sodium is the major determinant of serum osmolality under normal conditions (beta = 0.778, p<0.000). 5, 7
• Correct serum sodium for hyperglycemia: add 1.6 mEq/L to measured sodium for each 100 mg/dL glucose above 100 mg/dL. 6
• Elevated corrected sodium suggests water deficit relative to solute. 7

Hematocrit

• Elevated hematocrit indicates hemoconcentration from volume depletion, though this is less specific than osmolality. 6
• Must be interpreted in context of baseline values and potential anemia. 6

Serum Bicarbonate

• Low serum bicarbonate (<15 mEq/L) combined with clinical parameters can predict dehydration severity. 8
• Particularly useful in gastroenteritis-related dehydration in children. 8

Critical Interpretation Caveats

Essential Pre-Interpretation Steps

• Always check that serum glucose and urea are within normal ranges before interpreting osmolality as dehydration, as abnormalities in these affect osmolality interpretation. 1, 9
• In hyperglycemic states (DKA/HHS), calculate effective osmolality: 2[measured Na] + glucose/18. 6

Tests to Avoid

• Do NOT use clinical signs (skin turgor, mouth dryness, weight change) or urine tests (specific gravity, color, osmolality) to assess hydration status—these carry a Grade A recommendation against use with 83-100% consensus. 1, 9
• Bioelectrical impedance must not be used (Grade A recommendation with 100% consensus). 1

Monitoring During Treatment

• Recheck serum osmolality every 2-4 hours during active treatment. 6, 9
• Ensure osmolality change does not exceed 3 mOsm/kg/hour to prevent cerebral edema. 9
• Monitor electrolytes, glucose, BUN, and creatinine every 2-4 hours alongside osmolality. 6