What is the calculation for serum osmolality to determine dehydration?

Serum Osmolality Calculation for Dehydration Assessment

The standard formula to calculate serum osmolality for determining dehydration is: 2 × [Na (mEq/L)] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8, with a threshold >300 mOsm/kg indicating dehydration. 1

Primary Calculation Formula

The most widely recommended equation across clinical guidelines is:

Calculated Osmolality = 2 × [Na] + (Glucose/18) + (BUN/2.8)

Where all values use standard units: sodium in mEq/L, glucose in mg/dL, and BUN in mg/dL. 1, 2

• This formula is endorsed by the American Diabetes Association for hyperglycemic crises and has high-level evidence supporting its use 1, 2
• The multiplication factor of 2 for sodium accounts for its accompanying anions (primarily chloride and bicarbonate) 2
• Normal plasma osmolality ranges from 275-295 mOsm/kg 2, 3

Dehydration Thresholds

For identifying dehydration, use these specific cut-offs:

• >300 mOsm/kg = Dehydration present - This threshold is based on rigorous research separating hydrated from dehydrated states and is associated with increased mortality and disability risk 1, 4
• >320 mOsm/kg = Severe hyperosmolar state (diagnostic criterion for Hyperosmolar Hyperglycemic State in diabetics) 1, 4

Alternative Formula for Geriatric Populations

When managing older adults specifically, an alternative validated equation can be used:

Osmolarity = 1.86 × (Na + K) + 1.15 × glucose + urea + 14 (all measured in mmol/L)

• This formula has been validated specifically in European cohorts of older adults with and without diabetes, poor renal function, and various health conditions 1
• Use an action threshold of >295 mmol/L for this equation to trigger intervention in older adults 1, 2
• This lower threshold (295 vs 300) provides 85% sensitivity and 59% specificity for detecting low-intake dehydration in elderly patients 1

Effective Osmolality (Tonicity)

For assessing true dehydration (excluding urea, which freely crosses cell membranes):

Effective Osmolality = 2 × [Na] + (Glucose/18)

• This formula excludes BUN because urea is an ineffective osmole that doesn't cause water shifts across cell membranes 1, 2, 3
• The American Diabetes Association specifically recommends this calculation for managing hyperglycemic crises 1
• Normal tonicity range is 275-295 mOsm/kg 3

Critical Clinical Caveats

Direct measurement is superior to calculation:

• Directly measured serum osmolality using freezing point depression is the gold standard and should be used when available 1, 4
• Calculated osmolality is acceptable for screening but may miss unmeasured osmoles (alcohols, toxins, mannitol) 2, 3
• The osmolal gap (measured minus calculated) should be 0 ± 2 mOsm/L; elevated gaps suggest unmeasured substances 2, 3

Correct sodium for hyperglycemia first:

• For every 100 mg/dL glucose above 100 mg/dL, add 1.6 mEq to the measured sodium value before calculating osmolality 1, 4
• This correction is essential because hyperglycemia causes osmotic water shifts that dilute serum sodium 1

Interpretation requires context:

• Ensure glucose and BUN are within normal range before interpreting elevated osmolality as dehydration 1, 4
• If glucose or BUN are markedly elevated, these should be normalized first to accurately assess hydration status 1

Do NOT use unreliable clinical signs:

• Skin turgor, mouth dryness, urine color, urine specific gravity, and weight change are unreliable for assessing dehydration, especially in older adults 1, 4
• These have inadequate diagnostic accuracy and should not guide clinical decisions 1, 4

Monitoring During Treatment

When treating dehydration:

• The rate of osmolality decrease should not exceed 3 mOsm/kg/hour to prevent cerebral edema 1, 4
• Recheck serum osmolality every 2-4 hours during active rehydration 4
• In pediatric patients with DKA, fluid replacement should achieve osmolality decrease not exceeding 3 mOsm/kg/hour over 48 hours 1