How do I calculate a patient's serum osmolality?

How to Calculate Serum Osmolality

Use the formula: Calculated Osmolality = 2 × [Na (mEq/L)] + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8, which is the American Diabetes Association's recommended standard for clinical practice. 1

The Standard Clinical Formula

The most widely accepted calculation uses three readily available laboratory values 1:

• Sodium (Na) is multiplied by 2 to account for its accompanying anions (chloride and bicarbonate), measured in mEq/L 1, 2
• Glucose is divided by 18 to convert from mg/dL to mOsm/kg 1, 3
• Blood Urea Nitrogen (BUN) is divided by 2.8 to convert from mg/dL to mOsm/kg 1, 3

This formula has high-level evidence supporting its use across clinical settings and provides acceptable accuracy for screening purposes 1.

Alternative Formula for Geriatric Populations

For older adults, a more comprehensive formula can be used 1, 2:

• Osmolarity = 1.86 × (Na + K) + 1.15 × glucose + urea + 14 (all measured in mmol/L) 2
• This formula includes potassium and uses different conversion factors, with moderate strength of evidence 1
• Use an action threshold of >295 mmol/L to trigger intervention in geriatric patients 1, 4

Calculating Effective Osmolality (Tonicity)

When you need to assess true osmotic driving force across cell membranes, exclude BUN since urea freely crosses membranes 1, 4:

• Effective Osmolality = 2 × [Na] + Glucose/18 1, 4
• This is particularly important for diagnosing conditions like Hyperosmolar Hyperglycemic State (HHS), which requires effective osmolality ≥320 mOsm/kg 4

Normal Values and Clinical Thresholds

Interpret your calculated values against these established ranges 1, 2:

• Normal range: 275-295 mOsm/kg 1, 2
• >295 mmol/L: Action threshold for geriatric patients 1
• >300 mOsm/kg: Indicates low-intake dehydration requiring immediate intervention 1, 4
• ≥320 mOsm/kg: Diagnostic criterion for Hyperosmolar Hyperglycemic State 1, 4
• <275 mOsm/kg: Indicates hyposmolality, suggesting overhydration or SIADH 4

Critical Caveats and Pitfalls

Direct measurement is the gold standard and should be used when available, as calculated osmolality may miss unmeasured osmoles like alcohols, mannitol, or other toxins 1, 4.

The Osmolal Gap

• Calculate the osmolal gap by subtracting your calculated osmolality from the directly measured value 1, 3
• Normal gap: 0 ± 2 mOsm/L 1, 3
• An elevated gap (>10 mOsm/L) suggests the presence of unmeasured osmoles such as methanol, ethylene glycol, ethanol, or mannitol 3, 5

When Calculated Values May Be Misleading

• Hyperglycemia: Correct sodium for glucose elevation—add 1.6 mEq to measured sodium for each 100 mg/dL glucose above 100 mg/dL before calculating osmolality 4
• Pseudohyponatremia: Calculated osmolality helps confirm this diagnosis when measured osmolality is normal despite low sodium 5
• Do not rely on clinical signs like skin turgor, dry mouth, or urine color to assess osmolality, especially in older adults—these are unreliable 1, 4

Unit Conversions

If your laboratory reports values in different units 2, 3:

• Glucose: mg/dL ÷ 18 = mmol/L
• BUN: mg/dL ÷ 2.8 = mmol/L
• Urea: mg/dL ÷ 6 = mmol/L (if urea is reported instead of BUN)

Monitoring During Treatment

When actively correcting osmolality abnormalities 4:

• Recheck osmolality every 2-4 hours during active treatment 4
• Do not change osmolality faster than 3 mOsm/kg/h to avoid complications like osmotic demyelination syndrome 4
• Verify that glucose and urea are within normal ranges before attributing abnormal osmolality solely to hydration status 2, 4