Answer
Osmolal Gap Calculation: Step-by-Step Medical Guide
Given Laboratory Data
- Measured serum osmolality: 320 mOsm/kg
- Sodium (Na⁺): 140 mmol/L
- Glucose: 180 mg/dL
- BUN (Blood Urea Nitrogen): 28 mg/dL
Step 1: Convert Glucose and BUN to mmol/L
To apply the osmolality formula, glucose and BUN must be in mmol/L units:
- Glucose (mmol/L): 180 ÷ 18 = 10 mmol/L
- BUN (mmol/L): 28 ÷ 2.8 = 10 mmol/L
Step 2: Calculate Serum Osmolality (Calculated Value)
Formula: Calculated osmolality = (2 × Na⁺) + (Glucose ÷ 18) + (BUN ÷ 2.8)
= (2 × 140) + 10 + 10
= 280 + 10 + 10
= 300 mOsm/kg
Step 3: Determine the Osmolal Gap
Osmolal Gap = Measured Osmolality − Calculated Osmolality
= 320 − 300
= 20 mOsm/kg
Conclusion
The osmolal gap is calculated to be 20 mOsm/kg, which may indicate the presence of unmeasured osmoles such as alcohols, glycols, or other toxins in the bloodstream. This calculation is critical for clinical assessment in cases of suspected poisoning or metabolic imbalance.