Learning Objective: By the end of this lesson, the learner will be able to calculate the osmolar gap from a BMP, understand the basis of the calculation, and recognize common clinical scenarios that cause an elevated osmolar gap.
Osmolar Gap
The osmolar gap (OG) is the difference between measured osmolality and calculated osmolality:
- Normal OG: ≤ 15 mOsm/kg
Estimated Osmolality Formula
![\text{Calculated Osmolality (mOsm/kg)} \approx 2 \times [\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}](https://s0.wp.com/latex.php?latex=%5Ctext%7BCalculated+Osmolality+%28mOsm%2Fkg%29%7D+%5Capprox+2+%5Ctimes+%5B%5Ctext%7BNa%7D%5E%2B%5D+%2B+%5Cfrac%7B%5Ctext%7BGlucose%7D%7D%7B18%7D+%2B+%5Cfrac%7B%5Ctext%7BBUN%7D%7D%7B2.8%7D&bg=ffffff&fg=000&s=0&c=20201002)
Basis of the Formula
- Na⁺ is the most abundant extracellular osmole.
- Na⁺ is doubled to account for its accompanying anions (mostly Cl⁻).
- Conversion factors (18 and 2.8) transform glucose and BUN from mg/dL to mOsm/L.
Clinical Relevance
- An elevated osmolar gap helps narrow differential diagnoses.
- Common causes of elevated OG:
| Substance | Notes |
|---|---|
| Ethanol | Alcohol ingestion |
| Methanol | Toxic alcohol can cause visual disturbances |
| Ethylene glycol | Found in antifreeze, it can cause renal failure |
| Acetone | Ketoacidosis |
| Mannitol | Hyperosmotic therapy for cerebral edema |
Tip: An inebriated patient with a normal glucose and BUN may have an elevated OG due to ethanol or another alcohol.
