How is osmolality calculated from a standard chemistry panel, and what is the osmolal gap used for?

• A. Estimated osmolality = Na+ + glucose/18 + BUN/2.8; osmolal gap detects unmeasured osmotically active substances (e.g., alcohols, methanol).
• B. Estimated osmolality = 2Na + glucose/18 + BUN/2.8; osmolal gap detects hydration status.
• C. Estimated osmolality = Na + glucose + BUN; osmolal gap detects electrolyte abnormalities.
• D. Measured osmolality directly; osmolal gap is not used.

Answer: (A)

Osmolality reflects how many osmotically active particles are in serum, and the main contributors in a standard panel are sodium (and its accompanying anions) plus small solutes like glucose and urea nitrogen. A quick estimate converts these into milliosmoles per kilogram: roughly doubling the sodium value (to account for sodium salts), then adding glucose divided by 18 and BUN divided by 2.8. The osmolal gap is the difference between the measured osmolality and this calculated estimate. A larger gap points to the presence of other unmeasured osmotically active substances—such as ethanol, methanol, ethylene glycol, or isopropanol—that aren’t captured by the standard panel and can indicate poisoning or ingestion.