An elevated osmolal gap is most suggestive of the presence of which substances?

• A. Low osmolality
• B. Elevated BUN
• C. Unmeasured osmotically active substances such as ethanol or methanol
• D. Hypernatremia

Answer: (C)

An elevated osmolal gap occurs when there are substances in the blood that increase the measured osmolality but are not included in the calculated osmolality. The gap is found by subtracting the calculated osmolality (commonly based on sodium, glucose, and BUN) from the measured osmolality; a normal gap is roughly 0–10 mOsm/kg. When the gap is high, it points to unmeasured osmotically active substances, with ethanol and methanol being classic examples. These alcohols contribute to the measured osmolality but are not accounted for in the calculation, inflating the gap.

The other options don’t explain a high gap. Low osmolality would lessen the gap or keep it normal. Elevated BUN and hypernatremia increase the components used to calculate osmolality and don’t create an unmeasured solute effect, so they don’t produce a prominent elevated gap.