Osmolal gap is defined as the difference between measured osmolality and what other value?

• A. Ideal osmolality
• B. Calculated osmolality
• C. Plasma osmolality
• D. Molar osmolality at 4°C

Answer: (B)

Osmolal gap measures how much of the blood’s osmotic concentration is unaccounted for by the main measured solutes. It is found by subtracting the calculated osmolality (estimated from the common solutes) from the measured osmolality. The calculated osmolality is typically estimated using the main contributors: about 2 times the sodium concentration plus glucose divided by 18 plus BUN divided by 2.8, in the usual mg/dL units. If the measured osmolality is higher than the calculated value, the gap is positive, suggesting the presence of other osmotically active substances in the blood, such as certain alcohols or glycols. Normal gaps are generally around 0 to 10 mOsm/kg; a larger gap can indicate an intoxication or other unmeasured solutes.