American Board of Internal Medicine (ABIM) Certification Practice Exam

A plasma osmolal gap greater than which value is considered abnormal?

1. 5

2. 10

3. 15

4. 20

The correct answer is: 10

A plasma osmolal gap greater than 10 mOsm/kg is considered abnormal. The osmolal gap is calculated by subtracting the measured osmolality from the calculated osmolality. Under normal circumstances, this gap should be relatively small, usually less than 10 mOsm/kg. When the gap exceeds this value, it can indicate the presence of unmeasured osmoles in the plasma, which might suggest conditions such as intoxication (with substances like ethanol or methanol), renal failure, or other metabolic disturbances. Therefore, a value higher than 10 reflects an abnormality that warrants further investigation to understand the underlying cause of the increased osmolal gap.