The osmolar gap, also known as the osmolal gap, is the difference between the measured osmolality of a patient’s serum and the calculated osmolality. Calculating the osmolar gap can help clinicians identify the presence of unmeasured osmotically active substances in the blood, such as certain toxins or metabolic byproducts.

Here’s the step-by-step calculation:

Calculating Osmolality

First, calculate the osmolality using the following formula:

Calculated osmolality (mOsm/kg) = (2 x sodium) + (glucose / 18) + (BUN / 2.8)

Where:

• Sodium is in mmol/L
• Glucose is in mg/dL
• BUN (blood urea nitrogen) is in mg/dL

Calculating Osmolar Gap

Next, subtract the calculated osmolality from the measured osmolality:

Osmolar gap (mOsm/kg) = Measured osmolality – Calculated osmolality

A normal osmolar gap is typically less than 10 mOsm/kg. An elevated osmolar gap may indicate the presence of unmeasured osmotically active substances.

Keep in mind that this calculation is a simplified version and does not take into account other factors that can affect osmolality. It’s essential to consult with a healthcare professional for accurate interpretation and diagnosis.