Serum Osmolal Gap Calculator

How it is calculated

The tool estimates serum osmolality from routine chemistry and subtracts it from the measured value: calculated = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 and gap = measured − calculated. Sodium is in mmol/L (mEq/L), glucose and BUN are in mg/dL, and the measured osmolality from the lab is in mOsm/kg; the divisors 18 and 2.8 convert the two mg/dL values to mOsm/kg. Results are rounded to whole numbers. A negative gap is possible and simply means the measured value is below the calculated one. All four values are required. Sodium must be between 100 and 200, so a value in the wrong unit (such as 14) gives no result; glucose may be 0 to 2000 mg/dL and BUN 0 to 300 mg/dL, and the measured osmolality must lie between 150 and 500 mOsm/kg; anything outside these limits gives no result. Ethanol is not part of the formula, and the tool says nothing about what a gap means. Educational use only.

Common mistakes

• Entering glucose or BUN in mmol/L. The formula divides by 18 and 2.8 for mg/dL, so SI values must be converted to mg/dL first. A glucose of 7 (mmol/L) instead of 126 mg/dL is accepted and turns the example gap of 17 into 24.
• Entering sodium in a unit other than mmol/L (mEq/L). A value like 14 instead of 140 is rejected, so no result appears.
• Using a calculated or estimated osmolality in the measured box. The first box must be the laboratory's directly measured osmolality.
• Reading a negative gap as an error. It can happen from measurement variation; read the size and sign against the lab's own reference range.
• Treating the number as a diagnosis. Alcohols, other substances and sample timing matter, and only grossly impossible entries are rejected (glucose above 2000, BUN above 300, osmolality outside 150 to 500), so a value in the wrong unit can still pass; check the units against the lab report.