Osmolar Gap

What is this formula?

The Osmolar Gap is the difference between measured serum osmolality and calculated serum osmolarity. It is used to detect the presence of unmeasured osmotically active substances in the blood.

When to use it

Use this formula when evaluating suspected toxic alcohol ingestion, unexplained metabolic acidosis, hyperosmolar states, or abnormal laboratory findings.

Example

Data:

MOsm = 320 mOsm/kg

COsm = 295 mOsm/kg

Formula:

OG = MOsm − COsm

Substitution:

OG = 320 − 295

Result:

OG = 25 mOsm/kg

The osmolar gap is 25 mOsm/kg.

Applications

- Toxicology

- Emergency medicine

- Nephrology

- Critical care medicine

- Evaluation of unexplained metabolic acidosis

Note

The osmolar gap is a screening indicator rather than a definitive diagnostic test. Elevated values may suggest the presence of ethanol, methanol, ethylene glycol, isopropanol, mannitol, or other unmeasured osmoles. Reference ranges vary among laboratories, but values greater than approximately 10 mOsm/kg are often considered abnormal.