The difference between the measured osmolality and the calculated osmolality of a patient’s serum is known as the osmolar gap. Calculated osmolality is typically derived from commonly measured serum components, such as sodium, glucose, and blood urea nitrogen (BUN). A typical calculation uses the formula: 2[Sodium] + [Glucose]/18 + [BUN]/2.8, where concentrations are expressed in mg/dL (or mmol/L for sodium). The measured osmolality is obtained via laboratory analysis, using methods like freezing point depression. The gap is then determined by subtracting the calculated osmolality from the measured osmolality.
Elevated gaps can indicate the presence of unmeasured osmotically active substances in the serum. This can be clinically significant in the diagnosis of conditions such as toxic alcohol ingestion (e.g., methanol, ethylene glycol), ketoacidosis, or severe hyperlipidemia. Monitoring the gap provides clinicians with a valuable tool in assessing the patient’s condition and guiding appropriate treatment strategies. Historically, the assessment has been a valuable diagnostic clue in identifying otherwise occult causes of altered mental status or metabolic disturbances.
