Serum osmolal gap

Serum osmolal gap (also known as osmolar gap or osmolality gap) is a calculated value derived from the osmolality of a patient's blood serum. It is used in medical diagnostics to identify certain types of metabolic disorders and poisoning.

Definition

The serum osmolal gap is the difference between the measured serum osmolality and the calculated serum osmolality. The calculated osmolality is typically derived from the concentrations of sodium, glucose, and blood urea nitrogen (BUN) in the blood serum.

Clinical significance

The serum osmolal gap is used in the diagnosis of various medical conditions. An elevated osmolal gap can indicate the presence of unmeasured osmoles in the blood, such as ethanol, methanol, ethylene glycol, or propylene glycol. This can be indicative of poisoning or metabolic disorders.

In addition, the serum osmolal gap can be used to monitor the progress of treatment for these conditions. A decreasing osmolal gap can indicate that the unmeasured osmoles are being metabolized and excreted from the body.

Calculation

The serum osmolal gap is calculated using the following formula:

Osmolal gap = Measured osmolality - Calculated osmolality

The calculated osmolality is typically derived using the following formula:

Calculated osmolality = 2 * (Sodium concentration) + (Glucose concentration / 18) + (BUN concentration / 2.8)

Limitations

While the serum osmolal gap is a useful diagnostic tool, it has several limitations. It is not specific for any particular substance, and an elevated osmolal gap can be caused by a variety of substances. In addition, the accuracy of the osmolal gap can be affected by errors in the measurement of serum osmolality or the concentrations of sodium, glucose, and BUN.

See also

• Osmolality
• Blood tests
• Metabolic disorders
• Poisoning