Osmolargap

Osmolargap, also known as osmolal gap, is a laboratory test used in medicine to evaluate the cause of elevated osmotic pressure in a patient's blood, particularly in the context of acute kidney injury and diabetes insipidus. It is calculated by subtracting the sodium concentration from the calculated osmolality of the blood.

The osmolal gap is determined by comparing the calculated osmolality of the blood to the measured sodium concentration. Normally, the concentrations are equivalent, but in cases of elevated osmolal gap, the molar size of the solute may differ from that of sodium. This discrepancy occurs when substances such as methanol, ethylene glycol, or other organic solutes increase the osmolality of the blood without displacing sodium.

Elevated osmolal gap can be indicative of several conditions, including diabetic ketoacidosis, alcohols, salicylates and sulfonamide toxicity, ethylene glycol poisoning, and ingestion of mannitol or other sugar alcohols. The presence of an elevated osmolal gap can help clinicians differentiate between these various conditions, thereby facilitating a timely diagnosis and initiating the appropriate treatment.

The osmolal gap is often used in conjunction with other laboratory tests, such as serum ketone measurements and blood ethanol assays, to confirm the presence and cause of abnormal osmolal elevation. Use of the osmolal gap in clinical practice can aid in the detection of diagnostic dilemmas, such as the differentiation between diabetic ketoacidosis and lactic acidosis, and therefore is considered a useful tool in guiding emergency department treatment of patients presenting with elevated osmotic pressures.