osmolar gap
The osmolar gap is a diagnostic tool which can help identify the presence of some foreign solute in the body fluids. In the CICM fellowship SAQs, it mostly identifies young women who have ingested a toxic alcohol.
definition
difference between measured osmolality and calculated, i.e. (2× Na+ + glucose + urea)
(in UK units, if it matters)
- not to be taken literally mathematically - different units
Normal <10
what's it caused by?
Normal anion gap and high osmolar gap
implies a solute (i.e. osmotically active) that does not dissociate at physiological pH
therefore this includes:
- glycine (TURP syndrome)
- polyethylene glycol (in some IV meds including lorazepam)
- soon after ethylene glycol, methanol intoxication (see the deranged physiology link)
- mannitol, sorbitol
high anion gap AND high osmolar gap
→ What is the anion?
- Toxicological causes
- Methanol intoxication (the anion is formic acid)
- Ethylene glycol intoxication (the anions are glycolic acid and oxalic acid)
- Diethylene glycol intoxication (the anion is 2-hydroxyethoxyacetic acid, HEAA)
- Propylene glycol intoxication (the anions are pyruvate, lactate and acetate)
- Salicylate intoxication (the anions are salicylate and lactate)
- Any toxin causing massive lactic acidosis, eg. isoniazid
- Endocrine and metabolic disturbances
- Lactic acidosis
- Alcoholic or diabetic ketoacidosis
- Acute kidney injury