Name 5 causes of an increased osmolar gap with an elevated anion gap metabolic acidosis.
The concentration of solutes in the plasma determines the plasma osmolality. It is primarily determined by the concentration ofNaCl, NaHCO3, glucose and urea.
The formula to calculate plasma osmolality (Posm) is:
Calculated Posm = 2 * [plasma Na] + [glucose] / 18 + [BUN] / 2.8
When glucose and BUN are measured in mg/dL
And
Calculated Posm = 2 * [plasma Na] + [glucose] + [BUN]
When glucose and BUN are measured in mmol/L
The osmolal gap is the difference between the measured osmolality and the calculated osmolality.
Therefore,
Osmolal gap = measured osmolality – calculated osmolality
Any difference is accounted for by unmeasured osmoles. Causes of an osmolal gap include such molecules as:
- ethanol,
- methanol,
- ethylene glycol,
- isopropyl alcohol,
- glycerol,
- mannitol,
- sorbitol,
- and acetone.
A metabolic acidosis can be seen along with an osmolal gap in the following situations:
- Methanol intoxication – the methanol accounts for the osmolal gap, and an anion-gap metabolic acidosis develops due to the production of formic acid.
- Ethylene glycol intoxication – the ethylene glycol accounts for the osmolal gap, and an anion-gap metabolic acidosis develops due to the production of glycolic acid and oxalic acid.
- Lactic acidosis – the osmolal gap is thought to be due to the accumulation of glycogen breakdown products, the acidosis is due to the lactic acidosis
- Diabetic/Alcoholic ketoacidosis – the osmolal gap is due to acetone and the acidosis is due to ketoacid production
