What is the role of plasma osmolality in determining the cause of high anion gap (AG) acidosis?

Updated: Dec 08, 2020
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Plasma osmolality and the osmolar gap can be helpful in determining the cause of high AG acidosis. Plasma osmolality can be calculated using the following equation:

Posm = [2 × Na+]+[glucose in mg/dL]/18+[BUN in mg/dL]/2.8

Posm can also be measured in the laboratory, and because other solutes normally contribute minimally to serum osmolality, the difference between the measured and the calculated value (osmolar gap) is no more than 10-15 mOsm/kg. In certain situations, unmeasured osmotically active solutes in the plasma can raise the osmolar gap (eg, mannitol, radiocontrast agents).

The osmolar gap can also be a clue to the nature of the anion in high-AG acidosis because some osmotically active toxins also cause a high-AG acidosis. Methanol, ethylene glycol, and acetone are classic poisons that increase the osmolar gap and AG; measuring the osmolar gap can help narrow the differential diagnosis of high-AG acidosis.

Causes of AG metabolic acidosis are discussed in more detail below.

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