Osmolar Gap

osmolar gap

If the anion gap is high, check osmolar gap.

Measured osmolarity is determined by freezing point depression of plasma. The more stuff plasma has dissolved in it, the lower the freezing point will be.

Calculated osmolarity is calculated by the formula 2Na + Glucose/18 + BUN/2.8 (these 3 are the major osmolytes in the plasma).

If there is a high difference between these values, it means there are osmolites present which are “imposters,” not physiologic. These can be:

  • Alcohols (methanol, ethanol, ethylene glycol, acetone, isopropyl alcohol)
  • Sugars (mannitol, sorbitol, glucose)
  • Proteins (gammaglobulins)
  • Lipids (triglycerides)

This is something I’m still learning about, but wanted to post to motivate myself to keep learning. Check back in a little while for a more complete explanation. Specifically, I’m still unsure of which causes of high gap acidosis produce a normal osmolar gap.

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