Our fellowship director asked me to do a lecture on osmolar gap. At first I thought that this was an odd topic as toxic alcohols, the standard reason for determining an osmolar gap are relatively rare findings and I was worried I'd be able to find enough to talk about for an hour.
I'm really happy how the lecture turned out. Not my best but pretty strong for a first crack at a new topic.
I structured the topic by looking at patients with low, normal and high anion gaps to go along with the high osmolar gap and started with a case of a high osmolar gap paired with a negative anion gap. I have only seen one negative anion gap and that was a case of hyperkalemia and hypoalbuminemia. This case comes from the Canadian Medical Association Journal. The low anion gap is from the unmeasured cation, lithium. The patient had a lithium level of 14.5 mmol/L.
Lithium is an unmeasured cation which expands the red box and decreases the anion gap.
The differential for a decreased anion gap.
The osmolar gap is driven up because the cation lithium is not part of the calculated osmolality but contributes to the measured osmolality. A unifying theme of osmolar gap is that adulterants that increase the osmolar gap always have relatively low molecular weights. Lithium carbonate does not disappoint with a molecular weight of only 74. Other intoxicants associated with an increased osmolar gap, likewise have a low molecular weight.
The case report then deals with the dialytic removal of lithium and the nature of lithium toxicity.
Here are the causes of an osmolar gap divided by anion gap:
Here it is:
Osmolar Gap
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