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.
Specific questions of this sort include the following examples:
The nature of the model answers suggests that the examiners expect their candidates to be able to generate a robust list of differentials. Descent into biochemical pfuffery will go unrewarded.
Essentially, this will be any substance administered into the bloodstream which does not dissociate at physiologic pH. Question 7.3 from the first paper of 2014 presented the candidate with such a scenario. Several possible explanations can be offered:
Question 7.2 from the first paper of 2014 asks for this specifically. And even more specifically, Question 19.2 from the first paper of 2009 aska about which toxins are responsible for this sort of picture.
As Jeffrey Kraut reminds us, timing is very important in the appearance and disappearance of the raised osmolar gap, particularly where it concerns the ingestion of toxic alcohols. All of these toxic alcohols are non-polar at physiological pH, and so cannot raise the anion gap - only the osmolar gap. Ergo, early in the intoxication only the osmolar gap will be raised. Later, as the intoxicated patient staggers vomiting around the emergency department, some of the toxic alcohol will have been matabolised into organic acids, raising the anion gap (and decreasing the osmolar gap). Ultimately, as they lay in their ICU bed, the extra osmoles will have been metabolised into organic acid anions, and the bicarbonate will have decreased stoichiometrically. Thus only the anion gap will be raised in late intoxication.
The uncorrected, of course.
The corrected sodium merely gives you the impression of what the sodium level would "naturally" be like if the extra osmoles were removed. However, the measured sodium is the actual serum sodium, and this is the concentration which is contributing to the osmolality of the solution.
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