We often talk about calculating the anion gap in the evaluation of patients. What about the osmolal gap? When do you calculate this? What's the differential diagnosis for an increased osmolal gap?
I recently came upon a nice 2011 review in the American Journal of Kidney Disease called "Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis". It's always nice to revisit and review this concept. You'll always learn something new. For instance, I didn't know that salicylates cause anion gaps as well as osmolal gaps.
So don't forget to calculate an osmolal gap for patients with unexplained an metabolic acidosis anion gap.
Graham has a great calculator at MDCalc to help you calculate the osmolal gap.
Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011 Sep;58(3):480-4. Pubmed .