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Calculator · Calculated osmolality, effective osmolality, and the gap
Osmolality and the osmolar gap
Two different numbers with the same name. Calculated osmolality includes urea and is what you compare with a measured value to get the gap. Effective osmolality excludes urea, because urea crosses cell membranes freely and pulls no water — and it is the one…
Before you use it
Use EFFECTIVE osmolality for HHS, and for the rate of fall. Using the calculated figure instead includes urea, which does not shift water, and will make a falling urea look like a falling osmolality — the correction then appears faster than it is. A raised osmolar gap (>10) points at an unmeasured osmole: ethanol, methanol, ethylene glycol, mannitol.
Source: JBDS-IP: Management of HHS in Adults (2022)
Checked, and shown
Every threshold on this page was checked against its primary source by a practising doctor. Here is what that involved, and what it found.