How this instrument works
Osmolality measures how strongly a fluid pulls water across a membrane, and in blood plasma that pull is dominated by sodium and the salts that travel with it. Because sodium and its accompanying anions make up the overwhelming majority of plasma's osmotic particles, a fast estimate starts by simply doubling the measured sodium — one term standing in for both sodium itself and the chloride and bicarbonate that roughly balance it. Glucose and urea nitrogen each add a smaller correction on top, since both molecules are also osmotically active and both are routinely available on the same basic metabolic panel.
This is the version commonly taught for quick emergency-department mental arithmetic — doubling sodium, then dividing glucose by 18 and BUN by 2.8 to convert each from mg/dL into the same osmotic units as sodium before adding them in. It trades a small amount of precision for speed, which is exactly the trade a clinician wants when trying to sort out an altered patient in the middle of a busy shift rather than waiting on a separate osmometer reading.
A normal calculated value runs roughly 275 to 295 mOsm/kg. The number becomes clinically powerful when it's compared against a directly measured osmolality from the lab: subtract the calculated figure from the measured one, and a gap larger than about 10 mOsm/kg — the osmolar gap — signals that some osmotically active substance is present in the blood that this simple formula never accounted for. That's a genuinely useful red flag, because a handful of the substances capable of producing a large gap, including methanol and ethylene glycol, are medical emergencies in their own right.
- Enter Sodium in mEq/L from a basic metabolic panel.
- Enter Glucose in mg/dL from the same panel.
- Enter BUN in mg/dL, also from the same panel.
- Read the calculated osmolality in mOsm/kg; if a lab-measured osmolality is also available, subtract this figure from it to get the osmolar gap.
Worked example — Na 150, glucose 90, BUN 14
A patient presents dehydrated with a sodium of 150 mEq/L, glucose of 90 mg/dL, and BUN of 14 mg/dL. Doubling sodium: 2 × 150 = 300. The glucose term: 90 / 18 = 5. The BUN term: 14 / 2.8 = 5. Adding all three gives 300 + 5 + 5 = 310 mOsm/kg — above the roughly 275-295 mOsm/kg normal range, consistent with the elevated sodium driving the whole result upward.
Now suppose the lab's own directly measured osmolality on that same blood sample comes back at 335 mOsm/kg instead of matching the calculated 310. The osmolar gap is 335 − 310 = 25 mOsm/kg — comfortably past the roughly 10 mOsm/kg threshold that raises suspicion for an unmeasured osmotically active substance, prompting a workup for possibilities like a toxic alcohol ingestion rather than dehydration alone.
Questions
Why does the formula start by doubling sodium?
Because sodium and the anions that electrically balance it — chiefly chloride and bicarbonate — together account for the large majority of the osmotically active particles in plasma. Doubling the measured sodium is a shorthand for counting both sodium and its paired anions at once, without needing to separately measure and add each one.
What is a normal plasma osmolality?
Roughly 275 to 295 mOsm/kg for most healthy adults. Values above that range usually reflect dehydration, uncontrolled high blood sugar, or elevated BUN from reduced kidney function; values below it often point toward excess free water or a low-sodium state.
What is the osmolar gap, and why does it matter?
It's the difference between a lab's directly measured osmolality and the value this formula calculates from sodium, glucose, and BUN alone. A gap larger than roughly 10 mOsm/kg means some other osmotically active substance is in the blood that the calculation never accounted for — classically a toxic alcohol such as methanol or ethylene glycol, though it can also reflect severe kidney failure, mannitol infusion, or a lab processing artifact. It is one of the few rapid clues available before specific alcohol levels return from the lab.
Is this the most accurate way to calculate osmolality?
No — it's the fast version, favored specifically for speed at the bedside. A separate, more precisely regression-fitted formula exists (1.86 × Na + glucose/18 + BUN/2.8 + 9) that was statistically derived against directly measured values and tends to track them more closely. Both formulas are legitimate; this one simply trades a little accuracy for arithmetic simple enough to do in your head.
Can a normal osmolar gap rule out a toxic alcohol ingestion?
Not entirely. Early in an ingestion, or once a toxic alcohol has been substantially metabolized into its more dangerous acidic byproducts, the gap can narrow toward normal even though the patient remains critically ill — the parent alcohol molecule is what drives the gap, and its breakdown products don't contribute the same way. A normal gap lowers suspicion but doesn't eliminate it if the clinical story still points toward ingestion.
References
- Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008;8:5 (PubMed)
- StatPearls — Physiology, Plasma Osmolality and Oncotic Pressure (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.