Calculated Osmolality & Osmolar Gap Calculator for Dogs and Cats
Calculate osmolality from sodium, potassium, glucose, and BUN, and compute the osmolar gap against a lab-measured value - a key early screening tool for ethylene glycol toxicity.
Glucose & urea/BUN units
Sodium, Na (mEq/L = mmol/L)
Potassium, K (mEq/L = mmol/L)
Glucose (mg/dL)
BUN (mg/dL)
Formula
Calculated osmolality = 2 × (Na + K) + Glucose/18 + BUN/2.8
Measured osmolality, mOsm/kg (optional)
Enter the lab-measured value to also calculate the osmolar gap. Osmolality is reported in mOsm/kg everywhere, so no unit conversion is needed for this field.
Osmolar gap and anion gap are complementary in suspected ethylene glycol toxicity: the osmolar gap is typically elevated early after ingestion (from the unmetabolized glycol itself), while the anion gap rises later as the glycol is metabolized into toxic acid metabolites. A patient can have a normal anion gap but an elevated osmolar gap in the first couple of hours after ingestion - don't rule out ethylene glycol toxicity based on a normal anion gap alone if ingestion was recent.
How to use this tool
Calculated osmolality formula
Calculated osmolality (mOsm/kg) = 2 × (Na + K) + glucose/18 + BUN/2.8 (or 2 × Na + glucose/18 + BUN/2.8 if excluding potassium). Worked example: Na 145, K 4.5 (included), glucose 100, BUN 20 → 2×(145+4.5) + 100/18 + 20/2.8 ≈ 299 + 5.6 + 7.1 ≈ 312 mOsm/kg.
Osmolar gap
Osmolar gap = measured osmolality − calculated osmolality. A gap near zero is expected and normal. An elevated gap (commonly cited threshold: roughly 10-15 mOsm/kg, with gaps over 25-30 being strongly suggestive) indicates unmeasured osmotically active substances in the blood - most urgently, ethylene glycol (antifreeze) toxicity, but also exogenous mannitol administration or severe lipemia/hyperproteinemia (which can cause a falsely elevated gap via lab measurement artifact).
Osmolar gap vs anion gap in ethylene glycol toxicity
These two values are complementary: the osmolar gap is typically elevated early after ingestion (from the unmetabolized glycol itself), while the anion gap rises later as the glycol is metabolized into toxic acid metabolites. A patient can have a normal anion gap but an elevated osmolar gap in the first couple of hours after ingestion - don't rule out ethylene glycol toxicity based on a normal anion gap alone if ingestion was recent.
Frequently asked questions
How is calculated osmolality determined?
Calculated osmolality (mOsm/kg) = 2 × (Na + K) + glucose/18 + BUN/2.8 (or just 2 × Na if excluding potassium). Worked example: Na 145, K 4.5 (included), glucose 100, BUN 20 → 2×(145+4.5) + 100/18 + 20/2.8 ≈ 299 + 5.6 + 7.1 ≈ 312 mOsm/kg.
What does an elevated osmolar gap mean?
It signals the presence of osmotically active substances in the blood that aren't accounted for by the standard formula's inputs. The most clinically urgent cause in small animal practice is ethylene glycol/antifreeze toxicity, but mannitol administration and severe lipemia/hyperproteinemia can also elevate it.
Why does the osmolar gap matter more early after suspected ethylene glycol ingestion than the anion gap does?
Unmetabolized ethylene glycol itself is an osmotically active molecule that raises the gap immediately, while the anion gap doesn't rise until the glycol is metabolized into glycolic and oxalic acid over the following hours. Checking both, and understanding their different time courses, avoids missing an early case.
Can severe lipemia cause a falsely abnormal result?
Yes - very high lipid or protein content in a blood sample can interfere with some osmolality measurement methods and calculations, so interpret an unexpected gap in the context of the whole sample and patient.