Acid-base & blood gas

Osmolar gap

Diagnostic adjunct in suspected toxicosis (ethylene glycol, methanol, isopropanol), DKA / HHS workup, and monitoring of mannitol therapy. Supports both US units (Na mEq/L, glucose and BUN mg/dL) and SI units (Na mmol/L, glucose and urea mmol/L). A normal gap does NOT rule out ethylene glycol in late presentations because the parent compound is metabolized to organic acids within 12–18 hr; anion gap and clinical context remain primary in those cases.

A normal gap does not rule out ethylene glycol

Ethylene glycol is metabolized within 12–18 hr of ingestion in dogs (less in cats), and the osmolar gap may close by then even as the patient deteriorates from worsening metabolic acidosis and renal injury. In late presentations, anion gap and clinical context are primary; a normal osmolar gap cannot rule out toxicosis. Use this calculator within the early window (1–8 hr post-ingestion in dogs, 1–3 hr in cats) for maximum sensitivity.

Sodium (mEq/L)

Reference 142–158 mEq/L in dogs, 149–162 in cats (DiBartola Ch. 9). Note: mEq/L and mmol/L are numerically identical for Na (monovalent ion).

Glucose

mg/dL mmol/L

Conversion: 18 mg/dL = 1 mmol/L. Default mg/dL matches most US veterinary labs.

BUN / urea

mg/dL mmol/L

Conversion: BUN 2.8 mg/dL = urea 1 mmol/L (two nitrogens per urea molecule, atomic mass 14). Choose mg/dL if your panel reports BUN, mmol/L if it reports urea.

Measured osmolality (mOsm/kg) (optional, for the osmolar gap)

Direct reading from the lab's freezing-point or vapor-pressure osmometer. Without this, only the calculated osmolality is shown.

Ethanol (mg/dL) (optional, if administered or measured)

If ethanol antidote therapy is in progress or ethanol ingestion is suspected, include it in the calculated osmolality to avoid mistaking it for an unmeasured osmole. Conversion: 4.6 mg/dL = 1 mmol/L.

Awaiting input

Enter Na, glucose, and BUN (or urea) to compute calculated osmolality. Add a measured osmolality from the lab to compute the osmolar gap.

Reference

Formula

Calculated osmolality estimates serum osmolality from the three largest contributors: sodium (with its obligate paired anions), glucose, and urea. The osmolar gap is the difference between a directly measured osmolality and this calculated value.

US units

$$\text{Osm}_{\text{calc}} = 2 \times [\text{Na}^+] + \frac{[\text{glucose}]}{18} + \frac{[\text{BUN}]}{2.8}$$

Sodium in mEq/L. Glucose and BUN in mg/dL. The division constants are molar-mass conversions to mmol/L. Ethanol contribution, when relevant, adds [ethanol]/4.6.

SI units

$$\text{Osm}_{\text{calc}} = 2 \times [\text{Na}^+] + [\text{glucose}] + [\text{urea}]$$

All values in mmol/L. The 2× multiplier on sodium accounts for obligate paired anions (chloride and bicarbonate primarily); K, Ca, and Mg contributions are conventionally omitted because they are small and approximately constant.

Osmolar gap

$$\text{Gap} = \text{Osm}_{\text{measured}} - \text{Osm}_{\text{calc}}$$

Gap (mOsm/kg)	Interpretation
< 10	Within normal limits. Does not rule out ethylene glycol in late presentations.
10–20	Borderline. Possible early or late ethylene glycol toxicosis, mannitol therapy, mild measurement variance.
> 20	Significantly elevated. Ethylene glycol toxicosis is the dominant differential in dogs and cats unless another osmotic agent (mannitol, ethanol antidote, IV propylene glycol vehicle) is known to be present.

Reference range for serum osmolality

Approximately 290–310 mOsm/kg in dogs and cats (DiBartola Ch. 9). Above 320 mOsm/kg defines hyperosmolarity; above 350 risks irreversible CNS injury.

Sources

Source citations

DiBartola SP, ed. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 4th ed. St. Louis, MO: Elsevier Saunders; 2012. Ch. 3 (Disorders of Sodium and Water). Reference ranges, calculated- osmolality formula, and clinical interpretation.
Thrall MA, Grauer GF, Mero KN. Clinicopathologic findings in dogs and cats with ethylene glycol intoxication. J Am Vet Med Assoc. 1984;184(1):37–41. Foundational veterinary study on the osmolar gap in ethylene glycol toxicosis.
Connally HE, Thrall MA, Hamar DW. Safety and efficacy of high-dose fomepizole compared with ethanol as therapy for ethylene glycol intoxication in cats. J Vet Emerg Crit Care. 2010;20(2):191–206. Feline diagnostic and treatment window.
Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. New York: McGraw-Hill; 2001. Foundational reference for the formula and the osmolar gap concept.