Serum osmolality in cats and dogs

What it shows

Osmolality is the number of osmotically active particles in a kilogram of water. The laboratory can measure it with an osmometer, which reads the freezing point of the sample, or calculate it: twice the sum of sodium and potassium, plus glucose, plus urea, all converted to the same units.

Not every particle moves water. Sodium is the main “effective” particle, and glucose joins it when insulin is lacking; urea crosses cell membranes freely and so barely shifts water between cells and blood. The body senses changes in effective osmolality of only a few per cent and answers with thirst and with antidiuretic hormone (ADH), which makes the kidneys keep water.

The osmolal gap is the measured value minus the calculated one. In health it is small; a large gap means extra particles that are not in the formula, or a falsely low sodium reading.

Higher than reference — what it may be a sign of

• Too little water for the amount of sodium: no access to water or inability to drink, loss of pure water in diabetes insipidus, or loss of fluid poorer in salt than blood through the gut, kidneys or skin. Too much sodium — salt poisoning or concentrated saline — raises it too.
• High glucose in diabetes mellitus: glucose adds particles and draws water out of cells, so sodium may even look low while osmolality is high.
• High urea in kidney disease raises both measured and calculated values, but urea is an “ineffective” particle and does not dehydrate cells the way sodium does.
• Ethylene glycol (antifreeze): within the first three hours after a toxic intake, dogs and cats have markedly high osmolality with an increased osmolal gap, together with acidosis and a raised anion gap. If poisoning is suspected, go to a vet at once.
• Mannitol given by a vet also widens the gap.

Lower than reference

Low osmolality means too few effective particles, mainly too little sodium for the amount of water. Causes include water gain — drinking a lot, low-salt fluids, the kidneys holding back water (inappropriate ADH, heart failure, nephrotic syndrome, liver cirrhosis) — and loss of electrolytes with dehydration. Low sodium usually, but not always, means low osmolality: with high glucose, sodium can be low while osmolality is high, and in marked lipaemia or very high protein some analysers read sodium falsely low (pseudohyponatraemia).

Look at together with

• Sodium — the main contributor; osmolality is read as the balance of sodium and water.
• Glucose and urea — the other parts of the calculated value.
• Ethylene glycol test and anion gap — when the osmolal gap is wide.
• Urine osmolality — urine that cannot be concentrated or diluted stays close to blood, about 300–320 mOsm/kg; comparing the two shows how the kidneys handle water.
• Triglycerides and total protein — marked lipaemia or very high protein can widen the gap through a falsely low sodium.

Units and conversion

Osmolality is reported in mOsm/kg — milliosmoles per kilogram of water. Measured and calculated values share this unit, so the gap is a simple subtraction. In osmolarity (per litre) the number is practically the same. In the app the row keeps the number and unit printed on the form. Nothing needs converting.

Reference range

The reference range is on your lab’s form. As a teaching guide, measured osmolality in healthy dogs is about 290–310 mOsm/kg and in cats about 290–330 mOsm/kg; the calculated value is typically about 280–310 mOsm/kg. In health the osmolal gap is under 10 mOsm/kg; a larger gap points to unmeasured particles or a falsely low sodium. A calculated value alone cannot reveal a gap — for that the lab must also measure osmolality with an osmometer.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine) — Fluid changes (osmolality, osmolal gap, tonicity).
2. eClinPath (Cornell University College of Veterinary Medicine) — Sodium.
3. eClinPath (Cornell University College of Veterinary Medicine) — Urinalysis: concentrating ability.
4. Regehr T. Ethylene Glycol Toxicosis in Animals. Merck Veterinary Manual, professional version, full review January 2022 (last updated September 2024).