A newly derived formula outperforms published formulae for estimating urine osmolality in healthy dogs receiving intravenous fluids.

A new formula using urine sodium, potassium, urea nitrogen, and glucose reliably estimates urine osmolality in healthy dogs.

This prospective observational study derived and validated a new formula for estimating urine osmolality (Uosm) in dogs using readily available urinary analytes. Nine healthy purpose-bred Beagles received isotonic or hypotonic IV fluids in a randomized crossover design with a 10-day washout period, generating 89 urine measurements via indwelling urinary catheters at baseline and every 12 hours. The proposed formula — Uosm = 2 × sodium + 1.8 × potassium + urea nitrogen/2.5 (mg/dL) + glucose/3.9 (mg/dL) — was derived using linear mixed models. Agreement with directly measured Uosm was strong, with a concordance correlation coefficient (CCC) of 0.96 (95% CI, 0.94–0.97) and a fixed bias of +7.99 mOsm/kg. Passing-Bablok regression confirmed no proportional or constant bias. When benchmarked against previously published estimation formulae, the new formula demonstrated the highest CCC and lowest fixed bias. The authors acknowledge that the study population was limited to healthy dogs under controlled fluid administration, and further validation in patients with renal, endocrine, or other systemic disease is needed. Clinically, this formula offers a practical alternative when direct osmometry is unavailable, enabling assessment of renal concentrating ability, fluid balance, and water handling using standard urine chemistry values routinely obtainable in practice.

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