Potassium (K⁺) supplementation of isotonic crystalloid fluids in daily fluid therapy is commonly performed, yet its accuracy in veterinary medicine is undetermined.

To investigate the accuracy of K⁺ supplementation in isotonic crystalloid fluids.

None.

Observational study. 210 bags of fluid supplemented with KCl being administered to hospitalized dogs and cats intravenously (IV) were sampled over a 3-month period. Measured K⁺ concentration ([K⁺]) was compared to the intended [K⁺] of the bag. In a second experiment, 60 stock fluid bags were supplemented to achieve a concentration of 20 mmol/L K⁺, mixed well and [K⁺] was measured. In another 12 bags of 0.9% NaCl, K⁺ was added without mixing the bag, and [K⁺] of the delivered fluid was measured at regular time points during constant rate infusion.

The measured [K⁺] was significantly higher than intended [K⁺] (mean difference 9.0 mmol/L, range 6.5 to >280 mmol/L, P < .0001). In 28% of clinical samples measured [K⁺] was ≥5 mmol/L different than intended [K⁺]. With adequate mixing, K⁺ supplementation of fluids can be accurate with the mean difference between measured and intended [K⁺] of 0.7 (95% CI −0.32 to 1.7) mmol/L. When not mixed, K⁺supplementation of 20 mmol/L can lead to very high [K⁺] of delivered fluid (up to 1410 mmol/L).

Inadequate mixing following K⁺ supplementation of fluid bags can lead to potentially life threatening IV infused [K⁺]. Standard protocols for K⁺ supplementation should be established to ensure adequate mixing.

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