BACKGROUND: Biochemistry analyzers, which utilize indirect potentiometry, are used to determine serum electrolyte concentrations in dogs. Artifactual increases or decreases in these electrolyte concentrations can be caused by alterations in the serum water fraction (SWF). Severe hypo- and hyperproteinemia cause changes in SWF, which can then result in incorrectly reported serum sodium concentrations. OBJECTIVES: The goals of this study were to determine an average actual SWF (SWF METHODS: Serum samples from 115 canine patients were analyzed for electrolytes measured by both indirect and direct potentiometry. Total protein, albumin, triglycerides, and cholesterol were also determined. Each serum sample was then lyophilized to determine SWF RESULTS: The mean SWF CONCLUSIONS: Applications of correction formulae are expected to limit the misinterpretation of electrolyte data from indirect potentiometry when altered SWF occurs. A case example of the potential utility of these correction formulae is presented.