Formulae to correct sodium concentrations for serum water fraction in cases of hypo- and hyperproteinemia in dogs.

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_ACTUAL) in dogs and establish formulae to correct serum sodium concentration measured by indirect potentiometry in hypo- and hyperproteinemic patients.

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_ACTUAL. A canine-specific formula to estimate SWF (SWF_EST-CAN) was developed using a multivariable linear model and compared with the human-estimated formula (SWF_EST-HUM).

Results: The mean SWF_ACTUAL in this population of dogs was 92.7%, which was significantly different (p < .0001) than the mean (95.1%) calculated using SWF_EST-HUM. The formula derived from SWF_EST-CAN recapitulated SWF_ACTUAL more accurately than SWF_EST-HUM. Based on a slope closer to 1.0, the corrected sodium concentrations calculated using the canine formula correlated marginally better with the serum sodium measured by direct potentiometry than those calculated using the human formula.

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.