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

Background Biochemistry analyzers in many high-throughput laboratories use indirect potentiometry to determine serum electrolyte concentrations, which involves a pre-analytical dilution step that may be associated with artifactual increases or decreases in electrolyte concentrations under circumstances of altered serum water fraction (SWF). Severe hypo- and hyperproteinemia, conditions that cause altered SWF, are recognized but under-emphasized causes of falsely measured serum sodium concentrations. Objectives The goals of this study were to determine the average actual SWF (SWFA) and establish formulae to correct serum sodium concentration measured by indirect potentiometry in hypo- and hyperproteinemic cats. Methods Serum samples from 112 feline patients were analyzed for electrolytes (measured by both indirect and direct potentiometry), total protein, albumin, triglycerides, and cholesterol. Each serum sample was also lyophilized to determine the SWFA. A feline-specific formula to estimate SWF (SWFE-FEL) was developed and evaluated with a multivariable linear model. Results The mean SWFA in this population of cats was 91.2%, which was significantly different (P < .0001) than the mean (93.9%) calculated using the human estimated formula (SWFE-HUM). The formula devised for the SWFE-FEL better recapitulated the SWFA than did the SWFE-HUM, and the corrected sodium concentrations calculated using the feline formula were better correlated with serum sodium measured by direct potentiometry than those determined using the human formula. Conclusions Application of feline-specific formulae is expected to limit the misinterpretation of electrolyte data from indirect potentiometry when altered SWF occurs. To demonstrate this, a case example of a hypoproteinemic cat is provided.