The effect of urea on refractometric total protein measurement in dogs and cats with azotemia

BackgroundWhile protein is the predominant solute measured in plasma or serum by a refractometer, nonprotein substances also contribute to the angle of refraction. There is debate in the current literature regarding which nonprotein substances cause factitiously high refractometric total protein measurements, as compared to the biuret assay. ObjectivesThe purpose of the study was to determine if the blood of azotemic animals, specifically with increased blood urea concentration, will have significantly higher refractometric total protein concentrations compared to the total protein concentrations measured by biuret assay. MethodsA prospective case series was conducted by collecting data from azotemic (n=26) and nonazotemic (n=34) dogs and cats. In addition, an invitro study was performed where urea was added to an enhanced electrolyte solution at increasing concentrations, and total protein was assessed by both the refractometer and spectrophotometer. Statistical analysis was performed to determine the effect of urea. ResultsThe refractometric total protein measurement showed a positive bias when compared to the biuret protein measurement in both groups, but the bias was higher in the azotemic group vs the nonazotemic group. The mean difference in total protein measurements of the nonazotemic group (0.59g/dL) was significantly less (P<.01) than the mean difference of the azotemic group (0.95g/dL). The invitro experiment revealed a positive bias with a proportional error. ConclusionsThis study demonstrated that increasing concentrations of urea significantly increased the total protein concentration measured by the refractometer as compared to the biuret assay, both invivo and invitro.