title: The effect of urea on refractometric total protein measurement in dogs and cats with azotemia.
abstract: BACKGROUND
  While 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.
  
  
  OBJECTIVE
  The 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.
  
  
  METHODS
  A prospective case series was conducted by collecting data from azotemic (n = 26) and nonazotemic (n = 34) dogs and cats. In addition, an in vitro 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.
  
  
  RESULTS
  The 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 (0.59 g/dL) was significantly less (P < .01) than the mean difference of the azotemic group (0.95 g/dL). The in vitro experiment revealed a positive bias with a proportional error.
  
  
  CONCLUSIONS
  This study demonstrated that increasing concentrations of urea significantly increased the total protein concentration measured by the refractometer as compared to the biuret assay, both in vivo and in vitro.
authors.affiliation: Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
authors.forename: Kelsey P
authors.initials: KP
authors.lastname: Legendre
authors.affiliation: Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
authors.forename: Mary
authors.initials: M
authors.lastname: Leissinger
authors.affiliation: Finn Pathologists - CVS Diagnostic Laboratories, Harleston, Norfolk, UK.
authors.forename: Viviana
authors.initials: V
authors.lastname: Le Donne
authors.affiliation: Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
authors.forename: Britton J
authors.initials: BJ
authors.lastname: Grasperge
authors.affiliation: Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Gaunt
mesh_terms: D000818:Animals; D053099:Azotemia / Q000097:blood / Q000662:veterinary*; D001663:Bilirubin / Q000097:blood; D001737:Biuret; D001798:Blood Proteins / Q000032:analysis; D002371:Cat Diseases / Q000097:blood*; D002415:Cats; D004283:Dog Diseases / Q000097:blood*; D004285:Dogs; D011446:Prospective Studies; D012031:Refractometry / Q000662:veterinary; D013053:Spectrophotometry / Q000662:veterinary; D014508:Urea / Q000097:blood*
chemical_list: D001798:Blood Proteins; D001737:Biuret; D014508:Urea; D001663:Bilirubin
keywords: 
  BUN
  ; cat; dog; interference; refractometer; total protein