UNCORRECTED PROOF Laboratoire Ve´biotel, 41bis Av Aristide Briand, Arcueil, France Comparison of Plasma/Serum Urea and Creatinine Concentrations in the Dog: A 5-Year Retrospective Study in a Commercial Veterinary Clinical Pathology Laboratory C. Me ´daille 1,4 , C. Trumel 2 , D. Concordet 3 , F. Vergez 2,3 and J. P. Braun 3 Addresses of authors: 1 Laboratoire Ve´biotel, 41bis Av Aristide Briand, 94110 Arcueil, France; 2 De´partement des Sciences cliniques des animaux de sport et de loisirs and; 3 De´partement des Sciences biologiques et fonctionnelles & UMR ENV-INRA 181 Physiopathologie et Toxicologie Expe´rimentales, Ecole Nationale Ve´te´rinaire, 23 Chemin des Capelles, 31076 Toulouse Cedex3,France; 4 Correspondingauthor:Tel.:+33149121102;fax:+33149121110;E-mail:christine.medaille@wanadoo.fr With 6 figures and 1 table Received for publication: October 30, 2003 Summary Serum/plasma urea (S/P-urea) and creatinine (S/P-creatinine) concentrations are routinely assayed as indirect markers of glomerular filtration rate and have been reported to be highly correlated. The aim of this study was to evaluate the rela- tionship between the two analytes in a large number of unse- lected samples submitted to a commercial laboratory. In 4799 pairs of results, the correlation was high (r ¼ 0.795) and was not affected by sex or age. The relationship between the two analytes was best represented by a second-order polynomial equation. However, the dispersion of results was large and there was a high percentage of cases (27.5%) where S/P-urea wasincreasedandS/P-creatininenormal(£120 lmol/lforthis laboratory), while there was a low percentage of cases where S/P-creatinine (1.6%) was increased and S/P-urea normal (£8 mmol/l for this laboratory). The discrepancy between increasesinS/P-ureaandS/P-creatininemightnotonlyreflect ahighincidenceofnon-renalfactorsofvariationforS/P-urea butalsoaneffectofthesizeormusclemassofthedogsonthe limit of decision for S/P-creatinine. In dogs with normal S/P- urea, there was a significant effect of the size on the 0.975 quantile of S/P-creatinine, ranging from 106 lmol/l in very small dogs to 133 lmol/l in large and very large dogs. This study shows that isolated increases in S/P-urea could be mis- leading for the diagnosis of renal diseases and that the refer- ence intervals of S/P-creatinine should be re-evaluated according to breed or muscle mass of dogs. Introduction Routine evaluation of renal function in dogs is based on urinalysis and measurement of plasma indirect markers of the reduction of glomerular filtration rate (GFR). Most of the studiesagreethatserum/plasmacreatinineconcentration(S/P- creatinine) reflects changes in GFR more accurately than serum/plasma urea concentration (S/P-urea) because of mul- tiple extrarenal factors which can influence S/P-urea, such as effects of a meal, starvation, gastrointestinal haemorrhage, dehydration, etc. (Finco, 1995a,b; Polzin et al., 2000 2 ). S/P- creatinine has fewer extrarenal factors of variation (see review in Braun et al., 2003), the main one being muscle mass as reported in greyhounds (Feeman et al., 2003). However, in routine practice large biochemical profiles are analysed in which both analytes are measured and changes of renalfunctionindiseasedanimalsarefrequentlymonitoredby measurement of both S/P-creatinine and S/P-urea. The correlation between S/P-urea and S/P-creatinine has been reported to range from 0.4 to 0.8 (Gabrisch, 1973 3 ; RamadanandTadic,1985 4 ;Harapinetal.,1993),andtheS/P- (urea/creatinine) ratio differed significantly from one dog to another so that it had little clinical relevance (Finco and Duncan,1976).Moreover,simultaneousmeasurementofboth analytes increases the probability that only one of them is increased above the cut-off point, thus making the medical decision more complicated. The aim of this retrospective study was to describe the relationship between S/P-urea and S/P-creatinine in a large number of unselected canine serums and plasmas submitted over 5 years to a commercial veterinary clinical pathology laboratory, and to analyse the possible effects of sex, age and size/breed. Material and Methods Samples All 4949 serums or heparin plasmas of dogs submitted for measurement of S/P-urea and/or S/P-creatinine from June 98 toMarch2003bymorethan125veterinarypractitionersinor closetoPariswereincludedinthestudy.Recommendationby thelaboratorytotakesamplesonlyfromfastinganimalsmay not have been respected in some cases which were impossible to identify. Age and sex of dogs were reported in 3943 and 4707 cases, respectively; breed was indicated in 4194 dogs which were classified as very small, small, medium, large and very large accordingtomeansizeineachbreedortoestimatedweightfor mixed-breed dogs. Motives for analyses were most often not indicated,thustherelativeproportionofdiseasedandhealthy animalsaswellasthenatureofdiseasecouldnotbetakeninto account. Analytical techniques Analyses were performed with a FP901 spectrophotometer (LabsystemOY,Helsinki,Finland)andaKonelab20(Thermo ClinicalSystems,Espoo,Finland).Ureawasdeterminedbyan enzymatic procedure (BioMe´rieux, Marcy l’Etoile, France) and creatinine by Jaffe´ Õs reaction (BioMe´rieux). Accuracy of J V A 6 1 6 B Dispatch: 17.5.04 Journal: JVA CE: T.Balaji Journal Name Manuscript No. Author Received: No. of pages: 5 PE: Kavitha U.S. Copyright Clearance Center Code Statement: 0931–184X/2004/5103–0001 $15.00/0 www.blackwell-synergy.com J.Vet.Med.A 51, 1–5 (2004) Ó 2004 Blackwell Verlag, Berlin ISSN 0931–184X