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Evaluation of factors that affect analytic variability of urine protein-to-creatinine ratio determination in dogs

Gabriele Rossi DVM, PhD1, Luca Giori DVM2, Simona Campagnola DVM3, Andrea Zatelli DVM4, Eric Zini DVM, PhD5,6,7, and Saverio Paltrinieri DVM, PhD8
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  • 1 Department of Veterinary Pathology, Hygiene and Public Health, University of Milano, 20133 Milan, Italy.
  • | 2 Department of Veterinary Pathology, Hygiene and Public Health, University of Milano, 20133 Milan, Italy.
  • | 3 Department of Veterinary Pathology, Hygiene and Public Health, University of Milano, 20133 Milan, Italy.
  • | 4 Clinica Veterinaria Pirani, Via Majakowski 2/L,M,N, 42100 Reggio Emilia, Italy.
  • | 5 Clinic for Small Animal Internal Medicine, Vetsuisse Faculty, University of Zurich, 8057 Zurich, Switzerland.
  • | 6 Istituto Veterinario di Novara, Strada Provinciale 9, Granozzo Con Monticello, NO 28060 Italy.
  • | 7 Department of Veterinary Clinical Sciences, University of Padua, 35020 Agripolis, Legnaro, Italy.
  • | 8 Department of Veterinary Pathology, Hygiene and Public Health, University of Milano, 20133 Milan, Italy.

Abstract

Objective—To determine whether preanalytic and analytic factors affect evaluation of the urinary protein-to-creatinine (UPC) ratio in dogs.

Sample—50 canine urine samples.

Procedures—The UPC ratio was measured to assess the intra-assay imprecision (20 measurements within a single session), the influence of predilution (1:10, 1:20, and 1:100) for urine creatinine concentration measurement, and the effect of storage at room temperature (approx 20°C), 4°C, and −20°C.

Results—The coefficient of variation at room temperature determined with the 1:20 predilution was < 10.0%, with the highest coefficients of variation found in samples with a low protein concentration or low urine specific gravity. This variability could result in misclassification of samples with UPC ratios close to the thresholds defined by the International Renal Interest Society to classify dogs as nonproteinuric (0.2), borderline proteinuric (0.21 to 0.50), or proteinuric (> 0.51). A proportional bias was found in samples prediluted 1:10, compared with samples prediluted 1:20 or 1:100. At room temperature, the UPC ratio did not significantly increase after 2 and 4 hours. After 12 hours at room temperature and at 4°C, the UPC ratio significantly increased. The UPC ratio did not significantly change during 3 months of storage at −20°C.

Conclusions and Clinical Relevance—The intra-assay precision of the UPC ratio was sufficiently low to avoid misclassification of samples, except for values close to 0.2 or 0.5. The optimal predilution ratio for urine creatinine concentration measurement was 1:20. A 1:100 predilution is recommended in samples with a urine specific gravity > 1.030. The UPC ratio must be measured as soon as samples are collected. Alternatively, samples should be immediately frozen to increase their stability and minimize the risk of misclassification of proteinuria.

Contributor Notes

Presented in abstract form at the 20th European College of Veterinary Internal Medicine Companion Animals Congress and the 13th European Society of Veterinary Clinical Pathology Congress, Toulouse, France, September 2010.

The authors thank Dr. Giuliano Ravasio and Ugo Gandolfi for technical assistance.

Address correspondence to Dr. Paltrinieri (saverio.paltrinieri@unimi.it).