Background

Serum protein analysis by capillary electrophoresis (CE) is a well-established laboratory method used for the diagnosis and follow up of infectious, inflammatory, immune-mediated, and neoplastic conditions in human and veterinary medicine (Jenkins, 2009; Giordano and Paltrinieri, 2010). CE is one of the most frequent techniques used to separate serum proteins, as it is a simple, adaptable, and quick technique, and does not require a large sample amount. The CE laboratory technique is based on the separation of charged molecules by their electrophoretic mobility in an alkaline buffer at a specific pH. The separation occurs according to the electrolyte pH and electroosmotic flow, yielding different electrophoretic fractions (Osatinsky, 2007). In serum, the phoretogram is commonly divided into five different fractions, from low to high molecular weight and charge: albumin, alpha₁ globulin, alpha₂ globulin, beta globulin, and gamma globulin (Gay-Bellile et al., 2003; Tappin et al., 2011).

Recently, the analysis of proteins in urine by CE has proven to be a suitable method in human medicine to detect the presence of characteristic electrophoretic patterns in renal and extrarenal disorders, such as myelomas (Jenkins, 1997; Theodorescu et al., 2005; Mischak et al., 2010). Although quantitative proteinuria can only be assessed by calculation of the urine protein/creatinine ratio (UPC), electrophoretic techniques can be used as a qualitative method to assess the loss of proteins through the urine, as different patterns can be identified (Lees et al., 2005). CE associated with mass spectrophotometry techniques is also useful to identify peptide biomarkers associated with chronic kidney disease (Pelander et al., 2019). The use of urine to identify such abnormalities represents a great advantage over the use of blood, because urine can be collected in quantity, does not require trained staff for its collection, and urinary proteins remain stable for at least 3 days at 4ºC or 3 months at -20°C (Théron et al., 2017). They can provide information about kidney functionality earlier than blood biomarkers, such as SDMA or Cystatin C (Yalçin and Çetin, 2004; Pelander et al., 2019).

Comparison with other electrophoretic techniques that evaluate proteinuria could be challenging because the principle of migration is different. In sodium dodecyl sulfate agarose/polyacrylamide gel (SDS-AGE or SDS-PAGE) electrophoretic techniques, particle migration is only based on their molecular weight. Initially, it might be easier to identify proteins that migrate in the different phoretogram fractions (Yalçin and Çetin, 2004; Zini et al., 2004; Giori et al., 2011; Lavoue et al., 2015; Chacar et al., 2017; Hokamp et al., 2017). Nevertheless, the result of the technique presented in this manuscript is a profile, where abnormalities associated with protein excretions can be easily detected when compared against curves from healthy dogs and any other mammals.

The aim of this study is to establish a standardized protocol to prepare urine adequately, to evaluate the proteins in dog urine by CE on an instrument normally used for serum samples. It can be a useful tool to assess pathological proteinuria in dogs, alongside quantitative methods such as UPC.