Cystatin C is a serum protein with low molecular mass, which has been suggested as a marker to assess renal function in the dog. This protein is regularly assessed using particle-enhanced turbidimetric immunoassay (PETIA) and particle-enhanced nephelometric immunoassay (PENIA), in which rabbit anti-human cystatin C antibodies are used. The purpose of this work was to compare the results of cystatin C analysis obtained by PETIA and PENIA assays in the dog. Forty dogs of different genders and breeds were classified into four groups of 10 animals each based on serum creatinine concentrations (4 stages of chronic kidney disease). Serum cystatin C concentration was measured using PETIA and PENIA assays, the results were compared, and correlation with serum urea and creatinine concentrations was established. The correlation coefficient for results obtained using PETIA and PENIA assays was r = 0.706. Serum cystatin C concentrations obtained in PETIA had a lower correlation coefficient with creatinine concentrations than those found in PENIA (r = 0.614 and r = 0.904, respectively); similarly, serum cystatin C was less correlated with serum urea concentration in PETIA than in PENIA (r = 0.463 and r = 0.636, respectively). The results obtained in this study suggest that the nephelometric assay is more sensitive and was shown to be more closely correlated with other renal function indicators than the PETIA assay.