The concept of the cardio–renal axis is based on the assumption that kidney dysfunction is a secondary consequence of heart disease, a mechanism observed in dilated cardiomyopathy (DCM). Traditional serum markers are not always effective, especially in the early stages of kidney disease. Thus, there is a need to find more sensitive measures of renal impairment during heart failure. The selected biomarkers, cystatin C, KIM-1 (kidney injury molecule-1), and NGAL (neutrophil gelatinase-associated lipocalin), were evaluated in a study for their efficacy as indicators of early kidney damage in dogs with dilated cardiomyopathy. The serum concentrations of these biomarkers and their creatinine ratio were measured and analyzed to determine their diagnostic values. This study’s findings suggest that during the asymptomatic phase of dilated cardiomyopathy, only NGAL concentration and the NGAL/creatinine ratio may serve as diagnostic markers for early-stage kidney injury. The above markers can, therefore, be used in the early diagnosis of kidney diseases in dogs with asymptomatic DCM.