Chronic kidney disease (CKD) is a common disease of older cats that has a progressive course and a high mortality rate. The development and the progression of CKD depend upon the presence of complications such as proteinuria or hypertension. Therefore an early diagnosis of CKD or of hypertension and proteinuria associated with CKD may allow veterinarians to prevent progression of the disease and ultimately improve the quality of life of affected feline patients. In people, several recognized biomarkers serve as early, measurable indicators of CKD or of disease progression. Unfortunately only some of these have been deeply investigated in cats with CKD. Therefore, this study will assess how biomarkers in blood [such as homocysteine (Hcy), endothelin-1 (ET-1), aldosterone, angiotensin II], or in urine [such as urinary protein to creatinine (UPC) ratio, presence of tubular proteins, or alpha-1-macroglobulin (A1M)], may allow an early diagnosis of CKD or identify cats at risk of severe worsening of the disease. To this aim, privately owned cats routinely admitted to their clinical services will have samples tested to assess the serum or urinary levels of these biomarkers. Biomarker levels in cats with CKD of differing severity will be compared with indicator values from cats with non-azotemic CKD and with clinically healthy cats. Moreover, non-azotemic cats at risk to develop CKD will have repeated samples tested over the next 18 months. This planned process will determine the time-based appearance of azotemia and changes in the biomarker levels thus allowing the identification of an early indicator of CKD.