To assess the presence of this marker, a study was carried out with twenty-eight hospitalized and critically ill dogs at the Veterinary Hospital. Right at the time of admission, when using the glomerular filtration rate to analyze the levels of Cystatin C, we already observed values ​​below the reference, an early indication of an acute kidney injury. Even through this method it was possible to identify more cases than the conventional method described by the International Society of Renal Interest (IRIS) and serum creatinine.