Kidney disease is defined as the presence of functional or structural abnormalities in one or both kidneys (Figure 1). It is recognized by either reduced kidney function or kidney damage.1 In dogs and cats, loss of two-thirds or more of functional nephrons is associated with loss of adequate urine concentrating ability, whereas loss of three-quarters or more of functional nephrons results in azotemia.
Photo by Gregory Kindred
This article focuses on the serum chemistry profile, complete blood count, and urinalysis findings that indicate kidney disease. Correctly interpreting these findings will help you formulate an effective diagnostic and therapeutic plan and provide prognostic information.
Azotemia is defined as an abnormal concentration of urea, creatinine, and other nonprotein nitrogenous substances in blood, plasma, or serum. Azotemia can be associated with several fundamentally different causes. Because nonprotein nitrogenous compounds (including urea and creatinine) are endogenous substances, abnormally elevated serum concentrations may be caused by an increased rate of production (by the liver for urea; by muscles for creatinine) or by a decreased rate of clearance (primarily by the kidneys).1
Azotemia should not be used as a synonym for kidney disease since it may be caused by lower urinary tract abnormalities or other nonrenal factors, and kidney disease can be present in the absence of azotemia. Although blood urea nitrogen (BUN) and creatinine concentrations are commonly used as crude indices of the glomerular filtration rate (GFR), meaningful interpretation of these parameters depends on recognizing and evaluating prerenal, primary renal, and postrenal factors that may reduce GFR.
Creatinine is a nonenzymatic breakdown product of phosphocreatine in muscle, and daily creatinine production is determined largely by individual muscle mass. In dogs and cats, creatinine excretion is accomplished almost exclusively by glomerular filtration, and the creatinine concentration is inversely related to GFR.2
A serum creatinine concentration is the most commonly used measure of severity of renal dysfunction and is the basis for staging chronic kidney disease (CKD) (Table 1). To optimize accurate staging of CKD, serum creatinine concentrations should be evaluated on two or more occasions when the patient is well-hydrated.
Table 1: International Renal Interest Society (IRIS) Stages of Chronic Kidney Disease in Dogs and Cats