Chronic kidney disease (CKD) is the most common disease affecting the kidneys of dogs and cats. It may be recognized by reduced kidney function or the presence of kidney damage. CKD is defined as kidney damage present for at least three months, with or without decreased glomerular filtration rate (GFR) or greater than 50% reduction in GFR persisting for at least three months (Polzin et al, 2005). Kidney damage is further defined as either 1) microscopic or macroscopic pathologic changes detected by histologic or direct visualization of the kidneys or 2) markers of damage detected by blood or urine tests or imaging studies. In the past, multiple terms were used to define the severity of renal functional abnormalities including renal insufficiency, renal failure and uremia. However, there has not been uniform agreement on the specific definition of renal insufficiency vs. renal failure. Therefore, it has been proposed by the International Renal Interest Society (IRIS) to replace these terms with a scheme to classify severity of CKD into four stages. Staging is based initially on fasting blood creatinine concentration, assessed on at least two occasions in the stable patient (Table 37-1). The patient is then substaged based on proteinuria and blood pressure (Tables 37-1A and 37-1B). Two of the foundational assumptions inherent in this classification scheme are that the presence of CKD has been confirmed and that azotemia, if present, has been localized as renal in origin.
This classification scheme emphasizes the continuum of severity of renal injury of dogs and cats starting from those at risk for developing CKD, to those with documented presence of kidney damage but without azotemia in stage 1 CKD, to progressively more severe CKD with resultant increasing serum creatinine concentration for stages 2 to 4. Furthermore, by using the term “kidney disease” and staging the severity of disease, it is possible to facilitate understanding, communication and application of management guidelines for patients in each stage.
The goals of this chapter are to provide pathophysiologic concepts and practical nutritional management recommendations for dogs and cats with CKD. Nutritional management of patients with CKD includes measures to reduce signs of uremia and slow progression to later stages of disease. There is general agreement regarding nutritional management of CKD when overt signs exist; however, the role of nutritional intervention during earlier stages of CKD is less well defined. Thus, in a sense, the question is not whether to use nutritional management but when should it be initiated. Historically detection of CKD in its early stages was difficult however, recent advances in renal biomarkers (e.g. symmetric dimethylarginine (SDMA)) that correlate with decreasing glomerular filtration rate allow for detection of kidney disease well before azotemia develops (Relford et al, 2016). Armed with this new diagnostic tool, investigators are beginning to understand the role of nutritional intervention in patients with stage 1 CKD (Hall et al, 2016; Hall et al, 2016a). Given this new information and since there appears to be no harm in avoiding nutrient excess (e.g., phosphorus) during earlier stages, nutritional management should be considered in stage 1 CKD and is clearly indicated when serum creatinine exceeds 2 mg/dl (179 μmol/l) (Jacob et al, 2002; Ross et al, 2006). Similarly, significant and persistent renal proteinuria, even in the absence of azotemia, reflects marked renal damage and signals the need for nutritional management regardless of the CKD stage.