Clinical symptoms of renal dysfunction (polydipsia, polyuria) are not evident until a large portion of renal tissue has been destroyed. Until this point, many cases can be undiagnosed. Chronic renal failure has many physiological effects. These include the decreased ability to excrete nitrogenous waste (and thus build up of azotaemia), sodium and phosphorus, and an increased loss of potassium and water-soluble vitamins. Other clinical symptoms also include systemic hypertension, secondary hyperparathyroidism and nonregenerative anaemia (Lane, 2005).
Most veterinary practices offer owners renal screening for older patients, as part of senior clinics or as part of preanaesthetic screening or before the start of pharmaceutical regimes (normally osteoarthritis treatments). Early identification of these cases is required to manage the different stages of chronic renal failure. Guidelines set out by the International Renal Interest Society (IRIS, 2019) help to establish which types of management are required at the different disease stages.
Diagnostics for staging
Initially, renal failure was diagnosed with serum concentrations of blood urea nitrogen (BUN) and creatinine. However, symmetric dimethylarginine (SDMA) has been shown to detect chronic renal failure in cats on average 17.0 months before serum creatinine concentration increased above the reference interval (Hall et al, 2014). SDMA monitoring is available for in-house analysis.
The IRIS (2019) renal scoring index identifies the progression of the disease to facilitate appropriate treatment and monitoring of the patient. The initial staging is based on a fasted plasma creatinine level and SDMA and then sub-staged dependent on proteinuria levels and arterial blood pressure (Table 1).
Table 1.
International Renal Interest Society staging of chronic renal disease in cats and dogs (IRIS, 2019)
| Stage 1 No azotaemia (normal creatinine) | Stage 2 Mild azotaemia (Normal or mildly elevated creatinine) | Stage 3 Moderate azotaemia | Stage 4 Severe azotaemia | |
|---|---|---|---|---|
| Creatinine in mg/dL (stage based on stable creatinine) | ||||
| Canine | Less than 1.4 (125 μmol/L) | 1.4–2.8 (125–250 μmol/L) | 2.9–5.0 (251–440 μmol/L) | Greater than 5.0 (440 μmol/L) |
| Feline | Less than 1.6 (140 μmol/L) | 1.6–2.8 (140–250 μmol/L) | 2.9–5.0 (251–440 μmol/L) | Greater than 5.0 (440 μmol/L) |
| SDMA in μg/dL (stage based on stable SDMA) | ||||
| Canine | Less than 18 | 18–35 | 36–54 | Greater than 54 |
| Feline | Less than 18 | 18–25 | 26–38 | Greater than 38 |
| UPC ratio (substage based on proteinurea) | ||||
| Canine |
|
|||
| Feline |
|
|||
| Systolic blood pressure in mm Hg (substage based on blood pressure) | ||||
|
||||
| SDMA, symmetric dimethylarginine; UPC, urine protein:creatinine ratio | ||||
Fasted blood samples should be recommended, as even a moderately high protein meal before sampling can elevate blood plasma creatinine levels. SDMA is unaffected by fasting times or body condition score, so can be more reliable marker to use. Repeat blood sampling should occur, as required, but should be performed more regularly if urinalysis shows changes in proteinuria levels.
Some owners can find obtaining urine samples difficult and guidance on methods on how to obtain these samples should be offered (PDSA, 2024), as these are the most useful diagnostic tools in the progression of renal failure. Urine concentration should be routinely measured with the use of a refractometer. The urine protein creatinine (UP/C) ratio should also be measured as required by the veterinary surgeon overseeing the case. Medications such as angiotensinconverting enzyme (ACE) inhibitors and blockers should only be instigated when proteinuria is present (Lecavalier et al, 2021). It should be noted that proteinuria can present at any stage of renal failure and is not directly linked to the level of azotaemia.
Management of chronic renal failure
The management of chronic renal failure can be split into nutritional and pharmaceutical regimes, alongside regular monitoring schedules (Table 2). This article will focus on the clinical nutrition of these animals.
Table 2.
Treatment recommendations from IRIS dependent on the staging of chronic renal failure (IRIS, 2019).
| Stage 1 | Stage 2 | Stage 3 | Stage 4 |
|---|---|---|---|
|
|
|
|
SDMA, symmetric dimethylarginine; UPC, urine protein:creatinine ratio
Clinical nutrition
Protein
The role of diet is very important in aiding in the longevity and quality of life of the animal. Diet has been shown to be the most important aspect in the management of animals with chronic renal failure (Elliott et al, 2000; Schauf et al, 2021). A nutritional assessment should be performed at every consultation with the patient (WSAVA Nutritional Assessment Guidelines Task Force Members et al, 2011). This includes nutritional history, weight, body condition score (BCS) and muscle condition score (MCS). By understanding the ideal nutrient requirements for an animal with renal failure, and at each IRIS stage, guidance can be offered when owners wish to prepare homemade diets or feed a non-renal lifestage commercial diet with added phosphate binders. In a study conducted by Hall et al (2016) it was suggested that non-azotemic cats with elevated serum SDMA (early renal insufficiency) fed a food designed to promote healthy aging are more likely to demonstrate stable renal function compared with cats fed owner's-choice foods. It also found that cats fed owner's-choice foods are more likely to demonstrate progressive renal insufficiency.
The choice of renal diet (early or advanced) will depend on IRIS staging. Differences in these diets will be aimed at the protein and phosphate levels. Monitoring of muscle condition score is required to ensure that lean muscle mass loss is not occurring.
Restricted or reduced protein and phosphates are the main characteristics of renal diets. There are many studies looking at the evidence behind the nutrients required for renal support and the diets themselves. The most often quoted is Plantinga et al (2005). The study demonstrated that the median survival time of 175 cats that received conventional diets was 7 months, whereas the median survival time of 146 cats given one of seven renal diets was 16 months. The cats on the most effective of the diets had a median survival time of 23 months and those on the least effective diet had a median survival time of 12 months. The composition of the seven diets was comparable, except that the most effective diet had a particularly high content of eicosapentaenoic acid (EPA). Scherk and Laflamme (2013) stated that phosphorus restriction seems to be of value in chronic kidney disease, but inadequate data are available to determine the degree of restriction needed.
Fats
Plantinga et al (2005) showed the importance of fats in the diet and that they can contribute towards an increase in life expectancy in animals with chronic renal failure. The cause of this was not investigated as part of the study but has been debated. Did the inclusion of omega-3 fatty acids help to decrease inflammation and hence increase longevity? Or were the additional calories helpful in maintaining lean body mass during renal cachexia and improving quality of life scores?
The addition of fats to the diet is beneficial, as it offers twice the energy per gram as carbohydrate and aids in palatability (Plantinga et al, 2005). The use of omega-3 essential fatty acids has also been shown to decrease inflammation in the kidney, lower hypertension and preserve renal function; therefore, veterinary diets are supplemented with EPA and docosahexaenoic (DHA). Omega-6 fatty acids appear to be detrimental in dogs with naturally occurring renal disease by acutely increasing glomerular filtration rate, increasing inflammation and the worsening of renal lesions (Bauer et al, 1997).
Water
Renal disease causes a progressive decline in urine concentrating capacity (Malkina, 2023). Dehydration, volume depletion, renal hypoperfusion and dietary salt intake stimulate urine concentration. Avoiding dehydration and renal hypoperfusion reduces the work of concentrating the urine and helps to maintain intrarenal protective mechanisms. Chronic renal failure patients must have unlimited access to fresh water and free choice consumption. This can be exceptionally important in cats, which have fastidious drinking habits (Sparkes et al, 2016).
Increasing water consumption can be achieved in different ways: feeding a moist diet rather than a dry diet and by increasing the availability of water. Increasing the number of bowls (including types) around the house and allowing water to stand for a period prior to being offered to some animals can be beneficial. This allows the chlorine in the water to evaporate off, which some animals prefer. Ideally phosphate-free hydration broths should be offered that can aid in increasing calorific intake alongside hydration without an increase in phosphate. It is important to discuss ways to monitor hydration levels with the client. Demonstrating monitoring of clinical signs can help the client take care of their pet. This can include testing for skin tenting, tacky mucus membranes and the colour of membranes. If pet owners know what is normal, when the animal becomes unwell or the renal disease progresses, they can flag this to the practice quicker – ultimately increasing the welfare of the animal.
Carbohydrates
The importance of dietary fibre in the management of renal disease and in maintaining the gastrointestinal microbiome should not be underestimated (Ranganathan and Anteyi, 2022). The animal itself cannot digest soluble fibre in the diet, but the microbes within the intestine can. Soluble fibre is fermented into short-chain fatty acids (SCFA) or volatile fatty acids (VFA). These products are an important energy source for the intestinal cells and can increase blood flow to the intestine.
Nitrogenous waste products in the blood are presented to the intestinal lumen, where urease, an enzyme produced by intestinal bacteria, hydrolyses the urea into ammonia and carbon dioxide. The ammonia is then used by the intestinal bacteria. This process means that nitrogenous waste products are excreted in faecal matter, rather than in urine by the kidneys. Dietary fibre may also be beneficial for improving gastrointestinal motility in dogs with renal failure (Makki et al, 2018; Kidney Disease: Improving Global Outcomes Diabetes Work Group, 2020). Colonic transit times can be decreased in moderate renal disease, as it alters duodenojejunal motility.
Phosphate binders
Supplements that can be used in cases of chronic renal diseases are designed to help reduce the phosphate levels in the diet. Phosphate binders that can be used include calcium carbonate, which can reduce the apparent digestibility of phosphorus more than two times than when fed a standard diet.
Chitosan (a derivative of chitin) has also been used in supplements for animals experiencing chronic renal failure. Chitosan acts as an absorbent in the intestines, and thus lowers the absorption of certain substances including phosphates and some uraemic toxins.
Feeding a renal diet
Objectively, the role of these veterinary diets is to help reduce azotaemia, hyperphosphatemia and to control secondary hyperparathyroidism – ultimately improving both the clinical and biochemical status of the animal. But a reduced dietary intake has often been blamed on the palatability of the diet. The effect of uraemia on the sense of taste and smell and the development of food aversions can all contribute greatly towards inappetence. Changing any animal from a high-salt, high-protein diet to a commercial renal diet can be very difficult. Changing via a transitional intermediate diet (senior or early renal diet) over a more prolonged period (usually 3–4 weeks) can be beneficial in some animals.
Cyclic use of diets can also be helpful. Instead of using all flavours of the diets all of the time, limit to one flavour, then when the pet's intake declines, the owner can then swap to a different flavour and appetite usually increases. A cycle of the different flavours, textures and sometimes brands ensures that the animal remains on a renal diet.
Conclusions
Feeding a renal patient can be very much a balancing game between achieving adequate calorie intake and consuming diets that have the required nutrient balance. Using the International Renal Interest Society staging can help veterinary nurses with recommending the correct diet.
KEY POINTS
- All animals need careful monitoring once placed on a diet or phosphate binder to ensure the desired results have occurred. Modification to feeding plans might be required.
- Individuals will respond to medications and nutritional management in dramatically different ways and therefore individualisation of care plans is required.
- Monitoring potassium and calcium are important, as are urea, creatinine and phosphate.