Table of Contents:
- Symptoms of Kidney Disease
- Evaluation and Testing for Kidney Disease in Cats
- Kidney Disease Staging and Substaging
- Treatment of Renal Disease
Kidney (renal) disease is extremely common in aging cats. It can be acute onset, but is typically seen as a chronic age-related change diagnosed later in the course of the disease.
Understanding kidney disease involves having an understanding of the anatomy and function of the kidneys. The kidneys filter blood from the circulating blood stream and remove waste to create urine. They are also responsible for helping to maintain a healthy water balance and work to regulate sodium, potassium, calcium, and phosphorus levels.
Kidneys also help to maintain a normal blood pressure and produce hormones to promote red blood cell formation. Without normal renal function, all of these essential processes can fail. The kidneys are made up of millions of nephrons, which contain a glomerulus responsible for filtering and the tubule, which helps transport waste and filter blood to the appropriate locations. The loss of functioning nephrons causes the remaining nephrons to work harder, which ultimately causes damage. One theory for the development of renal disease in cats is thought to be secondary to chronic interstitial nephritis (CIN), which is identified by advanced fibrosis, loss of nephrons, and sterile inflammation, leading to tubulointerstitial disease.
Symptoms of Kidney Disease
Clinical signs of chronic renal disease in cats tend to be subtle and progressive.
The most common signs are:
- Increased thirst
- Increased urination
- Weight loss
- Unkempt hair coat
- Decreased appetite
- Foul breath
Cats that exhibit symptoms should be evaluated by a veterinarian. There are other diseases in older cats that can mimic renal failure and should be addressed and determined as the underlying cause. Diseases that can mimic renal failure with similar clinical signs include:
- Diabetes Mellitus
- Urinary tract infection/kidney infection
- Hyperthyroidism (elevated thyroid levels)
- Hypercalcemia (elevated calcium)
If a cat is suspected to have underlying kidney disease, your veterinarian will perform a full physical exam. On exams, they assess hydration, palpate the kidneys, and look for muscle loss, any evidence of a heart murmur, and pale mucous membranes. Cats with chronic kidney disease will often be dehydrated, because the kidneys are not functioning appropriately and, therefore, are causing excess water to be lost in the urine. Because the kidneys are responsible for making a hormone called erythropoietin that assists in the creation of red blood cells, cats with kidney disease often have pale mucous membranes and suffer from anemia. Cats with underlying anemia can have a heart murmur and should be auscultated when performing a physical exam. Palpation may also present small, misshapen kidneys. If a cat has pain or oversized kidneys, they need to be assessed for renal cancer or a kidney infection (pyelonephritis).
Evaluation and Testing for Kidney Disease in Cats
Ultimately, kidney function is evaluated by performing further diagnostics, including:
- Complete blood count (CBC). This evaluates the red blood cell count, white blood cell count, and platelets. If a patient is anemic, their red blood cell count would reflect these changes. Evaluating the white blood cell count can give further indication if an underlying infection is present.
- Chemistry panel. This is the most important piece of diagnostics in determining kidney function. The two main chemistry values used to determine function are blood urea nitrogen (BUN) and creatinine. Creatinine is considered to be the more sensitive of these two values in regards to renal function. When the kidney function is failing, these values rise. Dehydration and urinary obstruction are two other reasons for elevated values and the significance of this elevation should be used in conjunction with a physical exam.
- Electrolytes. Cats with renal disease can have disturbances in their sodium, potassium, phosphorus, and calcium levels.
- Urinalysis. Submitting a sample of urine to an outside lab can assist in determining kidney function. The urine specific gravity (USG) is a measurement of the kidney’s concentrating ability. As kidneys fail, they lose the ability to concentrate urine. In cats where kidney values are elevated due to dehydration, their USG should be very high, indicating the kidneys are conserving all the water they can. Isosthenuria is the term for inappropriate urine concentrating ability, where the USG is between 1.008 – 1.012. Normal USG in healthy, appropriately-hydrated cats is >1.035.
- Serum symmetric dimethylarginine concentration (SDMA) is a biomarker that can help determine early signs of renal dysfunction. SDMA is run from a blood sample on cats and is commonly part of yearly screening blood work. SDMA is a surrogate marker for the glomerular filtration rate (GFR), which is an indicator of how well the kidneys are working. An elevated SDMA is a more sensitive function indicator, and elevations of SDMA in cats with normal creatinine values indicates reduced renal function (known as stage 1 CKD). SDMA is a great tool to monitor yearly blood work in cats and can raise suspicion of reduced renal function before changes in kidney values levels are noted.
- Ultrasound. Abdominal ultrasound is a useful diagnostic for cats with underlying renal disease. Cats with chronic disease have aging changes that present on ultrasound, as opposed to cats that have an acute injury. Determining if cats have acute, chronic, or acute-on-chronic kidney disease is very important in managing the disease, expected prognosis, and outcome. Ultrasound is also valuable to look for kidney stones, blockages, bladder stones, infections, and types of cancer.
- Blood pressure measurements. Any cats that have suspicion of renal disease should have their blood pressure checked. The kidneys help in blood pressure regulation, and in advanced stages of kidney disease, animals can develop hypertension (elevated blood pressure). Hypertension can cause additional injury to the kidneys, as well as damage a cat’s retinas or brain.
- Urine protein to creatinine ratio (UP/C). This helps determine the amount of protein loss through the kidneys. Research has shown that the magnitude of protein loss in urine is associated with more rapid progression of renal disease and higher mortality.
Kidney Disease Staging and Substaging
Chronic kidney disease has been categorized by The International Renal Interest Society (IRIS). This society has helped to create a standardized staging system for cats and dogs with renal disease. Having an accepted staging system creates a coherent and easy-to-follow method all veterinarians can use that avoids subjective variance. The staging system is primarily based on blood creatinine levels, with a subset of staging based on urine protein creatinine ratio and blood pressure.
|Stage||Blood Creatinine (umol/l & mg/dL)||SDMA (ug/dL)|
|2||140 – 250, 1.6 – 2.8||18 – 25|
|3||251 – 440, 2.9 – 5.0||26 – 38|
|0.2 – 0.4||Borderline proteinuric|
|Systolic Blood Pressure||Blood Pressure Substage||Risk of Future Target Organ Damage|
|140 – 159 mmHg||Prehypertensive||Low|
|160 – 179 mmHg||Hypertensive||Moderate|
Treatment of Renal Disease
Treatment of chronic renal failure is multifactorial and highly dependent on each patient.
A few keys to treatment include:
- Maximization of hydration. Cats with chronic renal disease live in a state of dehydration. Improving hydration helps blood perfusion to the kidneys and improves general well-being. Improving hydration can be done passively by making more water bowls available or adding water to moisten dry kibble. Active ways to improve hydration can be at home through subcutaneous fluid therapy. Your veterinarian can prescribe prescription fluids that are given at a set frequency and amount based on your cat’s needs. Subcutaneous fluids should never be initiated without a veterinarian’s recommendation, since fluid therapy can have negative side effects to those with underlying heart disease. Never inject medications or fluid into your cat without a veterinarian’s recommendation and guidance.
- Managing hypertension. Blood pressure medications are initiated in cats with hypertension to help stop additional organ damage. These medications are prescribed by your veterinarian and serial blood pressure monitoring helps to determine the ideal dose and frequency.
- Managing anemia. Cats with renal disease lack erythropoietin, which is a hormone that regulates red blood cell formation. In cats with chronic anemia, there are injections administered by your veterinarian to supply your cat with this hormone. This is a human erythropoietin derivative.
- Managing electrolytes. Cats with chronic kidney disease tend to have low potassium and elevated phosphorus due to lack of renal regulation. Those with hyperphosphatemia are prescribed phosphate binders to minimize phosphorus absorption through their diet. Cats with hypokalemia are prescribed potassium supplements. Serial electrolyte monitoring is needed to fine tune the perfect mixture for each cat.
- Changes in diet. There are a variety of vet-approved diet options. Prescription diets that are formulated for cats with chronic kidney disease tend to be low in protein to minimize the strain on the kidneys, as well as low in phosphorus and sodium. Wet, canned food is also preferred, since it helps maximize hydration. There have been multiple studies that have shown that cats with chronic renal failure have a longer life span when fed a prescription-formulated “renal diet,” compared to an equal subset population that was fed a maintenance cat food diet.
Ultimately, working with your veterinarian to create the best treatment plan for your cat is recommended. Frequent monitoring of renal values, electrolytes, and blood pressure are needed to monitor progression of disease and success of treatment.