Introduction
In order to understand why our pets get kidney disease and its impact on their health, it is important to know what kidneys do. In this three-part series, we will discuss what the kidneys do, what causes kidney failure and how we diagnose it, treat it, manage it and monitor your pet who is living with kidney disease.
Kidney Function and Structure
Kidneys are made up of units called nephrons; these are responsible for separating the products that our bodies need from those that our bodies have to excrete. When our kidneys need to dispose of waste or toxins these products are dissolved in water and passed when our pets urinate. Kidneys are also efficient at conserving and balancing electrolytes like potassium, which if low can lead to muscle weakness.
Hydration and Waste Management
Kidneys also play an important role in hydration or water conservation. When your pet is dehydrated, the kidneys are still able to manage toxin excretion, doing it using less water. Conversely, if a pet consumes a large amount of water, the kidneys will remove it, which prevents dilution of the bloodstream.
Balancing Electrolytes and Blood Pressure
Balancing calcium and phosphorus in the blood is another function of the kidney. If these are off balance, it can cause weakening of the bones. Kidneys also balance pH, which requires fine regulation, and if not managed properly, medical intervention is necessary. Blood pressure is regulated by the kidneys, and they need to be working efficiently to prevent high blood pressure, which can damage blood vessels in the body, including those in the kidney.
Hormonal Functions and Protein Conservation
Kidneys produce a hormone that regulates red blood cells by telling the bone marrow to produce more. Through the nephron, kidneys filter toxins but conserve important proteins that circulate in the bloodstream. Both of these functions play a role in causes of anemia and low protein levels.
Diagnosing Kidney Disease
Kidney disease is most often diagnosed by blood and urine testing. We perform these tests if your pet isn’t well (e.g., increased thirst, weight loss, muscle weakness, vomiting, or anorexia), but we also perform these tests as part of our senior wellness testing or pre-anesthesia testing prior to a surgical or dental procedure.
Challenges with Traditional Testing
The challenge with traditional blood and urine testing is that they don’t tell us how much kidney function has been affected. In most pets, numbers don’t start to go above average range until 75% of the kidney function has been lost. Blood tests in the normal range indicate that the kidneys are currently able to do their job, but we don’t know if they have already lost some ability to function.
Advanced Testing Methods
To assess kidney function, the primary tests we perform are urinalysis, BUN and Creatinine blood testing, electrolyte testing, phosphorus testing, and looking for anemia and the newer SDMA (symmetric dimethylarginine test). These tests are all valuable, but some have advantages over others.
Urinalysis and Blood Testing
A urinalysis is performed to look at urine specific gravity and to make sure that the kidneys aren’t leaking proteins. Urine specific gravity can be affected by other causes of increased thirst, and a pet may have a low specific gravity and usually have functioning kidneys. Urine protein can be falsely elevated if the pet has a bladder infection or other inflammation and it isn’t specific to the kidney. BUN (blood urea nitrogen) and creatinine are blood tests that are often performed together. BUN is created when the liver breaks down proteins, and creatinine is a product of normal muscle breakdown. These waste products are typically eliminated from the body, and lab testing showing high levels of BUN and Creatinine can indicate that the kidneys are not functioning correctly.
The Role of SDMA
SDMA is a newer test and it has been proven to be a more reliable indicator of kidney function than the previously mentioned tests. SDMA is considered a kidney-specific biomarker. It is excreted by the kidneys and reflects the filtering capacity of the kidney. SDMA will increase much earlier than creatinine (as early as a 25% decrease in function vs 75% for creatinine) and is impacted by fewer non-kidney factors.
Upcoming Improvements
Currently, we are sending SDMA testing to an external laboratory, but expect that it will soon be a routine part of most blood panels that we do in the clinic. This will hopefully help us pick up renal disease much earlier and extend the quality and lifetime of our pets who are affected by kidney disease. In the next part of this series, we will discuss what we can do if there are elevations in SDMA or BUN and Creatinine. Early detection is essential and we are excited to have a new tool to help with this.
Managing Kidney Disease
Your Pet has been diagnosed with Kidney Disease, now what? In previous blogs we discussed the diagnosis of renal disease, now we will discuss therapies for these pets. Some of this information was collected from the Cornell Feline Health Center Website, although it applies to both cats and dogs. We will focus on chronic kidney disease (CKD) as we tend to see that more frequently. There is no specific cure for CKD, but treatment can improve and prolong the lives of pets with this disease. Treatment is aimed at minimizing the buildup of toxic waste products in the bloodstream (BUN, Creat and Phosphorus), maintaining good hydration, correcting electrolyte disturbances, supporting good nutrition, controlling blood pressure, monitoring anemia, reducing the amount of protein lost in the urine and slowing the progression of the disease.
Dietary Management
Diet is a scientifically proven aspect of CKD treatment. Therapeutic diets that are restricted in protein, phosphorus, and sodium content and high in water-soluble vitamins, fiber, and antioxidant concentrations may prolong life and improve quality of life in pets with CKD. One of the concerns is that often these pets are already feeling poorly and have some degree of anorexia. Although most of the companies that make this type of diet strive for palatability, it is often difficult to get these pets to eat. It is important to make a gradual transition to a therapeutic diet and to consider food temperature, texture, and flavor. I often tell owners that I want their pet to eat; although therapeutic diets are beneficial, nutrition is more important.
Medications and Supplements
Blood pressure and urinary protein loss are usually controlled with an oral medication called an angiotensin-converting enzyme inhibitor (semintra, fortekor). Anemia in pets with CKD may be treated by replacement therapy with erythropoietin, a hormone that the kidneys usually produce, which stimulates red blood cell production. Although a number of other therapies, including phosphate binders, potassium supplementation, nutritional supplements, and administration of fluids either intravenously in the hospital or subcutaneously (which can be done in cats at home), have the potential to help with CKD, these approaches have not been fully validated. At Westwood Hills, we find that potassium supplements and fluids help pets feel much better. Hemodialysis, the removal of toxic waste products from the bloodstream by specially designed equipment that is done in people, is not done routinely in animals and needs more controlled studies to find out if it is effective. Kidney transplantation has only been done experimentally at a few select research hospitals.
Prognosis and Outcomes
Pets respond variably to treatment for CKD, so the prognosis for CKD in affected animals is difficult to predict. Some studies suggest that pets that lose more protein in their urine have less favorable prognoses. Pets that are still eating well and have no problems with a switch to therapeutic diets and taking medications tend to have better outcomes with respect to quality and length of life.
Written by by Jane Corkum, DVM