# Understanding Cat Blood Tests: Interpreting Renal Markers BUN, Creatinine and SDMA ## Key Takeaways - BUN (blood urea nitrogen) is a general indicator of kidney function and hydration, but its elevation can also be influenced by diet, liver function, and gastrointestinal bleeding, making it less specific for renal disease alone. - Creatinine is a more specific indicator of kidney function as it is a waste product of muscle metabolism produced at a relatively constant rate, but it can be slow to rise in early kidney disease and is affected by muscle mass and dehydration. - SDMA (symmetric dimethylarginine) is a sensitive biomarker for decreased kidney function, often rising earlier than creatinine, and is less influenced by muscle mass or diet, allowing for earlier detection of renal decline. - Veterinarians interpret BUN, creatinine, and SDMA in conjunction with a complete physical examination and urinalysis, as a high BUN with normal creatinine may suggest pre-renal causes like dehydration, while elevated creatinine and SDMA strongly indicate renal dysfunction. - Acute Kidney Injury (AKI) is often caused by toxins (e.g., lilies, antifreeze), severe infections, shock, or urinary obstruction, whereas Chronic Kidney Disease (CKD) is progressive and commonly associated with age, genetics, prior kidney injury, and hypertension. - Management of CKD focuses on dietary modification (low phosphorus, controlled protein), ensuring hydration, controlling blood pressure, managing nausea, and treating anemia, with regular monitoring via blood tests, urinalysis, and blood pressure measurements. --- When your veterinarian recommends a blood test for your [cat](/knowledge/veterinary-medicine/clinical-methods/cat), the focus often lands on kidney health. The renal markers BUN (blood urea nitrogen), creatinine, and SDMA (symmetric dimethylarginine) are the core trio used to evaluate how well your cat's kidneys are filtering waste. This article explains what these markers measure, why they matter, and how to interpret the results in the context of your cat's overall health. The kidneys perform several vital roles, including filtering metabolic waste, regulating hydration, maintaining electrolyte balance, and producing hormones. When kidney function declines, waste products accumulate in the blood. Understanding the nuances of BUN, creatinine, and SDMA helps you and your veterinarian detect kidney disease early, stage it accurately, and manage your cat's quality of life. ## At a Glance: Key Renal Markers in Cats | Marker | What It Measures | Why It Is Used | Key Limitation | | :--- | :--- | :--- | :--- | | **BUN (Blood Urea Nitrogen)** | The amount of urea nitrogen in the blood. Urea is a waste product from protein metabolism. | It is a general indicator of kidney function and hydration status. | Can be influenced by diet, liver function, and dehydration. A high BUN is not always due to kidney disease. | | **Creatinine** | A waste product from muscle metabolism. It is produced at a relatively constant rate. | It is a more specific indicator of kidney function than BUN. Elevated levels usually suggest decreased filtration. | Can be affected by muscle mass and can be slow to rise in early kidney disease. A significant portion of kidney function must be lost before creatinine rises above the reference range. | | **SDMA (Symmetric Dimethylarginine)** | A molecule released during protein breakdown. It is primarily excreted by the kidneys. | It is considered an earlier and more sensitive biomarker for decreased kidney function than creatinine. | It can be affected by non-renal factors in some cases, but it is generally more specific for kidney function. | ## Anatomy and Physiology of Feline Kidney Function To understand blood tests, it helps to know how the kidneys work. The functional unit of the kidney is the nephron. Each nephron consists of a glomerulus, which is a tiny cluster of blood vessels that filters blood, and a series of tubules that reabsorb water and nutrients and secrete waste products into the urine. The glomerular filtration rate (GFR) is the amount of blood filtered by the glomeruli per minute. This is the best overall measure of kidney function. When GFR decreases, substances that are normally cleared by the kidneys begin to accumulate in the blood. This is called azotemia. The markers BUN, creatinine, and SDMA are all used as indirect indicators of GFR. A decrease in GFR leads to an increase in these markers, but the relationship is not always linear, and each marker has its own strengths and weaknesses. ## Understanding BUN (Blood Urea Nitrogen) BUN is a measure of the amount of nitrogen in the blood that comes from urea. Urea is a byproduct of protein metabolism. The liver produces urea from ammonia, which is generated when the body breaks down proteins from the diet or from body tissues. The urea is then released into the bloodstream and filtered out by the kidneys. ### Causes of Elevated BUN in Cats An elevated BUN, also known as azotemia, can result from several different mechanisms. It is not a specific test for kidney disease on its own. - **Pre-renal Causes:** These are conditions that reduce blood flow to the kidneys, even though the kidneys themselves may be healthy. The most common cause is dehydration. Other pre-renal causes include shock, heart failure, and high blood pressure. In these cases, the kidneys are receiving less blood, so they filter less urea, leading to a buildup in the blood. - **Renal Causes:** These are conditions that directly damage the kidneys themselves, such as chronic kidney disease (CKD), acute kidney injury, infections, or toxins. When the nephrons are damaged, they cannot filter waste effectively. - **Post-renal Causes:** These are conditions that block the flow of urine out of the body, such as a urethral obstruction (a urinary blockage) or a ruptured bladder. If urine cannot leave the body, the waste products in it are reabsorbed into the bloodstream. ### Factors That Can Influence BUN BUN is not solely a kidney marker. It can be influenced by other factors, which is why it must be interpreted alongside other tests and a physical examination. - **Diet:** A high-protein diet can cause a mild increase in BUN. Conversely, a low-protein diet can cause BUN to be lower than expected. - **Liver Function:** Since the liver produces urea, severe liver disease can lead to a low BUN, even if the kidneys are failing. - **Gastrointestinal Bleeding:** Blood in the digestive tract is broken down into protein, which can lead to an increased BUN. ## Understanding Creatinine Creatinine is a waste product generated from the normal breakdown of muscle tissue. The amount of creatinine produced each day is relatively constant for an individual cat, based on its muscle mass. It is freely filtered by the glomerulus and is not reabsorbed to any significant degree by the tubules. This makes creatinine a better indicator of GFR than BUN. ### The Relationship Between Creatinine and Kidney Function As kidney function declines, the GFR decreases, and creatinine levels in the blood rise. However, the relationship is not linear. Because the kidneys have a large functional reserve, a cat can lose a significant amount of kidney function before the creatinine level rises above the reference range. In many cases, a cat may have lost up to 75% of its kidney function before a significant increase in creatinine is detected. ### Limitations of Creatinine as a Marker - **Muscle Mass:** Creatinine is directly related to muscle mass. A very muscular cat may have a higher baseline creatinine, while a cat with significant muscle wasting (cachexia) may have a lower baseline creatinine. This means that a creatinine level that is within the normal range might still be elevated for a specific cat with low muscle mass. - **Dehydration:** Like BUN, creatinine can be affected by dehydration. When a cat is dehydrated, the blood becomes more concentrated, and creatinine levels can rise even if the kidneys are functioning normally. - **Delayed Rise:** Creatinine is a late indicator of kidney disease. By the time creatinine levels are elevated, the kidney disease is often advanced. A recent study explored the correlation between changes in serum creatinine concentration and a specific ultrasound finding in azotemic cats. The study found that a renal cortical anisotropic backscattering artifact (CABA) was significantly positively correlated with clinical outcomes in cats with moderate to severe azotemia, with an odds ratio of 8.57. This suggests that imaging, along with blood tests, can provide additional prognostic information for cats with significant kidney disease. [1] ## Understanding SDMA (Symmetric Dimethylarginine) SDMA is a newer biomarker that has become a standard part of feline blood panels. It is a methylated form of the amino acid arginine that is released into the bloodstream during protein breakdown. SDMA is primarily excreted by the kidneys through filtration. Its main advantage over creatinine is that it is more sensitive and can detect a decrease in GFR earlier in the course of kidney disease. ### Why SDMA is a Valuable Tool - **Early Detection:** SDMA concentrations rise earlier than creatinine in the course of kidney disease. Studies have shown that SDMA can detect a reduction in GFR when creatinine levels are still within the normal reference range. This allows for earlier intervention and management. - **Less Affected by Muscle Mass:** SDMA is not significantly affected by muscle mass, making it a more reliable indicator of kidney function in cats with varying body conditions. - **Specificity:** While not perfect, SDMA is more specific for kidney function than BUN. It is less influenced by diet or liver disease. ### What SDMA Results Mean An elevated SDMA level indicates that the kidneys are not filtering waste products effectively. In the context of diagnosing and staging chronic kidney disease, SDMA is used in conjunction with creatinine and other tests. A persistently elevated SDMA, even with a normal creatinine, is often the first sign of early kidney dysfunction. ### SDMA and Its Role in Staging Kidney Disease Veterinary guidelines, such as those from the International Renal Interest Society (IRIS), now incorporate SDMA into their staging system for CKD. A cat with a normal creatinine but a persistently elevated SDMA may be classified as having Stage 1 kidney disease. This early diagnosis provides a valuable opportunity to implement management strategies, such as dietary changes and blood pressure monitoring, that may slow the progression of the disease. ## How Veterinarians Use These Markers Together No single blood test is perfect. The most accurate assessment of kidney function comes from evaluating BUN, creatinine, and SDMA together, along with a complete physical examination and urinalysis. - **BUN and Creatinine:** These are often evaluated together. A high BUN with a normal creatinine may suggest a pre-renal cause like dehydration. A high BUN and high creatinine are more indicative of a renal or post-renal problem. - **Creatinine and SDMA:** When creatinine is elevated, SDMA is used to confirm that the elevation is due to kidney disease and to help stage the disease. When creatinine is normal but SDMA is elevated, it points towards early kidney disease that might otherwise be missed. - **Urinalysis:** A urinalysis is essential to assess the kidney's ability to concentrate urine. In kidney disease, the urine is often dilute (low specific gravity). The presence of protein in the urine can also be an indicator of kidney damage. In a study on [feline morbillivirus](/knowledge/viruses/pet-viruses/feline-morbillivirus), a virus that can establish persistent infections in the feline kidney, researchers found that infected cats produced exceptionally high titres of neutralizing antibodies. While this research is focused on a specific virus, it highlights the ongoing investigation into various factors that can affect feline kidney health. [2] ## Causes and Risk Factors for Kidney Disease in Cats Kidney disease in cats can be acute (sudden) or chronic (long-term). ### Acute Kidney Injury (AKI) AKI is a sudden loss of kidney function that develops over a period of days. It is often caused by: - **Toxins:** Ingestion of lilies, antifreeze (ethylene glycol), or certain medications like non-steroidal anti-inflammatory drugs (NSAIDs). - **Infections:** Severe bacterial infections that affect the kidneys. - **Shock or Trauma:** Any condition that severely reduces blood flow to the kidneys. - **Urinary Obstruction:** A blockage in the urethra, which is more common in male cats, can cause a rapid buildup of waste products. ### Chronic Kidney Disease (CKD) CKD is a progressive and irreversible loss of kidney function that develops over months or years. It is one of the most common diseases in older cats. The exact cause is often unknown, but it can be related to: - **Age:** The risk of CKD increases significantly in cats over 7 years of age. - **Genetics:** Certain breeds, such as Persians, Abyssinians, and Siamese, may be predisposed. - **Previous Kidney Injury:** An episode of AKI can lead to long-term damage and the eventual development of CKD. - **High Blood Pressure (Hypertension):** Hypertension can be both a cause and a consequence of kidney disease. Untargeted metabolic profiling of cat urine and plasma in hypertensive cats has identified biochemical changes that could advance our understanding of the pathogenesis of hypertension and its link to kidney disease. [3] ## Veterinary Examination and Diagnostics If your cat is showing signs of kidney disease, such as increased thirst, increased urination, weight loss, or poor appetite, your veterinarian will perform a thorough evaluation. ### The Importance of a Complete History and Physical Exam The first step is a detailed history and a full physical examination. Your veterinarian will look for signs of dehydration, feel for abnormal kidney size or shape, and check for other health issues. They will also ask about your cat's diet, water intake, urination habits, and any potential exposure to toxins. ### Recommended Diagnostic Tests A comprehensive diagnostic plan for a cat with suspected kidney disease includes: 1. **Complete Blood Count (CBC):** This test evaluates red blood cells, white blood cells, and platelets. It can help identify anemia, which is common in cats with CKD, or signs of infection. 2. **Serum Biochemistry Profile:** This panel includes BUN, creatinine, and SDMA, along with other important markers like electrolytes (sodium, potassium, phosphorus, calcium) and glucose. Electrolyte imbalances are common in kidney disease. 3. **Urinalysis:** This test is critical for assessing urine concentration and checking for the presence of protein, blood, or crystals. A urine culture may be recommended if a urinary tract infection is suspected. 4. **Blood Pressure Measurement:** High blood pressure is a common complication of kidney disease. Your veterinarian will want to measure your cat's blood pressure to diagnose and manage hypertension. 5. **Imaging:** In some cases, abdominal ultrasound or X-rays may be recommended to evaluate the size, shape, and structure of the kidneys. As mentioned earlier, ultrasound can also provide prognostic information in cases of azotemia. [1] ## Evidence-Based Management of Chronic Kidney Disease While CKD is not curable, it is manageable. The goal of treatment is to slow the progression of the disease, manage clinical signs, and maintain a good quality of life. ### Dietary Management Dietary modification is the cornerstone of CKD management. A kidney-friendly diet is typically: - **Restricted in Phosphorus:** High phosphorus levels can worsen kidney damage. Restricting dietary phosphorus helps to slow disease progression. - **Restricted in Protein:** Reducing protein intake can decrease the workload on the kidneys and reduce the buildup of urea (BUN). However, the protein provided must be of high quality to maintain muscle mass. - **Increased in Omega-3 Fatty Acids:** These can help reduce inflammation in the kidneys. - **Added Potassium:** Cats with CKD often lose potassium in their urine, leading to low blood potassium levels. Supplementation may be necessary. ### Managing Clinical Signs - **Hydration:** Ensuring adequate water intake is vital. This can be encouraged by providing fresh water, using a water fountain, or feeding canned food, which has a high moisture content. In some cases, subcutaneous (under the skin) fluid therapy may be recommended. - **Controlling Blood Pressure:** If your cat has hypertension, medication like amlodipine may be prescribed. Untreated hypertension can cause further damage to the kidneys, as well as to the eyes and brain. - **Managing Nausea and Vomiting:** CKD can cause a buildup of toxins that lead to nausea and a poor appetite. Anti-nausea medications and appetite stimulants can be very helpful. - **Treating Anemia:** The kidneys produce a hormone called erythropoietin, which stimulates the bone marrow to make red blood cells. In CKD, this hormone is deficient, leading to anemia. Treatment may involve medications to stimulate red blood cell production. ### Monitoring and Follow-Up Regular monitoring is essential for cats with CKD. Your veterinarian will likely recommend periodic blood tests to check BUN, creatinine, SDMA, and electrolytes. Urinalysis and blood pressure measurements will also be repeated at regular intervals to assess the progression of the disease and the effectiveness of treatment. ## Unsafe Home Remedies and What to Avoid When your cat is diagnosed with kidney disease, you may be tempted to try home remedies. However, some can be harmful. - **Do Not Give Human Medications:** Never give your cat human medications, especially NSAIDs like ibuprofen or naproxen. These are highly toxic to cats and can cause acute kidney failure. - **Do Not Use Unproven Supplements:** Avoid giving your cat over-the-counter supplements without first consulting your veterinarian. Some supplements can interfere with medications or worsen kidney function. - **Do Not Make Sudden Diet Changes:** While a kidney diet is beneficial, changing your cat's food abruptly can cause them to refuse to eat. Cats can develop hepatic lipidosis (fatty liver disease) if they stop eating for even a few days. Any dietary change should be done gradually and under veterinary guidance. ## Prevention and Early Detection While not all forms of kidney disease can be prevented, there are steps you can take to protect your cat's kidneys. - **Regular Veterinary Checkups:** Annual or semi-annual wellness exams are crucial, especially for senior cats. These exams should include blood tests (including SDMA) and a urinalysis to detect early signs of kidney disease. - **Prevent Toxin Exposure:** Keep your cat indoors or supervise them outdoors to reduce the risk of exposure to toxins like lilies and antifreeze. Ensure all medications and household chemicals are stored safely out of reach. - **Encourage Water Intake:** Provide fresh, clean water at all times. Consider a pet water fountain, as many cats prefer moving water. Feeding a diet that includes wet food can also increase your cat's overall water intake. - **Manage Underlying Conditions:** Conditions like hypertension and diabetes can damage the kidneys over time. Managing these conditions effectively can help protect kidney function. ## Prognosis and Quality of Life The prognosis for a cat with kidney disease varies greatly depending on the underlying cause, the stage of the disease at diagnosis, and the response to treatment. - **Acute Kidney Injury:** The prognosis for AKI is guarded and depends on the cause and the severity of the damage. Some cats can recover with aggressive treatment, but others may progress to chronic kidney disease. - **Chronic Kidney Disease:** CKD is a progressive disease, but many cats can live comfortably for months or even years with appropriate management. The goal is to maintain a good quality of life. Your veterinarian will work with you to develop a management plan that addresses your cat's specific needs. ## Limitations and When to Contact a Veterinarian This article provides a general overview of interpreting renal markers in cats. It is educational and is not a substitute for veterinary diagnosis or treatment. There are important limitations to consider: - **Individual Variation:** Every cat is different. What is a normal value for one cat may be abnormal for another. Factors like age, breed, and muscle mass can influence baseline values. - **Breed-Level Information Cannot Predict Individual Outcomes:** While certain breeds may be predisposed to kidney disease, this does not mean that every cat of that breed will develop it. Conversely, any cat, regardless of breed, can develop kidney disease. - **Test Results Are a Snapshot:** A single blood test is a snapshot in time. Serial testing is often needed to understand trends and make an accurate diagnosis. - **Other Health Conditions:** Other diseases, such as hyperthyroidism, diabetes mellitus, and heart disease, can affect kidney function and complicate the interpretation of blood tests. **You should contact your veterinarian immediately if your cat shows any of the following signs:** - Increased thirst and urination - Loss of appetite or weight loss - Vomiting or diarrhea - Lethargy or weakness - Bad breath (halitosis) - Mouth ulcers - Straining to urinate or producing no urine (this is a medical emergency) ## Frequently Asked Questions ### What is the difference between BUN and creatinine in a cat's blood test? BUN (blood urea nitrogen) is a waste product from protein breakdown in the liver, while creatinine is a waste product from muscle metabolism. BUN is more easily influenced by factors like diet and dehydration, whereas creatinine is a more specific indicator of kidney function. ### Why is SDMA considered a better test for early kidney disease in cats? SDMA is considered better for early detection because its concentration in the blood rises earlier than creatinine when kidney function declines. It is also less affected by a cat's muscle mass, making it a more accurate reflection of true kidney filtration rate. ### My cat's creatinine is high but SDMA is normal. What does this mean? This pattern can be seen with non-renal factors that increase creatinine, such as dehydration or high muscle mass. However, it is important to discuss this with your veterinarian, as they may recommend further testing to rule out kidney disease. ### Can a cat's diet affect the results of BUN and creatinine tests? Yes, a high-protein diet can cause a mild increase in BUN. Creatinine is less affected by diet but is influenced by muscle mass. A cat with low muscle mass may have a lower baseline creatinine. ### What is the normal range for SDMA in cats? The reference range for SDMA in cats is generally considered to be 0 to 14 µg/dL. Values above 14 µg/dL are often indicative of decreased kidney function, but your veterinarian will interpret this in the context of your cat's overall health. ### My cat is drinking more water. Does that mean they have kidney disease? Increased thirst (polydipsia) and increased urination (polyuria) are common signs of kidney disease, but they can also be caused by other conditions like diabetes mellitus or hyperthyroidism. A veterinary examination and blood and urine tests are necessary to determine the cause. ### How often should I have my senior cat's blood tested for kidney disease? For senior cats (over 7 years of age), it is generally recommended to have blood tests and a urinalysis performed at least annually. If your cat has a health condition like kidney disease, your veterinarian may recommend more frequent monitoring, such as every 3 to 6 months. ### What should I do if my cat's blood test shows early kidney disease? If your cat is diagnosed with early kidney disease, work with your veterinarian to develop a management plan. This may include transitioning to a kidney-friendly diet, ensuring adequate hydration, monitoring blood pressure, and scheduling regular follow-up appointments to track the disease's progression.