BUN in Cats: Normal Range, What High Levels Mean & When to Worry
Last reviewed: April 2026
Normal BUN in cats is 17–35 mg/dL. Values of 36–50 are mildly elevated (often dehydration or early kidney disease), 51–80 are moderately elevated (significant dehydration or moderate CKD), and above 80 mg/dL indicates severe kidney disease or acute kidney injury requiring urgent care. The BUN:creatinine ratio — not BUN alone — tells you whether the cause is dehydration (ratio >30:1) or true kidney disease (ratio 10–30:1).
Normal BUN: 17-35 mg/dL
High BUN can mean kidney disease OR dehydration. The BUN:creatinine ratio tells you which. Always interpret with creatinine and SDMA.
Worried about your cat's BUN levels?
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Check My Cat's BUNIf your cat's bloodwork shows elevated BUN, it could mean kidney disease, dehydration, or other issues. The key is understanding the BUN:creatinine ratio to determine whether your cat has true kidney problems or a reversible condition.
What Is BUN?
BUN (Blood Urea Nitrogen) is a waste product created when the body breaks down protein. It's normally filtered out by the kidneys and excreted in urine. When kidneys aren't working properly — or when other factors affect protein metabolism — BUN levels rise.
Unlike SDMA, which is kidney-specific, BUN is influenced by multiple factors: hydration, diet, GI bleeding, and liver function. This makes BUN less specific for kidney disease but useful for understanding the overall clinical picture.
BUN Severity Chart: Understanding Your Cat's Results
| BUN Level | Severity | What It Means | Typical Action |
|---|---|---|---|
| 17-35 mg/dL | Normal | Normal kidney filtration and hydration | Routine monitoring |
| 36-50 mg/dL | Mild | Possible dehydration, early CKD, or high-protein meal | Check creatinine/SDMA, assess hydration |
| 51-80 mg/dL | Moderate | Significant dehydration or moderate kidney disease | Rehydrate and recheck |
| 81-120 mg/dL | Severe | Advanced CKD or acute kidney injury | Urgent intervention, IV fluids |
| >120 mg/dL | Critical | Uremia, severe kidney failure, or obstruction | Emergency hospitalization |
Note: BUN alone doesn't diagnose kidney disease. Always interpret alongside creatinine and SDMA to distinguish true kidney disease from dehydration.
The BUN:Creatinine Ratio — Key to Diagnosis
The BUN:creatinine ratio helps distinguish true kidney disease from dehydration or other causes of elevated BUN.
- →BUN and creatinine rise proportionally
- →Suggests true kidney disease (CKD or AKI)
- →Kidneys are the primary problem
- →BUN rises more than creatinine
- →Suggests dehydration or GI bleeding
- →Often reversible with rehydration
Real-World Examples:
Example 1: Dehydrated Cat
BUN: 60 mg/dL, Creatinine: 1.5 mg/dL → Ratio: 40:1 (HIGH)
Interpretation: Pre-renal azotemia (dehydration). Rehydrate and recheck — BUN should normalize.
Example 2: Chronic Kidney Disease
BUN: 60 mg/dL, Creatinine: 3.0 mg/dL → Ratio: 20:1 (NORMAL)
Interpretation: True kidney disease. Both values elevated proportionally.
Example 3: GI Bleeding
BUN: 55 mg/dL, Creatinine: 1.2 mg/dL → Ratio: 46:1 (VERY HIGH)
Interpretation: Digested blood raising BUN. Check for black tarry stool or vomiting blood.
Worried about your cat's BUN levels?
Upload your cat's bloodwork to see BUN in context with creatinine and other kidney markers. Track changes over time to monitor kidney function.
Analyze My Cat's ResultsCommon Causes of High BUN in Cats
Chronic Kidney Disease (CKD)
Most common cause in senior cats. BUN and creatinine both elevated with normal ratio. See our CKD guide.
Dehydration
Reduced blood flow to kidneys. BUN rises more than creatinine (high ratio). Reversible with fluids.
Urinary Obstruction EMERGENCY
Blocked cats can't urinate, causing rapid BUN/creatinine rise with high potassium.
Acute Kidney Injury
Sudden damage from toxins (lilies, antifreeze), infection, or trauma. Rapid BUN rise over hours.
GI Bleeding
Blood in the digestive tract is digested as protein, raising BUN with normal creatinine.
High-Protein Diet
Very high dietary protein can mildly elevate BUN (36-45 mg/dL) with normal creatinine/SDMA.
Symptoms of High BUN (Uremia)
When BUN becomes very high, toxic waste products accumulate, causing uremia. Watch for:
- • Loss of appetite — often the first sign
- • Vomiting — from GI irritation
- • Bad breath — ammonia or urine-like odor
- • Lethargy — weakness, sleeping more
- • Mouth ulcers — painful sores on gums or tongue
- • Increased thirst/urination — with kidney disease
- • Weight loss — muscle wasting
- • Seizures or confusion — severe uremia (emergency)
Note: Mild BUN elevation often has no symptoms. Symptoms typically appear when BUN exceeds 80-100 mg/dL or with rapid rise.
Before the recheck, not after
A single high BUN is frequently dehydration, a recent high-protein meal, or bleeding into the gut rather than kidney disease. If nothing has been diagnosed yet, cover is still worth pricing. Plans start at $9/month.
How “pre-existing” actually works: insurers exclude anything your pet has already shown signs of before the policy starts or during its waiting period. A diagnosis is not required — a note in the record can be enough on its own.
Conditions that are curable generally stop counting once your pet has been free of symptoms and treatment for a set period, commonly 180 days. Chronic ones — kidney disease, diabetes, heart disease — do not. So the moment that matters is before the next panel rather than after it, and whether a value already flagged in the record counts is a judgement your insurer makes when a claim is filed. Read the policy's own definition before buying.
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What Happens Next?
If your cat's BUN is elevated, your vet will determine the cause:
- • Check creatinine and SDMA — both elevated = kidney disease
- • Calculate BUN:creatinine ratio — high ratio = dehydration/GI bleeding
- • Urinalysis — urine concentration shows kidney function
- • Assess hydration — gum moisture, skin elasticity
- • Check phosphorus — elevated in kidney disease
- • Abdominal imaging — ultrasound to visualize kidneys
Treatment depends on the cause:
- Dehydration: IV or subcutaneous fluids. BUN should normalize within 24-48 hours if kidneys are healthy.
- Chronic Kidney Disease: Kidney diet, phosphorus binders, subcutaneous fluids at home, blood pressure control. See CKD management.
- Urinary Obstruction: Emergency catheterization, IV fluids, electrolyte correction, pain management.
- Acute Kidney Injury: Aggressive IV fluids, treat underlying cause, hospitalization for monitoring.
Can BUN Go Back to Normal?
Yes, in many cases:
- • Dehydration: BUN normalizes within 24-48 hours of rehydration
- • Urinary obstruction: BUN drops rapidly after catheterization (if no permanent kidney damage)
- • Acute kidney injury: May normalize if caught early and treated aggressively
- • GI bleeding: BUN normalizes once bleeding stops
- • Chronic kidney disease: BUN will NOT normalize but can be managed/stabilized
Key Takeaway
If BUN normalizes after rehydration, your cat likely doesn't have significant kidney disease.
If BUN remains elevated despite adequate hydration, chronic kidney disease is likely and ongoing management is needed.
How to Lower BUN in Cats
The approach to lowering BUN depends entirely on the cause. Here's what helps for each situation:
If Dehydration Is the Cause (Very Common in Cats)
Cats evolved as desert animals and are notoriously poor drinkers. Chronic mild dehydration is extremely common:
- • Switch to wet food — Canned food is 70-80% water vs 10% in kibble. This is the single most effective change
- • Water fountains — Cats prefer moving water. A fountain often doubles water intake
- • Multiple water stations — Place bowls in several locations away from food
- • Add water to food — Mix extra water into canned food to make a "soup"
- • Low-sodium broth — Chicken or tuna water can encourage drinking
- • Subcutaneous fluids — Your vet may administer fluids for moderate-severe dehydration
With rehydration alone, BUN typically normalizes within 24-48 hours if kidneys are healthy.
If High-Protein Diet Is Contributing
Cats are obligate carnivores and need protein, but very high-protein diets can elevate BUN:
- • Check the protein source — Quality matters more than quantity for cats
- • Consider a moderate-protein diet — If BUN is consistently elevated without kidney disease
- • Fast before bloodwork — 8-12 hours fasting gives more accurate BUN reading
- • Recheck in 2-4 weeks — BUN should stabilize with dietary adjustment
If GI Bleeding Is the Cause
- • Identify the source — Ulcers, IBD, tumors, or hairball-related irritation
- • Gastroprotectants — Omeprazole, famotidine, or sucralfate as prescribed
- • Stop any NSAIDs — These can cause GI ulcers
- • BUN normalizes once bleeding stops
If True Kidney Disease Is Present
If BUN remains elevated despite rehydration (and creatinine/SDMA are also elevated), your cat likely has kidney disease:
- • BUN won't fully normalize — But it can be managed and stabilized
- • Prescription kidney diet — Hill's k/d, Royal Canin Renal, or Purina NF
- • Phosphorus binders — Reduce waste product buildup
- • Subcutaneous fluids at home — Many owners learn to give fluids for Stage 3-4 CKD
- • Regular monitoring — Track trends over time
For detailed kidney disease management and prognosis, see our guide to creatinine levels in cats and CKD management guide. For what to actually put in the bowl, see our guide to the best diet for cats with CKD.
When BUN Won't Lower
Feeding a Cat With Kidney Disease
A raised BUN on its own is not a reason to change food. As above, BUN climbs with dehydration, a recent high-protein meal, or bleeding into the gut, and none of those calls for a renal diet. Once your vet has actually diagnosed CKD, the picture changes and diet becomes the most effective thing you control.
The evidence-based choice: a prescription renal diet. Hill's Prescription Diet k/d is the most studied of them, holding phosphorus to roughly 0.3–0.5% on a dry matter basis against 1.0–1.5% in ordinary cat food. Phosphorus restriction is the single dietary factor with the strongest evidence behind it in CKD. These diets need your vet's authorization, which you can supply when ordering through Petco's veterinary range rather than making a separate trip.
Shop Prescription Renal Diets →If you don't have the authorization yet: a renal diet needs a vet's sign-off before any retailer will sell it to you. An online vet can review your cat's bloodwork and write it without a clinic visit, which is usually faster than waiting for an appointment.
Talk to a Vet Online →If a prescription diet is not an option yet: Smalls is one of the few fresh cat food companies publishing real nutrient data, running roughly 0.37–0.39% phosphorus as fed, about 1.13–1.39% dry matter. Set that against the numbers above and the limit is plain — it is not a renal diet and not a low-phosphorus food, it sits at the lower end of the ordinary range. What it offers is high moisture, portion control, and figures you can check against your vet's guidance. That makes it worth considering in two narrow cases: a Stage 1 cat being managed without a therapeutic diet, or a cat refusing renal food outright, where the real alternative is not eating. IRIS guidance puts a therapeutic diet from Stage 2 onward, so this stops being appropriate as CKD progresses.
See Smalls Recipes →The full guide to what to feed a cat with kidney disease →
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When to Worry About BUN
Seek immediate veterinary care if:
- • BUN is above 80-100 mg/dL
- • Your cat is vomiting repeatedly or not eating
- • Your cat is straining to urinate or not producing urine (EMERGENCY)
- • Your cat is lethargic, weak, or hiding
- • Bad breath with ammonia/urine smell
- • Seizures or disorientation (severe uremia)
Related Reading
SDMA in Cats
The most sensitive early kidney marker
Cat Kidney Values Explained
Complete guide to creatinine, BUN, and SDMA
Chronic Kidney Disease in Cats
Management and treatment
Senior Cat Health Screening
Recommended tests for older cats
High BUN in Dogs
Same test, different normal ranges and causes in dogs
Track Your Cat's BUN Levels Over Time
Upload your bloodwork to VetLens and instantly see:
- ✓ What your cat's BUN level means
- ✓ Automatic BUN:creatinine ratio calculation
- ✓ Whether it's dehydration or kidney disease
- ✓ How BUN compares with SDMA and creatinine
- ✓ Questions to ask your vet
Frequently Asked Questions
What is a normal BUN level for cats?
Normal BUN levels in cats typically range from 17-35 mg/dL. Values between 36-50 mg/dL are mildly elevated, 51-80 mg/dL are moderately elevated, and above 80 mg/dL are severely elevated.
What causes high BUN in cats?
High BUN can result from kidney disease, dehydration, high-protein diet, gastrointestinal bleeding, urinary obstruction, or certain medications. The BUN:creatinine ratio helps differentiate kidney disease from dehydration.
What BUN level is dangerously high in cats?
BUN levels above 80-100 mg/dL are considered dangerously high. Values above 120 mg/dL typically require immediate veterinary intervention and hospitalization.
Can dehydration cause high BUN in cats?
Yes. Dehydration is a common cause of elevated BUN without true kidney disease. The BUN:creatinine ratio will be high (above 30:1). Rehydration usually brings BUN back to normal if kidneys are healthy.
What is the BUN:creatinine ratio and why does it matter?
The ratio helps distinguish dehydration from kidney disease. Normal ratio is 10:1 to 30:1. A high ratio (above 30:1) suggests dehydration or GI bleeding. A normal ratio with both values elevated suggests true kidney disease.
How quickly can BUN return to normal after rehydration?
If elevated BUN is due to dehydration, it typically normalizes within 24-48 hours of rehydration. If BUN remains elevated despite adequate hydration, true kidney disease is likely.
Is BUN or SDMA more accurate for kidney disease?
SDMA is more specific for kidney disease because it's only filtered by kidneys. BUN is affected by hydration, diet, and GI bleeding. Use both together for the complete picture.
Can high-protein diet cause elevated BUN in cats?
Yes, a very high-protein diet can mildly elevate BUN (typically 36-45 mg/dL) without indicating kidney disease. Creatinine and SDMA remain normal in this case.
What symptoms indicate dangerous BUN levels?
Symptoms of uremia include vomiting, loss of appetite, bad breath (ammonia smell), lethargy, weakness, mouth ulcers, and in severe cases, seizures. These require immediate veterinary care.
Should BUN be tested fasting in cats?
Ideally yes. A recent high-protein meal can temporarily elevate BUN. Fasting for 8-12 hours provides the most accurate reading, though this isn't always practical.