BUN in Cats: Normal Range, What High Levels Mean & When to Worry
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?
See BUN with creatinine & SDMA in context
If 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.
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.
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
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.