BUN in Cats: Normal Range, What High Levels Mean & When to Worry

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

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.

Common Causes of High BUN in Cats

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:

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

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.

When to Worry About BUN

Frequently Asked Questions