BUN Levels in Cats: What's Dangerously High?

BUN (Blood Urea Nitrogen) measures waste products filtered by the kidneys. Normal: 10-30 mg/dL. Values above 80-100 mg/dL are dangerously high and indicate severe kidney disease or acute injury.

If your cat's bloodwork shows elevated BUN, it could mean kidney disease, dehydration, or other issues. Understanding BUN—especially alongside creatinine and SDMA—helps 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.

What BUN Tells Us:

Kidney Function: High BUN often indicates reduced kidney filtration
Hydration Status: Dehydration causes BUN to rise more than creatinine
Protein Metabolism: Diet and GI bleeding can affect BUN independently of kidneys
Disease Severity: Higher values often indicate more advanced kidney disease

Normal BUN Ranges for Cats

10-30 mg/dL: Normal range
31-50 mg/dL: Mildly elevated (monitor, investigate cause)
51-80 mg/dL: Moderately elevated (likely kidney disease or dehydration)
81-120 mg/dL: Severely elevated (advanced kidney disease)
120+ mg/dL: Dangerously high (immediate veterinary care needed)

Important: BUN should always be interpreted with creatinine and SDMA for accurate kidney assessment.

What Causes High BUN in Cats?

1. Chronic Kidney Disease (CKD)

Progressive loss of kidney function over time. Most common cause in senior cats.

Pattern: BUN and creatinine both elevated, often with high SDMA

Symptoms: Increased drinking/urination, weight loss, poor appetite

2. Dehydration (Pre-Renal Azotemia)

Reduced blood flow to kidneys causes BUN to rise without true kidney damage.

Pattern: BUN rises more than creatinine (high BUN:creatinine ratio)

Symptoms: Dry gums, sunken eyes, lethargy, decreased skin elasticity

3. High-Protein Diet

Very high dietary protein increases BUN production independently of kidney function.

Pattern: Mild BUN elevation (30-40 mg/dL), normal creatinine and SDMA

Note: Less common in cats than dogs; cats are obligate carnivores

4. Gastrointestinal Bleeding

Blood in the GI tract is digested as protein, raising BUN.

Pattern: Elevated BUN with normal or low-normal creatinine

Symptoms: Black tarry stool, vomiting blood, pale gums

5. Urinary Obstruction

Blockage prevents urine excretion, causing rapid BUN and creatinine rise.

Pattern: Both BUN and creatinine severely elevated, high potassium

Symptoms: Straining to urinate, crying in litter box, no urine production (EMERGENCY)

6. Acute Kidney Injury (AKI)

Sudden kidney damage from toxins, infection, or trauma.

Pattern: Rapid rise in BUN and creatinine over hours to days

Symptoms: Sudden lethargy, vomiting, decreased or no urine output

Is your cat's BUN high from dehydration or true kidney disease?

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Understanding the BUN:Creatinine Ratio

The BUN:creatinine ratio helps differentiate true kidney disease from dehydration or other causes of high BUN.

How to Calculate:

BUN:Creatinine Ratio = BUN (mg/dL) ÷ Creatinine (mg/dL)

Normal Ratio: 10:1 to 30:1

Indicates proportional rise in both values (typical of CKD)

High Ratio: Above 30:1

BUN rising more than creatinine → suggests dehydration, GI bleeding, or high-protein diet

Low Ratio: Below 10:1

Creatinine rising more than BUN → rare; may indicate severe muscle loss or liver disease

Real-World Examples:

Example 1: Dehydration

BUN: 60 mg/dL, Creatinine: 1.5 mg/dL

Ratio: 60 ÷ 1.5 = 40:1 (HIGH)

Interpretation: Likely pre-renal azotemia (dehydration). Rehydrate and recheck.

Example 2: Chronic Kidney Disease

BUN: 60 mg/dL, Creatinine: 3.0 mg/dL

Ratio: 60 ÷ 3.0 = 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: 55 ÷ 1.2 = 46:1 (VERY HIGH)

Interpretation: Check for melena (black stool) or vomiting blood.

What to Do If Your Cat's BUN Is Elevated

Step 1: Determine the Cause

• Check creatinine and SDMA (both elevated = kidney disease)
• Calculate BUN:creatinine ratio (high ratio = dehydration/GI bleeding)
• Perform urinalysis (urine specific gravity shows hydration/concentration ability)
• Check phosphorus (elevated in kidney disease)
• Assess hydration status (gum moisture, skin turgor, PCV/TP)

Step 2: Treatment Based on Cause

For Dehydration:

• IV or subcutaneous fluids
• Recheck BUN in 24-48 hours (should normalize)
• Address underlying cause (vomiting, diarrhea, heat exposure)

For Chronic Kidney Disease:

• Kidney diet (low protein, low phosphorus)
• Phosphorus binders if elevated
• Subcutaneous fluids at home
• Blood pressure monitoring/control
• Regular monitoring every 3-6 months

For Urinary Obstruction:

• EMERGENCY: Immediate catheterization
• IV fluids and monitoring
• Electrolyte correction (potassium)
• Pain management

Can BUN Go Back to Normal?

Yes, in many cases:

Dehydration: BUN typically normalizes within 24-48 hours of rehydration
Urinary obstruction: BUN decreases rapidly after catheterization if kidney damage isn't permanent
Acute kidney injury: May normalize if caught early and treated aggressively
Chronic kidney disease: BUN will NOT normalize but can be managed/stabilized with treatment
GI bleeding: BUN normalizes once bleeding stops

Key Point:

If BUN normalizes after rehydration, your cat likely doesn't have significant kidney disease. If BUN remains elevated despite hydration, chronic kidney disease is likely.

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 lethargic, weak, or hiding
• Your cat is straining to urinate or not producing urine (EMERGENCY)
• Bad breath (ammonia/urine smell)
• Seizures or disorientation (severe uremia)

Get Your Cat's BUN Results Explained

Upload your cat's bloodwork into VetLens and get:

✓ BUN interpretation with severity assessment
✓ BUN:creatinine ratio calculation and analysis
✓ Comparison with SDMA, phosphorus, and hydration markers
✓ Differentiation between dehydration and kidney disease
✓ Treatment questions to discuss with your vet

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Frequently Asked Questions

What is a normal BUN level for cats?

Normal BUN levels in cats typically range from 10-30 mg/dL. Values between 30-50 mg/dL are mildly elevated, 50-100 mg/dL are moderately elevated, and above 100 mg/dL are severely elevated and indicate significant kidney dysfunction.

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 pre-renal causes like dehydration.

What BUN level is dangerously high in cats?

BUN levels above 80-100 mg/dL are considered dangerously high and often indicate severe kidney disease or acute kidney injury. Values above 120 mg/dL typically require immediate veterinary intervention and possible hospitalization.

Can dehydration cause high BUN in cats?

Yes. Dehydration is a common cause of elevated BUN without true kidney disease. With dehydration, BUN rises more than creatinine (high BUN:creatinine ratio). Rehydration usually brings BUN back to normal if kidneys are healthy.