The Blood Urea Nitrogen (BUN) test is a common blood test that measures the amount of urea nitrogen in your bloodstream. Urea is a normal metabolic waste product made when your body breaks down protein from foods and from normal tissue turnover. Your liver converts ammonia (a toxic byproduct) into urea, and your kidneys filter urea out of the blood and remove it in urine.

Because urea is cleared mainly through the kidneys, BUN is often used as a general indicator of kidney function and kidney filtration. In simple terms:

Higher-than-normal BUN can mean your kidneys are not filtering waste efficiently, but it can also rise for other reasons such as dehydration, a high-protein diet, certain medications, or bleeding in the digestive tract. Lower-than-normal BUN is less common and may be seen with low protein intake, overhydration, or sometimes severe liver problems (since the liver makes urea).

BUN is usually interpreted alongside other labs—especially creatinine and the BUN-to-creatinine ratio—to help your clinician understand whether a result is more likely related to kidney disease (renal health), hydration status, or other causes. Very high levels can be associated with uremia, a buildup of waste products that can cause symptoms and may require urgent evaluation.

Clinicians commonly order a BUN test to evaluate renal health, check for possible kidney disease, and monitor how well the kidneys are clearing waste. It may be part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP), and it’s frequently used as a general diagnostic test in both routine and urgent settings.

A BUN test may be ordered if you have symptoms or signs that could suggest reduced kidney function or fluid imbalance, such as:

• Swelling in the legs, ankles, feet, or around the eyes
• Changes in urination (less urine, very frequent urination, foamy urine, or dark urine)
• Fatigue, weakness, nausea, vomiting, or poor appetite
• Shortness of breath or unexplained weight gain from fluid retention
• Confusion, itching, or a “metallic” taste in the mouth (possible signs of uremia when severe)

It’s also commonly used to help evaluate or monitor:

• Dehydration (for example from vomiting, diarrhea, fever, heavy sweating, or poor fluid intake)
• High blood pressure or diabetes (both increase long-term risk for kidney disease)
• Use of medications that can affect kidney filtration (such as some diuretics, ACE inhibitors/ARBs, certain antibiotics, and NSAIDs like ibuprofen—your clinician will decide what’s relevant)
• Possible gastrointestinal bleeding (digested blood can raise BUN)
• Hospital or emergency evaluation for acute illness where kidney function may change quickly

Because BUN can be influenced by hydration, protein intake, and other factors, your provider will interpret results in context and may repeat testing or add related labs (creatinine, eGFR, electrolytes, urinalysis) to clarify the cause.

• Chronic kidney disease (CKD)
• Acute kidney injury (AKI)
• Dehydration (low body water/volume depletion)
• Urinary tract obstruction (e.g., kidney stones, enlarged prostate causing blockage)
• Congestive heart failure or reduced blood flow to the kidneys
• Gastrointestinal bleeding (upper GI bleed)
• Severe infection or sepsis (can reduce kidney perfusion and function)
• Liver disease (may lower BUN due to reduced urea production)

• Monitoring kidney function and overall renal health over time
• Early detection and management of kidney disease risk in people with diabetes, hypertension, or a family history of kidney problems
• Checking hydration status and guiding safe fluid management during illness, exercise, or recovery
• Medication safety monitoring (ensuring drugs and doses are appropriate for current kidney filtration)
• General wellness screening when included in routine panels (BMP/CMP) alongside electrolytes and creatinine
• Tracking recovery and complications after surgery, hospitalization, or serious infection