Blood Urea Nitrogen (BUN)

Kidney

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Fasting Required

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Blood urea nitrogen test; evaluates kidney function and protein metabolism.

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Blood Urea Nitrogen (BUN) Test Guide

Why is it done?
- Test Purpose: Measures the amount of urea nitrogen in the blood to assess kidney function and protein metabolism. Urea is produced by the liver when proteins are broken down and normally filtered by the kidneys.
- Primary Indications: Evaluate kidney function and glomerular filtration rate (GFR); assess for acute or chronic kidney disease; diagnose urinary tract obstruction; evaluate dehydration and fluid balance; monitor patients on dialysis; assess liver function and protein catabolism; evaluate suspected poisoning or toxemia during pregnancy
- Typical Timing/Circumstances: Part of routine metabolic panel (CMP) during annual physical exams; ordered when kidney disease is suspected; during acute illness or hospitalization; when monitoring chronic kidney disease; prior to major surgery or medication initiation; during prenatal screening; when evaluating symptoms of uremia or kidney dysfunction
Normal Range
- Reference Values: Adults (19-70 years): 7-20 mg/dL (2.5-7.1 mmol/L) • Older Adults (>70 years): 8-23 mg/dL • Children: 5-18 mg/dL • Infants: 5-18 mg/dL Note: Reference ranges may vary slightly between laboratories; consult your specific lab's reference range
- Units of Measurement: mg/dL (milligrams per deciliter) in the United States; mmol/L (millimoles per liter) in international units
- Result Interpretation: Normal: Within the reference range indicates adequate kidney function and proper protein metabolism • Low BUN: Below 7 mg/dL (rare finding); may indicate liver disease, malnutrition, or over-hydration • High BUN: Above 20 mg/dL suggests possible kidney dysfunction, dehydration, high protein intake, or increased protein breakdown
- Clinical Significance: Normal BUN indicates the kidneys are effectively filtering metabolic waste; Abnormal values require correlation with other kidney function tests (creatinine, eGFR) and clinical presentation for accurate diagnosis
Interpretation
- Elevated BUN (>20 mg/dL): Most commonly indicates impaired kidney function; reflects decreased glomerular filtration; may suggest acute kidney injury (AKI) or chronic kidney disease (CKD); can indicate dehydration, urinary tract obstruction, or shock; elevated protein catabolism from burns, infections, or sepsis; may reflect high protein diet or gastrointestinal bleeding
- Low BUN (<7 mg/dL): Uncommon finding; may indicate severe liver disease or cirrhosis; can reflect malnutrition or starvation; over-hydration or excessive fluid administration; pregnancy (normal physiologic change); use of certain medications; increased urine output
- BUN/Creatinine Ratio: Normal ratio is approximately 10:1 to 20:1; elevated ratio (>20:1) suggests prerenal causes (dehydration, shock, congestive heart failure); lower ratio (<10:1) may indicate kidney disease or post-renal obstruction; helps differentiate cause of elevated BUN
- Factors Affecting Results: Age (increases with age); gender (slightly higher in males); dehydration status; protein intake; medications (aminoglycosides, NSAIDs, ACE inhibitors); muscle mass; liver function; recent exercise or stress; time of day (slight diurnal variation); recent meal or fasting state
- Clinical Patterns: Acute elevation with normal creatinine suggests dehydration or prerenal insufficiency; Progressive elevation with rising creatinine indicates progressive kidney disease; Isolated elevated BUN with low creatinine may indicate malnutrition or liver disease; Disproportionate elevation with mild creatinine increase suggests extrarenal cause
Associated Organs
- Primary Organ Systems: Kidneys (primary focus - urea filtration and excretion); Liver (production of urea from protein metabolism); Cardiovascular system (affects renal perfusion and function)
- Kidney-Related Diseases: Acute Kidney Injury (AKI); Chronic Kidney Disease (CKD) stages 1-5; Diabetic Nephropathy; Glomerulonephritis; Pyelonephritis; Polycystic Kidney Disease; Renal Artery Stenosis; Nephrotic Syndrome; Acute Tubular Necrosis (ATN)
- Cardiovascular/Hemodynamic Conditions: Congestive Heart Failure; Myocardial Infarction; Hypovolemic Shock; Septic Shock; Severe Dehydration; Renal Artery Stenosis; Aortic Stenosis
- Hepatic/Metabolic Conditions: Cirrhosis; Acute Liver Failure; Hepatitis; Portal Hypertension; Severe Malnutrition; Starvation
- Other Associated Conditions: Gastrointestinal bleeding; Urinary tract obstruction (stones, tumors, strictures); Prostate hyperplasia; Severe infections/Sepsis; Burns; Trauma; Medication toxicity (aminoglycosides, NSAIDs); Diabetes Mellitus; Hypertension; Preeclampsia/Eclampsia
- Potential Complications: Uremia (toxicity from elevated urea); Fluid overload and pulmonary edema; Hyperkalemia and cardiac arrhythmias; Metabolic acidosis; Bone disease (osteodystrophy); Anemia; Cardiovascular disease progression; Infection; Death if untreated
Follow-up Tests
- Recommended Follow-up Tests (if BUN is abnormal): Serum Creatinine (essential for kidney function assessment); Estimated Glomerular Filtration Rate (eGFR) calculated from creatinine and age; Cystatin C (alternative filtration marker); Urinalysis with microscopy; 24-hour urine protein collection; Urine creatinine; Electrolyte panel (sodium, potassium, chloride, bicarbonate)
- Advanced Imaging/Procedures: Ultrasound of kidneys (assess size, obstruction); CT scan of abdomen/pelvis (evaluate obstruction, kidney structure); Renal doppler ultrasound (assess renal artery stenosis); Renal biopsy (if glomerulonephritis suspected); Voiding cystourethrography (if obstruction suspected)
- Complementary Blood Tests: Phosphorus; Calcium; Albumin; Liver function tests (AST, ALT, bilirubin); Hemoglobin/Hematocrit (assess for anemia); Blood glucose; Thyroid function (TSH); Parathyroid hormone (PTH); Complete metabolic panel (CMP)
- Monitoring Frequency: Acute Kidney Injury: Daily or every 2-3 days until stabilized; Chronic Kidney Disease Stage 1-3: Every 3-6 months; CKD Stage 4: Every 1-3 months; CKD Stage 5 (on dialysis): Monthly or as indicated; Post-treatment monitoring: As directed by nephrologist; Routine health screening: Annually or as recommended by physician
- Tests for Differential Diagnosis: If prerenal cause suspected: Fluid status assessment, vital signs, central venous pressure; If renal cause: Kidney ultrasound, renal biopsy; If post-renal cause: Bladder scan, post-void residual, renal ultrasound; If hepatic cause: Liver function tests, PT/INR, albumin, hepatitis serology; If sepsis/infection: Blood cultures, complete blood count, lactate, procalcitonin
Fasting Required?
- Fasting Requirement: No - Fasting is NOT required for BUN testing. The test can be performed at any time of day regardless of food or fluid intake.
- If Part of Comprehensive Metabolic Panel (CMP): If other tests in the panel require fasting (glucose testing), then 8-12 hour fast is recommended for the entire panel; Water intake is generally allowed; BUN itself is not affected by fasting status
- Medications to Avoid: Do NOT discontinue any regular medications unless specifically instructed by your physician; Inform laboratory technician of all medications and supplements; Certain medications may affect results (NSAIDs, ACE inhibitors, aminoglycosides, diuretics) but should not be stopped without medical guidance
- Pre-Test Preparation Instructions: Continue normal eating and drinking habits; Stay well-hydrated (normal fluid intake); Avoid excessive exercise or strenuous activity immediately before testing; Inform phlebotomist of current medications; Wear loose, comfortable clothing with accessible arms; Arrive on time and remain calm (stress can slightly elevate values); If possible, consistent timing (morning preferred) if repeat tests planned
- Special Considerations: Extreme dehydration may increase values; Recent high-protein meal may slightly elevate results; Heavy exercise or trauma may temporarily elevate BUN; Pregnancy may show lower values; Elderly patients may have higher baseline values; For accurate baseline assessment, repeat testing at similar time of day is recommended

How our test process works!