Blood Urea Nitrogen (BUN): Definition and Clinical Significance
Core Definition
Blood urea nitrogen (BUN) is a laboratory test that specifically quantifies the concentration of urea nitrogen in blood, typically reported in milligrams per deciliter (mg/dL) in the United States. 1
Biochemical Basis
Urea nitrogen represents approximately 46.7% of urea's molecular weight, with the relationship expressed as: urea nitrogen = total urea ÷ 2.14 1
Urea is generated primarily in the liver through the urea cycle as a degradation product of protein metabolism 2
After hepatic production, urea is filtered by the kidneys, with 40-50% of filtered urea being reabsorbed in the proximal tubule, paralleling sodium and water reabsorption 3
Clinical Measurement Context
Standard Laboratory Reporting
BUN is the standard term used in clinical practice globally, particularly in North American medical systems 1
Some international laboratories may report total urea concentration instead, but the underlying measurement principle remains the same 1
Critical Sampling Considerations
In dialysis patients, proper timing and technique for BUN sampling is critical to avoid dilution with saline or heparin, which artificially lowers values. 1
For predialysis BUN: blood samples must be drawn before dialysis starts to prevent saline dilution or sampling after treatment has begun 4
For venous catheters: withdraw 10 mL of blood before sampling to ensure no heparin contamination 3
Both predialysis and postdialysis samples should be drawn during the same treatment session 4
Postdialysis BUN requires specific technique: slow blood flow to 100 mL/min for 15 seconds to avoid access recirculation, which causes overestimation of dialysis adequacy 4
Clinical Applications
Dialysis Adequacy Assessment
BUN measurements are used to calculate urea clearance (Kt/V), which quantifies dialysis adequacy in hemodialysis patients 1
BUN is required to compute protein catabolic rate (PCR) in dialysis patients, providing nutritional assessment 1
Confusing total urea with BUN leads to inaccurate PCR and Kt/V calculations 1
Renal Function Evaluation
BUN has been utilized to evaluate renal function for decades, though it reflects more than just kidney function alone 1, 2
BUN is affected by multiple factors including protein intake, hydration status, liver function, and tissue catabolism 1
BUN should not be used alone to monitor progression of renal failure, particularly in patients with diabetes 3
Prognostic Value Beyond Renal Function
In heart failure patients, BUN serves as a better predictor of outcome than creatinine or estimated GFR, reflecting congestion, fluid retention, and cardiac dysfunction. 3
BUN ≥20 mg/dL is a minor criterion for ICU admission in pneumonia patients and is incorporated into severity scoring systems (CURB-65) 3
In acute coronary syndromes, elevated BUN independently predicts mortality (HR 3.2 for BUN ≥25 mg/dL vs. ≤20 mg/dL) 3
Higher BUN levels are independently associated with adverse renal outcomes and progression to end-stage renal disease, even after adjusting for eGFR 3