BUN Measures Urea, Not Uric Acid
Blood Urea Nitrogen (BUN) specifically measures urea nitrogen concentration in blood—it has nothing to do with uric acid (urate). 1
What BUN Actually Measures
BUN quantifies the nitrogen component of urea molecules circulating in blood, typically reported in mg/dL in the United States. 1
The relationship is straightforward: urea nitrogen = total urea ÷ 2.14, because nitrogen comprises approximately 46.7% of urea's molecular weight. 1
Urea is generated by the urea cycle enzymes, primarily in the liver, as a byproduct of protein metabolism and ammonia detoxification. 2
Why the Confusion Exists
The confusion likely stems from similar-sounding names, but these are completely different molecules:
Urea is a small nitrogen-containing waste product from protein metabolism, measured by BUN 1
Uric acid (urate) is a purine metabolism byproduct, measured separately and associated with gout—this is NOT what BUN measures 3
Clinical Context
BUN has been utilized to evaluate renal function for decades, though it reflects more than just kidney function alone—it's affected by protein intake, hydration status, liver function, and tissue catabolism. 1, 2
In dialysis patients, clinicians obtain predialysis BUN and postdialysis BUN samples to calculate urea clearance (Kt/V), which quantifies dialysis adequacy. 1
Proper sampling technique is critical to avoid dilution with saline or heparin, which artificially lowers BUN values. 4, 1
Common Pitfall to Avoid
Never confuse BUN with uric acid levels. If you're evaluating for gout, hyperuricemia, or tumor lysis syndrome, you need a separate uric acid test—BUN will not provide this information. 3