What is the main difference between BUN and creatinine in assessing renal function?

• A. Creatinine is solely influenced by hydration status.
• B. BUN is a direct measure of GFR.
• C. Creatinine directly reflects GFR and muscle mass; BUN is influenced by hydration, protein intake, and catabolic state.
• D. BUN/creatinine ratio is not useful in distinguishing prerenal from intrinsic renal disease.

Answer: (C)

Creatinine provides a closer estimate of how well the kidneys are filtering, while BUN reflects factors outside filtration. Creatinine is produced at a relatively steady rate from muscle and is filtered by the glomeruli with little reabsorption, so its blood level rises mainly when GFR drops. BUN, formed from protein breakdown in the liver, is influenced by hydration status, protein intake, and catabolic state; in dehydration or high protein metabolism, BUN can rise even if the GFR hasn’t changed much. That makes BUN less specific for GFR and explains why the two markers behave differently. The BUN/creatinine ratio can help distinguish prerenal from intrinsic renal issues, but the core difference is that creatinine more directly reflects GFR, whereas BUN varies with non-renal factors.