Which statement about BUN and creatinine as markers of kidney function is correct?

• A. BUN is more specific than creatinine
• B. Creatinine is a more specific marker of GFR; BUN is influenced by protein intake and volume status; both can be affected by non-renal factors; eGFR is preferred for staging
• C. eGFR is not used for staging CKD
• D. Both BUN and creatinine are unaffected by diet or muscle mass

Answer: (B)

Creatinine is a more specific reflection of GFR because it is produced at a relatively constant rate and cleared mainly by filtration, so its serum level tracks how well the kidneys are filtering. BUN, on the other hand, rises not only with reduced filtration but also with factors like protein intake and hydration status, so its levels can change for reasons outside kidney function. Both markers can be influenced by non-renal factors—muscle mass and age affect creatinine; liver function, diet, and volume status affect BUN. To stage CKD accurately, clinicians use estimated GFR, which incorporates creatinine (along with age, sex, and race) to provide a standardized measure of filtration. Hence, the statement that creatinine is a more specific marker of GFR; BUN is influenced by protein intake and volume status; both can be affected by non-renal factors; and eGFR is preferred for staging, is correct.