Creatinine and BUN are not increased until GFR decreases by about what percentage?

• A. 25%
• B. 50%
• C. 75%
• D. 90%

Answer: (B)

The kidneys have a large functional reserve, so they can maintain normal filtration even after losing a substantial portion of nephrons. Creatinine is produced at a relatively constant rate and is cleared by the kidney; as long as remaining nephrons can hyperfiltrate to compensate, the serum creatinine stays near normal. This compensation holds until about half of the overall GFR is lost, at which point the rise in creatinine becomes noticeable. BUN behaves similarly in relation to overall filtration, though it can be influenced by factors like protein intake and hydration. So, when GFR declines by roughly 50%, both creatinine and BUN typically begin to rise, making 50% the threshold at which these markers become increased.