Creatinine constitutes what percent of NPNs?

• A. 3%
• B. 5%
• C. 7%
• D. 10%

Answer: (B)

Non-protein nitrogen in serum is made up of several nitrogen-containing waste products, with urea forming the largest share. Creatinine is a byproduct of muscle metabolism that is excreted by the kidneys, and its proportion relative to the other NPN compounds is small and fairly constant. In standard clinical chemistry, creatinine contributes about 5% of the total NPN. This is why 5% is the best choice. The other options describe smaller or larger shares that aren’t typically used in this context.