What percentage of total NPN is urea?

• A. 40-50%
• B. 20-30%
• C. 60-70%
• D. 10-20%

Answer: (A)

Non-protein nitrogen (NPN) in blood is the pool of nitrogen-containing compounds not bound in proteins, and the largest contributor to this pool is urea. Urea is produced in the liver from ammonia and amino groups, then cleared by the kidneys, making it the major NPN species. Because of its prominent role, urea accounts for roughly two-fifths to one-half of the total NPN, typically cited as about 40–50%. The rest of the NPN comes from other compounds such as creatinine, uric acid, ammonia, and various amino compounds. That’s why urea nitrogen measurements (BUN) reflect a major portion of the NPN, though the exact percentage can vary with physiology and health status.