Which non-protein nitrogen does not measure kidney function?

• A. Ammonia
• B. Urea
• C. Creatinine
• D. BUN

Answer: (A)

Ammonia is not used to assess kidney function because it mainly reflects liver metabolism and gut production rather than renal clearance. Ammonia is produced from amino acid breakdown and is rapidly detoxified in the liver via the urea cycle; its blood level can be influenced by liver disease, dietary intake, and gut bacteria, making it an unreliable indicator of how well the kidneys are filtering. In contrast, urea (measured as blood urea nitrogen) and creatinine are cleared by the kidneys in a way that mirrors glomerular filtration rate, so their blood levels are routinely used to evaluate renal function. Thus, the non-protein nitrogen that does not measure kidney function is ammonia.