Summary
Highlights
Urea is the major excretory product of protein metabolism, formed in the liver from nitrogen released during amino acid catabolism. It is used to evaluate renal function, assess hydration status, determine nitrogen balance, diagnose renal disease, and verify dialysis adequacy. Analytical methods commonly couple the urease reaction with glutamate dehydrogenase.
Nonprotein nitrogen (NPN) is traditionally used to monitor renal function. The term originated from the need to remove proteins from samples before analysis. NPN comprises about 15 compounds, including urea, uric acid, creatinine, and ammonia, which are crucial for clinical interest.
Uric acid is the end product of purine metabolism, formed in the liver. It's measured to confirm and monitor gout, prevent uric acid nephropathy during chemotherapy, assess inherited disorders of purine metabolism, detect kidney dysfunction, and diagnose renal calculi. Enzymatic methods using uricase are widely used.
Creatine is synthesized in the liver and transported to muscles, where it becomes creatine phosphate—a short-term energy store. Creatinine is released into circulation at a constant rate proportional to muscle mass. Plasma creatinine concentration, creatinine clearance, and GFR (glomerular filtration rate) are used to gauge renal function. Measured using the Jaffé reaction or kinetic Jaffé reaction, while creatine is measured by HPLC or an endpoint Jaffé method.
Ammonia is produced from amino acid catabolism and bacterial metabolism in the intestines, then excreted as ammonium ion by the kidneys. Clinical conditions related to ammonia include hepatic failure, Reye's syndrome, and inherited urea cycle enzyme deficiencies. High ammonia concentrations are neurotoxic and associated with encephalopathy. Ammonia is measured using enzymatic methods with glutamate dehydrogenase.