Estimation of Urea in Serum and Urine
Urea is the major nitrogen-containing metabolite product of protein catabolism in humans, accounting for more than 75% of the nonprotein nitrogen eventually excreted. The biosynthesis of urea is carried out by hepatic enzymes of urea cycle. It is formed in the liver from carbon dioxide and ammonia, passes into the extracellular fluid, and is excreted almost entirely by the kidneys. The measurement of urea is an important investigation in diagnosing kidney damage. Urea is released into the blood which is then cleared by kidneys. The normal range of blood urea is 15–40 mg/dl. On an ordinary diet, urea nitrogen forms about 80–90% of the total urine nitrogen, but on low protein diet, it falls toward 60%. The total daily excretion of urea is about 30–40 g. Increased urea production occurs on high protein diets or after gastrointestinal hemorrhage and when there is increased tissue breakdown as observed in starvation, trauma, and inflammation. The capacity of the normal kidney to excrete urea is high, and in the presence of normal renal functions, urea levels rarely rise above normal despite increased production. A plasma urea concentration above 15 mmol/L almost certainly indicates renal impairment. The plasma urea is the most useful test of “renal excretory function,” as it correlates well with the clinical consequences of retained metabolic products (uremia) in renal insufficiency.