Estimation of Urea in Serum and Urine
16
16.1 Theory
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 extracel-
lularfluid, 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. It is higher in men than women and more in adults than young. The
urea content over periods is influenced by the amount of protein in the diet and tends
to be lower in people on low protein diet. 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.
16.2 Specimen Requirements
Serum/plasma or 24 h urine sample with preservative is used. Avoid high concen-
tration of sodiumfluoride during enzymatic estimation in serum. Serum sample is
stable up to 24 h at room temperature and for several days at 4C. Dilute serum
(1:20) and urine samples (1: 1000) before use.
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_16
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