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Operator’s Manual – OPTI CCA-TS2 BUN-1
ANALYTES BUN/UREA
BUN (Urea)
Clinical Significance
1
Urea is produced in the liver as a by-product from the breakdown of amino acids. These are transaminated
and deaminated to ammonia, which is a toxin. Detoxification of ammonia occurs in the urea cycle where
two molecules of ammonia are joined to a molecule of carbon dioxide to form urea.
On an average protein diet, urinary excretion expressed as urea nitrogen is 12 to 20 g/day.
2
Abnormal Levels
The blood urea reflects the balance between production and excretion.
Causes of high blood urea levels (> 7.1 mmol/L urea, 20 mg/dl BUN).
These may result from increased production or decreased excretion. Causes of increased production
include a high protein intake, gastrointestinal bleeding with absorption of amino acids and peptides,
or increased tissue breakdown which may be due to serious illness, trauma or certain drugs such as
tetracyclines and glucocorticoids. Decreased excretion is associated with a low glomerular filtration
rate (GFR). This can be due to a number of reasons, which can be classified as pre-renal uraemia due to
dehydration, renal uraemia due to intrinsic failure in the kidney or postrenal uraemia due to an obstruction
to urine outflow.
Causes of low blood urea levels (< 2.1 mmol/L urea, 6 mg/dL BUN).
These are less common than high levels and can be due to decreased production or increased excretion.
Decreased production can be due to ingestion of a low protein diet, very severe liver failure and, in infants
only, inborn errors of the urea cycle. Increased secretion is due to an increased GFR. This can be due to
over-enthusiastic infusion of intravenous fluids, inappropriate ADH secretion or pregnancy.
Measurement Principle
The BUN (urea) optode measurement is based on the enzymatic hydrolysis of urea by the enzyme urease.
Urea + H
2
O + 2H
+
2 NH
4
+
+ CO
2
Urease
The ammonium ions are measured by an ammonium-selective fluorescence-based optical sensor (optode).
The amount of urea present is proportional to the ammonium concentration detected.
Measurement Range
Range Resolution (Low/High) Units
2.8 to 112.0 0.1/0.1 mg/dL
1 to 40 0.01/0.01 mmol/L