Clinical background:
The formation of urea and its excretion by the kidneys represents the major route (c. 75%) by which nitrogen is removed from the body. It is an end product of metabolism and in the kidneys the contribution of urea to medullary hypertonicity is important in determining the kidney’s ability to concentrate urine.
Urea has traditionally been used as a renal marker but has limited value for this purpose when compared to creatinine. Plasma/serum levels depend on the rate of its formation and the rate of excretion. Although the major excretory route is the kidneys about 10% is lost in sweat and via the gut. A variable quantity of filtered urea is reabsorbed so that the amount excreted is not an accurate measurement of the GFR.
Circumstances related to renal disease where urea may be useful are;
*Suspected pre-renal failure e.g. due to fluid depletion or cardiac failure.
*In monitoring the effects of renal replacement treatment
*Predicting the severity of acute pancreatitis.
Suspected inherited metabolic disease (low levels seen in urea cycle disorders)
Interpretation of urea levels is made more difficult by it being influenced by 3 factors;
*Its rate of synthesis (reflecting protein turnover)
*The volume of distribution (total body water)
*Rate of its excretion (determined by rate of glomerular filtration and tubular reabsorption).
Specimen container paediatric:
Serum (SST or plain tube)
Specimen container adult:
Serum (SST or plain tube)
Minimum volume paediatric:
0.5 mL blood
Minimum volume adult:
1 mL blood
Sample stability :
Unseparated sample:
7 days
Separated sample:
3 days at 15 to 25°C,
7 days at 2 to 8°C,
6 months at -20°C
This does not apply to samples from patients on Rasburicase.
Reference ranges:
0 – 4w: 0.8 – 5.5 mmol/L
4w – 1y: 1.0 – 5.5 mmol/L
1y – 16y: 2.5 – 6.5 mmol/L
16y and over: 2.5 – 7.8 mmol/L
Other info:
Lithium heparin sample also acceptable