The amount of nitrogen in blood that comes from the waste product urea is measures by blood nitrogen urea. When proteins are filtered by liver then it gives urea nitrogen as a waste product. Urea is a end product of nitrogen metabolism.
2 Ammonia + CO2 + 3ATP ---> urea + water + 3 ADP
Kidneys filter urea nitrogen to the blood and to urine. If BUN test gives higher values than the normal then it means kidneys are not working properly.
Many of time BUN test is performed with creatinine in order to have more accurate results and is termed as BUN to creatinine ratio (BUN: Creatinine) Creatinine is a product of muscle creatinine metabolism.
Urea and Creatinine are small molecules of 60dalton and 113dalton respectively. These small units are widely distributed in the body fluids.
Why BUN is done?
You may need a blood urea nitrogen test:
Æ If doctor suspects that patient have kidney damage.
Æ If kidney function needs to be evaluated.
Æ For the determination of effectiveness of dialysis treatment if patient is receiving hemodialysis or peritoneal dialysis.
Æ For the monitoring of many drugs like aminoglycosides, amphotericin B etc.
How To Prepare for BUN test:
Before having a BUN test, do not eat a lot of meat or other protein in the 24 hours.
How It Is Done
A health professional can perform this test. Following are the steps for BUN test:
· First of all wrap an elastic band around the patient’s upper arm to stop the blood flow.
· Use alcohol to clean the needle site.
· Then, put the needle in vein.
· Attach tube with needle to fill blood in it.
· When enough blood is collected then remove the band form patient’s arm.
· Place cotton ball over the site of needle, when needle is removed and bandage it.
Although there is a little chance of risk during drawing blood from vein. Few of risks are:
· Patient may have some bruise at needle site.
· Phlebitis may also occur.
· People with bleeding disorders may suffer from ongoing bleeding.
There are two methods used for BUN test:
· Diacetyl or Fearon reaction
· Enzymatic method
1- Diacetyl or Fearon Reaction:
This test gives yellow chromogen with urea and it is quantified by using photometry. Now, it is modified for use in auto analyzers and generally it gives relatively accurate values of results. But it is still in limited use because of its less specificity. It is because of spurious elevations with sulfonylurea compounds and by colorimetric interference from hemoglobin of blood when whole blood is used for testing.
2- Enzymatic method:
The more specific method is enzymatic method. In this method , enzyme urease converts urea into ammonia and carbonic acid. Both of these products that are in proportional to the concentration of urea in the given sample are assayed in a no. of systems, few of them are automated. One of the system checks decrease in absorbance at 340 mm when the ammonia get reacts with alpha-ketoglutaric acid. The rate of increase in conductivity of the solution in which urea is hydrolyzed is measured by the Astra system.
Normal Range of Blood urea Nitrogen in :
Adults: 10-20 mg/dl (3.6-7.1mmol/L)
Children: 5-18 mg/dl (1.4-5.7mmol/L)
Higher values of BUN indicates:
· Kidneys are not working well
· Patient is dehydrated.
· High protein diet
· Patient may have bleeding into stomach or intestine
· Heart failure
· Patient may in shock from burns or an accident.
· Patient’s urinary tract is blocked due to kidney stone. tumor, or from an enlarged prostate in men.
· Patient is taking a medicine that affects your BUN level.
· High BUN-to-creatinine ratios can occur with due to acute kidney failure, which may be caused by shock or severe dehydration. A very high BUN-to-creatinine ratio may be occur due to bleeding in digestive tract or respiratory tract
Low levels of BUN indicates:
· Liver damage
· Low protein diet
· Low level of BUN to creatinine ration can be caused by rhabdomylosis, cirrhosis. pregnancy.
BUN in Pregnancy:
A low BUN level can occur normally in the second or third trimester of pregnancy
Urea nitrogen , BUN (serum)
Units for BUN
3 - 13
2.5 - 7.1
2.5 - 4.3
1.1 - 4.6
1.1 - 3.9
What Affects the Test
Result interpretations are affected by following:
· By the use of medicines like naficllin, amphotericin, tetracycline, corticosteroids etc
· By using diuretics which cause dehydration
· Protein intake
· Protein catabolism
· State of hydration
· Hepatic urea synthesis