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.
Risks
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.
Methods:
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 Results:
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)
Abnormal
Results:
Higher values of
BUN indicates:
·
Kidneys
are not working well
·
Nephrotoxicity
·
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
·
Malnutrition
·
Over-hydration
·
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
|
Non-pregnant Adult
|
1st
Trimester
|
2nd
Trimester
|
3rd
Trimester
|
mg/dL
|
7 -20
|
7 -12
|
3 - 13
|
3 -11
|
mmol/L
|
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
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