Renal Function Tests
Analysis of blood and urine samples
can be essential for the evaluation of kidney (renal)
function. The following are some of the basic renal
urea nitrogen (BUN) provides a rough measurement
of the glomerular filtration rate, the rate at
which blood is filtered in the kidneys. Urea is
formed in the liver as an end product of protein
metabolism and is carried to the kidneys for excretion.
Nearly all kidney diseases cause inadequate excretion
of urea, elevating BUN levels in the blood. (Other
causes of high BUN levels include gastrointestinal
bleeding and steroid treatment.)
is a breakdown product of creatine, an important
component of muscle. The production of creatinine
depends on muscle mass, which varies very little.
Creatinine is excreted exclusively by the kidneys,
and its level in the blood is proportional to the
glomerular filtration rate. The serum creatinine
level (serum is the clear liquid that remains after
whole blood has clotted) provides a more sensitive
test of kidney function than BUN because kidney
impairment is the most common cause of elevated
clearance rate determines how efficiently the kidneys
are clearing creatinine from the blood and serves
as an estimate of kidney function. For this test,
urine and serum levels of creatinine are measured,
as well as the volume of urine excreted over a
24-hour period. The creatinine clearance rate is
then calculated and expressed as the volume of
blood, in milliliters, that can be cleared of creatinine
in one minute. A low creatinine clearance value
indicates abnormal kidney function.
Purpose of the Test
evaluate kidney function and aid in the diagnosis
of kidney disease.
monitor the progression of renal insufficiency.
BUN-to-creatinine ratio may aid in the evaluation
of a person's state of hydration.
Who Performs It
doctor, a nurse, or a lab technician draws the
diet rich in meats can cause transient elevations
of serum creatinine and creatinine clearance.
high-protein diet or dehydration elevates BUN levels.
may increase creatinine clearance.
medications may affect BUN levels, serum creatinine,
and creatinine clearance.
Before the Test
sure to inform your doctor of all medications,
herbs, or supplements you are taking. You may be
advised to discontinue some of these agents before
not eat an excessive amount of meat before the
creatinine clearance test, and avoid strenuous
physical exercise during the urine collection period
for this test.
What You Experience
sample of your blood is drawn from a vein, usually
in your arm, and sent to a laboratory for analysis.
perform the creatinine clearance test, timed urine
specimens are collected in a special container
over a 24-hour period.
Risks and Complications
After the Test
may leave the testing facility.
your normal diet and any medications withheld before
the test, according to your doctor's instructions.
may collect and clot under the skin (hematoma)
at the puncture site; this is harmless and will
resolve on its own. For a large hematoma that causes
swelling and discomfort, apply ice initially; after
24 hours, use warm, moist compresses to help dissolve
the clotted blood.
blood and urine samples are sent to a laboratory
for analysis. A physician will review the results
of the tests for any evidence of kidney disease
or other abnormalities.
and urine tests are usually the first step in assessing
potential kidney disorders.
results often necessitate additional imaging tests,
such as a renal ultrasound or nuclear scan, to
evaluate kidney structure and function.
an abnormality is found and the doctor can make
a definitive diagnosis, appropriate treatment will
Johns Hopkins Consumer Guide to Medical Tests. You
can order this
book now on our secure server.