Analysis of blood and urine samples can be essential for the evaluation of kidney (renal) function. The following are some of the basic renal function tests.
Blood 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.) It can be done to determine the amount of urea nitrogen in the blood.
Creatinine 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 almost the only cause of elevated creatinine. It can also be measured with a urine test.
Creatinine clearance rate determines how efficiently the kidneys are clearing creatinine from the blood and serves as an estimate of kidney function. For renal function 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 1 minute. A low creatinine clearance value indicates abnormal kidney function. It requires both a urine and blood sample.
Purpose of the Renal Function Tests
- To evaluate kidney function and aid in the diagnosis of kidney disease.
- To monitor the progression of renal insufficiency.
- The BUN-to-creatinine ratio may aid in the evaluation of a person’s state of hydration.
Who Performs It
- A doctor, a nurse, or a lab technician draws the blood sample
- A diet rich in meats can cause transient elevations of serum creatinine and creatinine clearance.
- A high-protein diet or dehydration elevates BUN levels.
- Exercise may increase creatinine clearance.
- Some medications may affect BUN levels, serum creatinine, and creatinine clearance.
Before the Renal Function Tests
- Be sure to inform your doctor of all medications, herbs, or supplements you are taking. You may be advised to discontinue certain of these agents before the test.
- Do 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
- A sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.
- To perform the creatinine clearance test, timed urine specimens are collected in a special container over a 24-hour period.
Risks and Complications
After the Renal Function Tests
- You may leave the testing facility.
- Resume your normal diet and any medications withheld before the test, according to your doctor’s instructions.
- Blood 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.
- Your 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.
- Blood and urine tests are usually the first step in assessing potential kidney disorders. Abnormal results often necessitate additional imaging tests, such as a renal ultrasound or nuclear scan, to evaluate kidney structure and function.
- If an abnormality is found and the doctor can make a definitive diagnosis, appropriate treatment will begin.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media