Kidney function tests | Description | Prevention | Causes and symptoms | Treatment
Definition
Kidney function tests are a variety of tests designed to test how well the kidneys are functioning.
Purpose
Kidney function tests are an important diagnostic tool for cases in which kidney disease is suspected. Some tests provide information only about how well the kidneys are functioning, while others offer some insight into the possible causes of the problem. Kidney function tests can be used to monitor individuals who are believed to be at high risk for developing kidney disease, such as individuals with diabetes. For these individuals one or more kidney function tests may be part of regular heath screenings. Early diagnosis and intervention can help drastically slow the advance of kidney disease. Kidney function tests can also be used to help monitor the progression of the disease and the success or failure of treatment in individuals already experiencing kidney disease.
Precautions
There are a variety of medications, as well as some food and drinks, that can interfere with the results of certain kidney function tests. In some cases strenuous exercise before the test may also cause inaccurate results. Individuals should inform their doctor of all medications, vitamins , and supplements, including herbs, that they are taking to help ensure the accuracy of the test results.
Description
Each healthy individual has two kidneys located towards the back of the body, slightly below the rib cage. They are shaped like beans, and each one is about the size of an adult's fist. The kidneys filter out waste products and extra water from the blood so that it can be excreted as urine. Healthy kidneys filter about 200 quarts of blood every day, and remove about 2 quarts of waste each day. When the kidneys are damaged they can no longer filter properly, and stop filtering out some things that should be filtered out, while accidentally filtering out some things, like protein, that should not be filtered out. When the kidneys stop filtering properly waste products can building up in the blood to toxic levels, eventually causing death . Significant health problems occur when the kidneys function at about one quarter of capacity, and kidney function below one-tenth of normal can be fatal without intervention in the form of dialysis or kidney transplant.
There are a variety of kidney function tests. Each measures levels of different substances filtered by the kidneys, and can provide different information about the level of kidney functioning and what may be the cause of any problems. The tests fall into two broad categories: blood tests and urine tests.
Blood Tests
· Blood Urea Nitrogen (BUN)-BUN is the urea nitrogen in the blood. Urea is produced when protein ingested as food is broken down by the body. As kidney function decreases the kidney's ability to filter out the urea nitrogen decreases, leading to increased levels in the blood.
· Serum Creatinine-This test measures the levels of creatinine, a product of the break down of creatine. Creatine is found in muscle tissue, and can accumulate in the blood through the normal breakdown associated with wear-and tear on the muscles, or through consuming animal protein. When the kidneys are not working well they cannot filter creatinine as effectively, leading to elevated levels in the blood.
Urine Tests
· Creatinine Clearance-This test is usually done using urine collected over a full 24 hours. It compares the urine creatinine levels with blood creatinine levels, so a blood draw is also required. Creatinine clearance provides information about the rate at which the kidneys are filtering blood.
· Urine Protein-This test is often done during a normal urinalysis as a first line test for kidney problems. Normally, the kidneys do not filter protein out into the urine. If, however, the kidneys are damaged, protein molecules can begin to slip through and are passed out of the body in the urine. Testing the urine for the presence of protein is a quick, inexpensive test, and is often done even for patients with no symptoms of kidney disease.
Preparation
A variety of medications, food, and beverages may interfere with the results of certain kidney function tests. Therefore the individual may be asked to stop taking certain medications for one or more days before the samples are collected. Depending on the type of test the individual may be asked to refrain from drinking any fluids after a certain time, or may be asked to drink extra fluids. The individual may be advised to avoid certain types of food, drinks, or exercise before the samples are collected.
Aftercare
No aftercare is generally required for kidney function tests.
Complications
No complications are generally expected from kidney function tests. Anytime blood is drawn there is a small risk of excessive bleeding, bruising , swelling, or infection at the puncture site.
Results
The results of kidney function tests can help the doctor determine the level of kidney functioning, the degree of disease, if present, and in some cases give information about the cause of the kidney problems. In general information about the results of kidney function tests is combined with a physical examination, health history , and other diagnostic tests to make a diagnosis
Blood Tests
· Blood Urea Nitrogen (BUN)-Normal levels of BUN are generally between 7 and 20 mg/dL, although normal levels can depend on the laboratory doing the testing. Higher levels of BUN indicate lower levels of kidney function.
· Serum Creatinine-Normal results are generally from 0.6 to 1.4 mg/dL. Women often have lower levels than men because women generally have a lower level of body musculature than men. Higher than normal levels indicate kidney disease, and the higher the level of creatinine the more progressed the disease.
Urine Tests
· Creatinine Clearance-The normal rate for men is 97 to 137 ml/minute. The normal range for women is 88 to 128 ml/minute. This rate has been found to decline with age, so normal functioning for seniors is generally less than that for younger adults. Normal rates must also be adjusted for body size, and can vary depending on the laboratory in which the tests are done. Lower than normal rates indicate decreased kidney function.
· Urine Protein-Protein in the urine above very trace amounts is considered abnormal. In addition to kidney disease, stress, exercise, and some medications can also sometimes cause protein in the urine. Therefore a positive test for urine protein is often repeated or followed up to with other function tests to determine if kidney disease is actually present.
Caregiver concerns
A doctor determines the need for kidney function tests based on symptoms, risk factors, a physical examination, and the patient's health history. After the doctor orders the test the patient will have the test explained to him or her by the doctor or a nurse familiar with the test procedure. He or she will provide the patient with information about what specific preparations, if any, are required for the test. If urine collection is needed, the patient collects the urine him or herself, unless unable to do so. If the patient is confined to bed the sample may be collected by a nurse using a catheter. If a blood sample is required it is drawn by a phlebotomist, an individual specially trained in drawing blood.
The blood and urine samples are labeled with the patient's information, then stored by the nurse as necessary to maintain their integrity. They are then sent to a laboratory for analysis. At the laboratory a laboratory technician performs a variety of chemical tests and examines the samples under a microscope as needed. The results of the tests are then summarized and returned to the doctor who ordered the test. The doctor or doctor's representative then shares the results with the patients, and the doctor determines what, if any, further diagnostic tests or treatment is required.
QUESTIONS TO ASK YOUR DOCTOR
· Are there any medicines I should stop taking before the test?
· Should I continue to eat and drink normally before the test?
· If the tests show abnormal results, what is the next step?
Resources
BOOKS
Cortes, Pedor, and Carl Erik Mogensen, eds. The Diabetic Kidney. Totowa, NJ: Humana Press, 2006.
Fischbach, Frances Talaska, and Marshall Barnet Dunnin III. A Manual of Laboratory and DiagnosticTests.
Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2009.
Li, Wei, David Frierman, and Ben Luna. Diseases of the Kidney and Bladder: Diagnosis and Treatment with Chinese Medicine. Boulder: Blue Poppy Press, 2006.
Nunez, Juan F. Macias, J. Stewart Cameron, and Dimitrios G. Oreopolous, eds. The Aging Kidney in Health and Disease. New York: Springer, 2007.
PERIODICALS
Collier-Ramirez, Shelly, and Paris Roach. “Monitor Your Kidney Function.” Diabetes Forecast 60.7 (June 2007):12.
Marsik, C., et al. “Classification of Chronic Kidney Disease by Estimated Glomerular Filtration Rate.”European Journal of Clinical Investigation 38.4 (April 2008): 253-260.
ORGANIZATIONS
American Clinical Laboratory Association, 1250 H Street, Suite 880, Washington, DC, 20005, (202) 637-9466, (202) 637-2050, info@clinical-labs.org, www.clinical-labs.org.
National Kidney Foundation, 30 East 33rd Street, New York, NY, 10016, (212) 889-2210, (800) 622-9010, (212) 689-9261, www.kidney.org.
Robert Bockstiegel
