Overview of Kidney Function Tests
You have two kidneys, each approximately the size of a human fist. They are located in the back of your abdomen, below the rib cage, one kidney on either side of your spine.
The kidneys play several vital roles in maintaining health. One of their most important jobs is to filter waste materials from the blood and expel them from the body as urine. The kidneys also help control the levels of water and various minerals in the body. In addition, they are critical to the production of:
- vitamin D
- red blood cells
- hormones that regulate blood pressure
If your doctor thinks your kidneys may not be working properly, you may need kidney function tests. These are simple blood and urine tests that can identify problems with your kidneys.
Kidney function tests may also be ordered if you have other conditions that can harm the kidneys, such as diabetes or hypertension (high blood pressure). They can be used to monitor these conditions.
Symptoms of Kidney Problems
A number of symptoms may indicate a problem with your kidneys. These include
- high blood pressure
- blood in urine
- frequent urges to urinate
- difficulty beginning urination
- painful urination
- swelling in the hands and feet due to a buildup of fluids in the body
A single symptom may not mean something serious. However, when occurring simultaneously, these symptoms suggest that your kidneys are not working properly. Kidney function tests can help determine the reason.
Types of Kidney Function Tests
To test your kidney function, your doctor will order a set of tests that can be used together to estimate your glomerular filtration rate (GFR). The GFR tells the doctor how quickly your kidneys are clearing waste from your body. Tests include the following.
A urinalysis screens for the presence of protein and blood in the urine. There are many possible causes of protein in your urine, not all of which are related to disease. Infection increases urine protein, but so does a heavy physical workout. Because of this, your doctor may want to repeat this test after a few weeks to see if the results are similar.
In addition to standard urine tests, your doctor may also ask you to provide a 24-hour urine sample (described below). This can be used to see how fast a waste product called creatinine is being cleared from your body. Creatinine is a breakdown product of muscle tissue.
Serum Creatinine Test
This blood test examines whether creatinine is building up in your blood. Creatinine is usually completely filtered from the blood by the kidneys. Therefore, a high level of creatinine suggests a kidney problem.
Normal lab readings in men are 0.7 to 1.3 mg/dL of blood. Normal lab readings for women are 0.6 to 1.1 mg/dL.
Blood Urea Nitrogen (BUN)
The BUN test also checks for waste products in your blood. BUN measures the amount of nitrogen in the blood. Urea nitrogen is a breakdown product of protein. However, not all elevated BUN tests are caused by kidney damage. Common medications, including large doses of aspirin and some types of antibiotics, can also increase your BUN. Therefore, it is important to tell your doctor about any medications or supplements that you take regularly. You may need to stop certain drugs for a few days before the test.
A normal BUN is approximately 6 to 20 mg/dL of blood. A higher value could imply several different health problems.
Estimated Glomerular Filtration Rate (GFR)
This test estimates how well your kidneys are filtering waste. The rate is calculated by taking several factors into account, such as:
- test results, specifically creatinine levels
Any result lower than 60 is a warning sign of kidney disease.
How the Tests Are Obtained
Kidney function tests usually require a 24-hour urine sample and a blood test.
24-Hour Urine Sample
A 24-hour urine sample is used for a creatinine clearance test. It gives your doctor an idea of how much creatinine your body expels over a single day.
On the day that you start the test, urinate into the toilet as you normally would when you wake up.
For the rest of the day and night, urinate into a special container provided by your doctor. Keep the container capped and refrigerated during the collection process. Make sure to clearly label the container and to tell other family members why it is in the refrigerator.
On the morning of the second day, urinate into the container when you get up. This completes the 24-hour collection process.
Cap and label the container, and follow your doctor’s instructions about where to drop it off. You may need to return it either to your doctor’s office or a laboratory.
Blood samples are collected by venipuncture for BUN and serum creatinine tests. These samples will probably be taken in a lab or doctor’s office.
The technician drawing the blood will tie an elastic band around your upper arm. This makes the veins stand out. The area over the vein will be cleaned. Then a hollow needle will be slipped through your skin and into the vein. The blood will flow back into a test tube that will be sent for analysis.
You may feel a sharp pinch or prick when the needle enters your arm. After the test is over, the technician will place gauze and a bandage over the puncture. The area around the puncture may develop a bruise over the next few days. However, you should not feel severe or long-term pain.
Treatment of Early Kidney Disease
If the tests show early kidney disease, your doctor will focus on treating the underlying condition. If it is hypertension, your doctor will prescribe medications to control the blood pressure. He or she will also suggest lifestyle and dietary modifications.
If you have diabetes, your doctor may want you to see an endocrinologist. This type of doctor specializes in metabolic diseases and can help ensure that you have the best blood-glucose control possible.
If there are other causes of your abnormal kidney function tests, such as kidney stones and excessive use of analgesics (pain killers), your doctor will take appropriate measures to manage those disorders.
If your tests are abnormal, you will probably need regular kidney function tests in the months ahead. These will help your doctor keep an eye on your condition.