About the Test
Purpose of the test
The purpose of a creatinine clearance test is to assess kidney function. In particular, creatinine clearance is used to determine the estimated glomerular filtration rate (eGFR), which describes how well the kidneys are filtering the blood.
Measuring creatinine clearance to evaluate kidney filtration may be done for many different reasons. It can be an important tool for the diagnosis, screening, and monitoring of kidney disease.
Testing that happens after symptoms have occurred is part of making a diagnosis. Testing before symptoms have appeared is known as screening. Testing to observe or follow a condition is monitoring.
Diagnosis
A creatinine clearance test may be part of the diagnostic process if you have symptoms that are consistent with a possible kidney problem such as urinary changes, itching, loss of appetite, and swollen feet. Creatinine clearance testing may also help diagnose whether kidney problems are present in people with more general symptoms like fatigue.
Screening
Some people are at higher risk of kidney problems, including people with diabetes, high blood pressure, or a family history of kidney disease. For these patients, kidney function tests may be done as screening in order to try to detect kidney problems before any symptoms have occurred.
In many cases, initial screening is done with a creatinine blood test. Specialized formulas have been established to estimate the glomerular filtration rate based solely on blood levels of creatinine. A creatinine blood test is easier to conduct because it does not require a urine sample. However, a creatinine clearance test that includes a urine sample may be used if the blood test alone has an abnormal result or if individual patient factors make the blood creatinine measurement less reliable.
Monitoring
Creatinine clearance tests can be used to observe how kidney function changes over time. This type of monitoring may be useful in multiple situations:
- After kidney disease has been diagnosed, testing can help track how the disease progresses.
- Testing before and after treatment for kidney disease can assess how well the treatment is working.
- When a drug or treatment with side effects that can affect kidney function is prescribed, a creatinine clearance test can check for possible kidney damage or abnormal kidney function.
What does the test measure?
A creatinine clearance test measures the amount of creatinine in a sample of blood taken from your arm and in a sample of all the urine you produce during a 24-hour period.
Creatinine develops as a result of the breakdown of creatine, a chemical that helps provide the muscles with energy. The body consistently produces creatinine as a byproduct of normal activity, and the tiny filters in the kidneys, called glomeruli, work to remove the creatinine from the blood. Creatinine then can be carried out of the body in the urine.
A creatinine clearance test helps show how well this process is working by measuring the amount of creatinine in both the blood and urine. By applying a specific formula, these blood and urine measurements can estimate the amount of blood being filtered by the glomeruli in the kidneys, which is the glomerular filtration rate (GFR).
To calculate creatinine clearance, the creatinine level in the urine is multiplied by the total volume of urine produced over 24 hours. This is then divided by the amount of creatinine in the blood. The final value is converted to milliliters of blood per minute (mL/min).
Online calculators are available to apply this formula and determine creatinine clearance. These calculators often include adjustments to account for the fact that creatinine levels are affected by body size. The final unit of measurement for creatinine is typically milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m2).
When should I get a creatinine clearance test?
Like other kidney function tests, creatinine clearance may be measured if you are showing signs of possible kidney impairment. In these cases, it is a type of diagnostic test that can help to identify the cause of your symptoms.
Creatinine clearance may be tested if you are at risk of having kidney problems. For example, people who are going to start using medications that affect the kidneys may take this test. In addition, testing can be done to try to find kidney disease earlier in people with diabetes, high blood pressure, or a family history of kidney disease.
Other types of kidney function tests can be used in many of these situations, but creatinine clearance may be suggested if you have a diet that is very high or low in protein or if you have either very little or a lot of muscle mass. For people with these individual factors, a creatinine clearance test is usually more accurate than a test of creatinine in your blood alone.
Creatinine clearance can be used as a follow-up test if another type of kidney function test shows an abnormal result. In these cases, the combination of creatinine measurement in both blood and urine offers a method to confirm the findings of the initial test.
Finding a Creatinine Clearance Test
How to get tested
A creatinine clearance test is normally prescribed by a doctor. The blood sample is taken in a laboratory, hospital, or medical office. The urine sample requires collecting all of your urine during a 24-hour period, so at least part of this sample collection is often done at home. Both the blood and urine samples are analyzed by a lab to measure creatinine and calculate the creatinine clearance.
Can I take the test at home?
Limited or no options are available for at-home test kits of creatinine clearance.
Some at-home test kits measure creatinine in either the blood or the urine alone, but a full creatinine clearance test is generally not offered in commercial at-home tests.
With a standard creatinine clearance test, much of the urine sample is taken at home because the test requires that you collect all your urine for a full 24 hours.
How much does the test cost?
There is not a fixed cost for a creatinine clearance test. Instead, the cost can depend on factors like where the test samples are taken and whether you have health insurance coverage.
The cost of testing may be made up of several separate charges. These can include fees for having your blood drawn, costs of the special containers for urine collection, charges by the laboratory that analyzes the samples, and charges from your doctor for one or more office visits.
Health insurance frequently provides coverage for these costs if the creatinine clearance test is prescribed by your doctor. However, you may still have copays or deductible payments that you have to pay out of pocket. To get a more specific breakdown of expected costs, contact your health insurance company and/or your doctor’s office.
Taking a Creatinine Clearance Test
Two separate samples are required to conduct a creatinine clearance test. A sample of blood is taken from a vein in your arm in a medical office, laboratory draw site, or hospital. For a urine sample, you personally collect all of your urine in special containers or bags over a 24-hour period. The blood sample must be taken within 24 hours of the day that you collect your urine sample.
Before the test
To prepare for your creatinine clearance test, you may need to follow specific instructions regarding your diet. While you don’t normally need to fast, you may be told not to eat meat, which can affect creatinine levels, for 24 hours before the test.
Some medications and dietary supplements can change your creatinine levels. For that reason, you should tell your doctor about any drugs or supplements that you usually take, and your doctor can tell you if you need to temporarily stop using any of them before the test.
Before the urine collection, you will normally be advised to continue your typical daily fluid intake. In addition, it may help to review the instructions for collecting the urine sample with your doctor or nurse so that you can obtain the sample properly.
During the test
A creatinine clearance test has two different components, and what you can expect during the sample collection is different for each.
Blood Sample
A routine blood draw is typically how a blood sample is collected for a creatinine clearance test. A nurse or technician will have you sit in a chair and will tie an elastic band around your upper arm. This enhances blood flow in your lower arm and makes it easier to find a vein. They will use an antiseptic to clean the skin near the vein and then insert a needle into your arm to withdraw a vial of blood.
The entire blood draw normally takes less than a few minutes. You may find that the procedure causes some brief pain, including a possible stinging feeling when the needle is inserted and taken out of your arm.
24-Hour Urine Sample
In order to properly measure your urine creatinine levels, it’s important to know the total volume of urine that you produce. For that reason, you’ll be instructed to do a full 24-hour urine collection.
Your doctor will normally provide specific bags or containers for you to use during the test. In most cases, on the first day of the test you’ll start by waking up and urinating in the toilet and flushing. After that, you will collect all your urine in the provided containers. The following day, you’ll urinate into a container after waking up, and that will be the end of the 24-hour period for sample collection.
During the test, you should stay hydrated and make sure to drink water as you normally would. You may also be asked to keep notes about the times when you urinate.
After the test
After a blood draw, you may have some bruising or pain in your arm, but this rarely lasts for long. A bandage is usually placed over the puncture site to stop any bleeding once the needle is removed. You can generally return to normal activities after the blood test is complete.
Once you have obtained the last urine sample in the 24-hour period, you will need to take the containers or bags to your doctor’s office or a lab, but there are no side effects or restrictions once the collection period is over.
Creatinine Clearance Test Results
Receiving test results
In most cases, you will have results from your creatinine clearance test within a few days after the lab has received both your blood and urine sample. The test results may be sent to you by mail and/or made accessible through an online health portal. Your doctor’s office may also contact you to review your results.
Interpreting test results
The results of a creatinine clearance test are usually reported in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m2).
The test report should list the reference range for creatinine clearance for the laboratory that did the analysis of your samples. It is important to look closely at this reference range because results can vary from laboratory to laboratory.
In addition to the creatinine clearance, the test report may also list other measurements including
- Serum creatinine: The amount of creatinine in your blood measured in milligrams per deciliter (mg/dL)
- Urine creatinine: The amount of creatinine in your urine measured in milligrams per deciliter
- Urine volume: The total amount of urine that you collected over 24 hours measured in liters (L)
Each of these individual measurements are used to calculate your creatinine clearance. The test report may also show your estimated glomerular filtration rate (eGFR), which is the amount of blood the kidneys filter per minute. The creatinine clearance test is a method of assessing eGFR.
There is no universal reference range for creatinine clearance, so interpretation of your test result requires consideration of the normal range for the lab that conducted your test as well as your overall health and whether you have any symptoms of kidney disease.
When creatinine clearance is abnormally low, it can be a sign that the kidney is not filtering the blood properly. This may also be shown as a reduced glomerular filtration rate. Impaired kidney function can be caused by injury or disease within the kidney itself. It can also be caused by a number of other health issues that can affect kidney function such as heart failure or dehydration.
Though less common, creatinine clearance can be abnormally high. This indicates a more rapid filtration of the blood and may occur in pregnancy and in some people with diabetes.
Having a creatinine clearance in the normal range is not a complete assurance that the kidneys are working properly. Other factors can affect interpretation of a normal result:
- In response to injury, cells in the kidney can ramp up filtration, which may prevent an underlying problem from being detectable on a creatinine clearance test.
- Not all creatinine in the urine is removed from the blood by kidney filtration. About 10-20% is removed by tubular secretion, which is a process in the kidney that helps the blood reabsorb water while removing waste products. Because the creatinine clearance relies on a measurement of creatinine in the urine, it overestimates kidney filtration by around 10-20%.
- With mild kidney disease, the extent of change in creatinine levels may not be enough to cause creatinine clearance to fall outside the normal range.
- Individual factors including total muscle mass, diet, and medications may affect creatinine levels.
In order to properly understand and interpret your creatinine clearance test, all of these factors must be taken into account along with other considerations related to your health history or other tests that you may have taken.
As a result, it is essential to review your creatinine clearance level with your doctor who can most directly explain the significance of the test result in your situation.
Are test results accurate?
A creatinine clearance test is generally considered to be an effective way to learn about how well the kidneys are working. The ability to compare creatinine concentrations in both blood and urine can improve the dependability of the test for medical decision-making.
Like any test, though, a creatinine clearance test is not perfect. Accuracy depends on proper sample collection, and some people have difficulties with the 24-hour urine collection. If urine is over- or under-collected, it will throw off the formula for calculating creatinine clearance.
The ability of creatinine levels to assess kidney health generally requires that kidney function be stable. If a person is hospitalized with kidney failure or another serious condition, creatinine clearance may not be able to effectively evaluate their kidney filtration. Creatinine levels in the blood can also be less dependable in some people with advanced kidney failure.
Creatinine clearance is also known to overestimate the glomerular filtration rate by about 10-20%. This occurs because some creatinine is removed from the blood by a different process called tubular secretion. For this reason, creatinine clearance is typically interpreted as the upper level estimate of the maximum possible level of GFR.
Finally, accuracy of creatinine clearance testing can be affected by factors that can vary from person to person. For example, the amount of muscle mass directly influences creatinine levels, so people with extremely high or low amounts of muscle mass, such as bodybuilders or amputees, can have abnormal results. Many elderly individuals will also have mild-to-moderately reduced eGFR. Thus, diet, age, sex, ethnicity, and medications can all affect creatinine levels as well.
Do I need follow-up tests?
The need for follow-up after a creatinine clearance test depends on your test result as well as your health situation.
If your creatinine clearance test is abnormal, your doctor may suggest repeating the test. This is especially true if it is believed that you may have over- or under-collected your 24-hour urine sample.
If the test shows signs of possible kidney impairment, other types of tests may be warranted. These could include other blood and urine tests, ultrasound or other imaging, and in some cases, a kidney biopsy.
Your doctor can suggest a specific follow-up plan based on your test results, symptoms, and other health considerations. In some cases, you may be referred to a doctor that specializes in kidney disorders known as a nephrologist.
Questions for your doctor about test results
An important part of your health care is being able to understand the tests you have taken and what the results mean. As you talk with your doctor, you can ask some of these questions to help in this process:
- What was the result of my creatinine clearance test?
- Did that result fall in the normal reference range for the laboratory that ran my test?
- If my creatinine clearance was abnormal, what do you think may have caused it?
- Would it be beneficial or necessary to repeat the creatinine clearance test?
- Are there other follow-up tests that you recommend? If so, what are their benefits and downsides?
