Creatinine Clearance Test
What is the creatinine clearance test?
The creatinine clearance test is an older test used to check your kidney function. This test allows your healthcare provider to look at samples of your urine and blood to see how much of the waste product — creatinine — is filtered out by your kidneys. Abnormal levels of creatinine in your urine and blood could point to an issue like kidney disease.
Done in two parts, the creatinine clearance test involves collecting your urine over a 24-hour period of time and then having your blood drawn. These samples are then tested to see how much creatinine was filtered through your kidneys over the 24-hour window. The results of the test will be used in a mathematic formula that figures out your creatinine clearance. Creatinine clearance is one way to estimate your glomerular filtration rate (GFR), or how well the kidneys are filtering your blood. The GFR is the main number used by your provider to determine how well your kidneys are working.
The creatinine clearance test is not used very often any more. It has largely been replaced by the estimated GFR (eGFR) equations using creatinine levels in the blood — which means a 24-hour urine collection is not needed.
What is creatinine?
Creatinine is actually a waste product of creatine. Creatine is a chemical that your body uses to supply your muscles with energy. As you muscles use energy the tissue that makes up your muscles breaks down. This natural breakdown of muscle tissue causes creatinine to be released into your bloodstream. This is when creatine becomes creatinine.
Normally, creatinine is filtered out of your blood by your kidneys. If you think of each organ in your body as having a job, the kidneys are the filters. They’re responsible for cleaning your blood. The material that’s removed from your blood leaves your body in your urine. If your kidneys aren’t working properly, than you may have higher levels of creatinine in your blood than you should.
Why do I need to have the creatinine clearance test?
Anyone can be at risk for a kidney disease at some point in their life. However, some people have a higher risk of developing a kidney problem than others. These risks can include:
- Age (you could be at a higher risk after age 60).
- High blood pressure (hypertension).
- Family history of kidney disorders.
- Heritage (there’s a higher risk for people of an African-American, Asian, Native American or Pacific Islander background).
When is the creatinine clearance test done?
The creatinine clearance test is done when your healthcare provider thinks that the eGFR result given with your blood creatinine level may not be accurate. Examples includes when people have very little or a lot of muscle on their bodies. However, there are ways to check this, such as with the eGFR result using cystatin C, another blood test. As a result, creatinine clearance is rarely used in modern practice.
Are there any symptoms associated with a high creatinine level?
Kidney disease tends to be silent and, for many people, there are no symptoms in the early stages. However, as the disease progresses, you may experience a few symptoms. These can include:
- Fatigue (feeling tired).
- Changes in how often you urinate.
- Loss of appetite.
Why is the creatinine clearance test done?
The creatinine clearance test is to confirm the level of kidney function reported with the eGFR level. Your healthcare provider will want to know how well your kidneys are working. The results of this test can lead to a diagnosis of kidney disease.
Is the creatinine clearance test done at home?
Part of the creatinine clearance test is done at home and the other part in a lab. You will collect your urine over a 24-hour period of time at home. During this time, you can still participate in your normal daily activities. You just need to stick to a schedule for collecting your urine samples and make sure not to miss any collections (do not flush any urine).
The second part of the test involves having your blood drawn. This will need to be done at a lab, healthcare facility or your provider’s office. Your provider will give you directions on where to go to for the blood test when you pick up your test materials. Often, you will drop off your urine collection when you go to get your blood drawn.
What do I need to do to prepare for the creatinine clearance test?
Before the creatinine clearance test, your healthcare provider will give you specific instructions. During the creatinine clearance test you will need to collect your urine for 24 hours and then your blood will be drawn.
Your provider will provide the container you will need to collect your urine sample and will tell you how to store it over the 24-hour period when you’re collecting it. It’s important to follow the instructions your provider gives you. Make sure you collect urine throughout the test. If you skip a few times or don’t follow the instructions, you may need to repeat the test.
You may also be asked to stop taking some medications. This will be done with your provider’s direct supervision. Some medications could affect the test’s accuracy, and so you might need to temporarily stop taking them. These medications could include:
- Antibiotics (Bactrim).
- Stomach acid pills (such as famotidine).
Make sure to let your provider know about any medication you are taking and make sure there is a complete list of all medications you are taking in your medical record. Never stop any medication without first talking to your healthcare provider.
Do I need to fast (not eat) before the creatinine clearance test?
In general, you can eat normally before and during the creatinine clearance test. However, you may be asked not to eat overnight. Your provider may also ask you not to eat meat before the test. This could change the results because there’s higher levels of creatine in meat, which would cause your body to have higher levels of creatinine during the test.
What happens during the creatinine clearance test?
You will do the creatinine clearance test over 24 hours. During this time, you will collect your urine each time you urinate. This will give your healthcare provider a good idea of your creatinine levels throughout the entire day.
Make sure to follow the directions from your healthcare provider closely. These instructions will include details about how to store your urine sample and where you will take it when the test is over.
After 24 hours of collecting your urine, you will need to have your blood drawn. This second part of the test looks at the amount of creatinine in your bloodstream. This is called serum creatinine. Your results from both of these tests will be plugged into a mathematic formula that determines your creatinine clearance. This rate tells your provider just how well your kidneys are filtering waste products out of your bloodstream.
What happens if I forget to collect one urine sample during collection?
It’s very important to collect urine samples throughout the entire day for the creatinine clearance test to be accurate. If you skip one urine collection, it could affect the test results. Call your healthcare provider’s office if you miss a collection to see if you should continue the test or stop and start over the next day.
Will I feel any pain during the creatinine clearance test?
The creatinine clearance test is generally painless. The first part of the test involves normal urination over a 24-hour period of time. You might experience some discomfort from the needle when you have your blood drawn. This is a necessary part of the test and happens quickly. Some people describe having their blood drawn as a stinging feeling. It’s possible to feel some throbbing or have a slight bruise after having your blood drawn. However, all of this discomfort goes away quickly.
Results and Follow-Up
Can the creatinine clearance test be wrong?
The creatinine clearance test is generally a reliable test. Creatinine clearance is only an estimate of GFR, and in some cases can provide a result that is higher than what your GFR really is. In addition, there are a few reasons that the results could be wrong. These include:
- If you didn’t collect all urine samples as directed.
- If the math formula used to figure out your creatinine clearance isn’t done correctly.