Summary

• Creatinine is a waste product that is filtered from your blood by your kidneys.
• Creatinine clearance is a way of assessing how much creatinine is removed from your body during a 24-hour period.
• The test involves having a blood test and collecting all your urine over 24 hours and having a blood sample taken.
• The test is not routinely used because of the inconvenience of collecting urine over 24 hours.
• It has largely been replaced by the estimated glomerular filtration rate (eGFR) test.
• Creatinine clearance is still used in specific cases where the body size is much larger or smaller body than standard and the eGFR may not be accurate.

What is creatinine?

Creatinine comes from the normal wear and tear of your muscles. It is a by-product of a chemical called creatine that is found in muscle cells and is used to produce the energy needed to contract muscles. Creatine breaks down as a matter of course and becomes creatinine.

As a waste product, creatinine is filtered by your kidneys. Almost all of it is filtered from your blood by your kidneys and passed into your urine to be removed. Measuring creatinine is a good way to find out if your kidneys are working properly and it is usually ordered along with other tests for checking kidney function. It is also used to monitor the effectiveness of treatment for kidney disease.

What is creatinine clearance?

Creatinine clearance measures how much creatinine your kidneys filter over a 24-hour period.

The test measures the amount of creatinine in a sample of blood taken from your arm and in all the urine you produce during a 24-hour period.

To calculate creatinine clearance, the creatinine level in your urine is multiplied by the total volume of urine you produce over 24 hours. This is then divided by the amount of creatinine in your blood multiplied by the time over which it has been collected. The result is converted to millilitres of blood per minute (mL/min).

Your height and weight can be measured so that surface area of your body can be calculated and the result of your test adjusted to the 'standard' body surface area of 1.73m2.

The creatinine clearance test is not used as often as it once was because of the inconvenience of collected urine over 24 hours. It has largely been replaced by a blood test called the estimated glomerular filtration rate (eGFR) together with the albumin-creatinine ratio (ACR).

Measured creatinine clearance is often used when someone has very large muscle mass (such as a body builder) or very small body size (such as children) where the eGFR and ACR may be less accurate.

Having the test

Sample

Blood sample and a 24-hour urine sample.

A 24-hour urine sample is a collection of all urine passed over a 24-hour period.

Preparation

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.

Before the urine collection, you will normally be advised to continue your typical daily fluid intake. Drinking water can temporarily decrease creatinine levels. 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.

Your results

Reading your test report

Your results will generally be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

The lower the result of the creatinine clearance test means that less blood is being filtered by the kidneys. This could be due to damage in the kidneys, but other tests will be needed to make a diagnosis. Other possible causes include congestive heart failure, dehydration, shock, and obstruction within the kidneys.

Any more to know?

Certain drugs, such as aminoglycosides, cimetidine, cisplatin, and cephalosporins can lower creatinine clearance measurement. Diuretics can increase the result.

If any of your urine samples are missed you will not have a complete collection, and the results of the test will not be valid. You should call either your doctor or the laboratory where you obtained your container to ask if you should discontinue the test and begin again another day.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods, medications or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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