What is being tested?
This test measures the concentration of creatinine in both a sample of blood and a sample of urine from a 24-hour urine collection. The results are used to calculate the amount of creatinine that has been cleared from the blood and passed into the urine over a 24 hour time period.
Creatinine is a chemical derived from creatine, a nitrogen-containing organic compound used by muscles to store and transfer energy. The amount of creatinine produced in the body is dependent on muscle mass and is constant for an individual. It is removed from the body as blood passes through the kidneys. By measuring creatinine in the blood and in the urine, it is possible to determine the amount of blood filtered by the kidneys in a measured period of time. A calculation is made based on the amount of creatinine in the urine, the time period of collection, and the amount of creatinine in the blood serum or plasma. The results are a measure of kidney function.
Since the amount of creatinine produced depends upon muscle mass, in some circumstances, e.g. in children, the calculation will need to be corrected for body surface area.
How is it used?
The creatinine urine test is used to help evaluate the rate and efficiency of kidney filtration. It is used to help detect kidney dysfunction resulting from either kidney disease or decreased blood flow to the kidneys.
Creatinine is normally filtered through the glomerulus - a group of blood vessels in the kidneys responsible for filtration. An abnormal kidney function test indicates that the person may have lost more than 30-40 per cent of their kidney's function, which may be a permanent or temporary loss.
When is it requested?
The test may be requested if you suffer from kidney disease or a condition known to effect kidney function such as congestive heart failure, shock, or diabetes. It may also be requested to assess kidney function prior to the administration of some toxic drugs as dose may be dependent on kidney function.
What does the result mean?
A decreased creatinine clearance may indicate a decrease in the amount of blood filtered by the kidney due to disease within the kidney cells or to decreased delivery of blood to the kidneys. Congestive heart failure, dehydration, shock, obstruction within the kidney, or acute or chronic kidney failure are among the possible causes.
Is there anything else I should know?
Certain drugs, such as aminoglycosides, cimetidine, cisplatin, and cephalosporins can decrease the creatinine clearance measurement. Diuretics can increase the result.
Creatinine clearance may also be measured before you are prescribed certain drugs which rely on good kidney function in order to allow them to be removed from the body effectively after they have carried out their purpose. Other straightforward methods of estimating the function of your kidneys now in routine use across Australia include the calculation of estimated glomerular function rate (eGFR) from a single blood sample taken from a vein in your arm.
Common questions
If you do not have a complete collection, the results 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.
There are other, more involved tests that have higher accuracy. However, these are more complex for the patient and doctor and involve injecting the patient with a tracer and are more expensive. For most medical purposes, the accuracy of the eGFR is sufficient however creatinine urine may be useful in patients with abnormal body composition, diet or other factors.
No. The test requires analysis and calculations by skilled personnel in an accredited laboratory.
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