When your results of a routine blood creatinine test and/or estimated glomerular filtration rate (eGFR) are not within the normal ranges; when you have signs and symptoms that may be due to kidney disease or damage or when you have a problem affecting the function of your kidneys, such as an obstruction within the kidney or acute or chronic kidney failure, or dysfunction due to another disease, such as congestive heart failure
You may be instructed to fast overnight or refrain from eating cooked meat; some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.
Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is filtered from the blood by the kidneys and released into the urine. A creatinine clearance test measures creatinine levels 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. This calculation allows for a general evaluation of the amount of blood that is being filtered by the kidneys in a 24-hour time period.
The amount of creatinine produced in the body is dependent on muscle mass and is relatively constant for an individual. The amount of creatinine removed from the blood depends on both the filtering ability of the kidneys and the rate at which blood is carried to the kidneys.
The amount of blood filtered per minute by the kidneys is known as the glomerular filtration rate (GFR). If the kidneys are damaged or diseased, or if blood circulation is slowed, then less creatinine will be removed from the blood and released into the urine and the GFR will be decreased.
GFR is difficult to measure directly. Therefore, it is recommended to estimate GFR by measuring the creatinine level in the blood and using the results in an equation to calculate estimated GFR. The calculation that takes into account several factors, such as age, gender and race of the person tested (see the article on Estimated Glomerular Filtration Rate).
Another, less common way to estimate GFR is to calculate creatinine clearance. There are several versions of the creatinine clearance calculation. All of them include the measurement of the amount of creatinine in a blood sample collected just before or after the urine collection, the amount of creatinine in a 24-hour urine sample, and the 24-hour urine volume. Since the amount of creatinine produced depends on muscle mass, some calculations also use a correction factor that takes into account a person's body surface area (using their height and weight).
How is the sample collected for testing?
The test requires a 24-hour urine collection and a blood sample drawn either at the beginning or end of the urine collection. The blood sample is drawn by needle from a vein in the arm. The person being tested will also usually be asked to provide their current height and weight.
Is any test preparation needed to ensure the quality of the sample?
You may be instructed to fast overnight or refrain from eating cooked meat. Some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.
How is it used?
A creatinine clearance test may be used to help detect and diagnose kidney dysfunction. It may be used in follow up to abnormal results on a blood creatinine test and estimated glomerular filtration rate (eGFR).
A creatinine clearance may also sometimes be used to detect the presence of decreased blood flow to the kidneys, as may occur with congestive heart failure.
In people with known chronic kidney disease or congestive heart failure, the creatinine clearance test may be ordered to help monitor the progress of the disease and evaluate its severity. It may also be used to help determine if and when kidney dialysis may be necessary.
When is it ordered?
The creatinine clearance test may be ordered when a healthcare practitioner wants to evaluate the filtration ability of a patient's kidneys. It may be ordered as follow up when a person has, for example, increased blood creatinine concentrations on a routine chemistry panel (CMP) or protein in the urine on a routine urinalysis. It may be ordered when there is a suspected kidney disorder because of certain signs and symptoms.
Signs and symptoms that may be an indication of kidney problems include:
- Swelling or puffiness, particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
- Urine that is foamy, bloody, or coffee-colored
- A decrease in the amount of urine
- Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
- Mid-back pain (flank), below the ribs, near where the kidneys are located
- High blood pressure
- Blood and/or protein in the urine
The creatinine clearance may also be ordered periodically when it is known that someone has a kidney disorder or decreased blood flow to the kidneys due to a condition such as congestive heart failure.
What does the test result mean?
A decreased creatinine clearance may suggest kidney disease or other conditions that can affect kidney function. These can include:
- Damage to or swelling of blood vessels in the kidneys (glomerulonephritis) caused by, for example, infection or autoimmune diseases
- Bacterial infection of the kidneys (pyelonephritis)
- Death of cells in the kidneys' small tubes (acute tubular necrosis) caused by, for example, drugs or toxins
- Prostate disease, kidney stone, or other causes of urinary tract obstruction
- Reduced blood flow to the kidney due to shock, dehydration, congestive heart failure, atherosclerosis, or complications of diabetes
For more on these, see Kidney Disease.
Increased creatinine clearance rates may occasionally be seen during pregnancy, exercise, and with diets high in meat, although this test is not typically used to monitor these conditions.
Is there anything else I should know?
People with one dysfunctional and one normal kidney will usually have normal creatinine clearance rates as the functional kidney will increase its rate of filtration in compensation.
Creatinine clearance rates tend to fall later in life as the glomerular filtration rate (GFR - the rate at which the glomeruli filter the blood) declines.
Certain drugs, such as aminoglycosides, cimetidine, cisplatin, and cephalosporins, can decrease the creatinine clearance measurement. Diuretics can increase the result.
Why do I have to collect my urine for 24 hours?
A 24-hour urine sample is required instead of a random urine sample because the amount of creatinine in the urine will vary somewhat during the course of a day. By collecting all urine for 24 hours, the amount of creatinine in the urine can be averaged over the entire day and will give a better indication of what is going on in the body.
What should I do if I forget to save one urine sample during the collection?
If you do not have a complete collection, the results will not be valid. You should call your healthcare practitioner's office or the laboratory where you obtained your collection container to ask if you should discontinue the test and begin again another day.
Are there other ways to estimate or determine the glomerular filtration rate (GFR) of my kidneys?
Yes. The GFR can be estimated with a simple blood creatinine test (see the article on eGFR). Given a measurement of the amount of creatinine in a blood sample, the healthcare practitioner will use a formula to estimate the rate at which the kidneys are filtering blood. The formulas take relevant factors into account, such as the person's age, weight, height, and ethnicity.
The collection of a 24-hour urine specimen needed for the creatinine clearance test can be impractical to obtain, so the eGFR calculations are usually the preferred method for evaluating kidney function. The creatinine clearance test specifically may be ordered when a healthcare practitioner suspects a problem with blood flow to the kidneys.