Creatinine Clearance Test

Creatinine is a substance produced in the body as a waste product when muscle tissue breaks down. Because it is a waste product, it needs to be removed from the body. More specifically, creatinine in the bloodstream must be filtered out and discarded on an ongoing basis. The body manages this function through the use of the kidneys to filter and clean the blood. A healthy person’s kidneys clear creatinine at a fairly predictable rate. The creatinine clearance test is a measurement used to determine whether a person’s kidneys are functioning well enough to clear creatinine from the blood at a fast enough rate. A young adult with properly functioning kidneys usually will have a creatinine clearance rate of approximately 125 milliliters per minute. As the creatinine is filtered, it is removed from the body by the production and release of urine.

Background

The creatinine clearance test is most often used to gauge how well a person’s kidneys are functioning. Two different steps can be used to measure creatinine clearance. The first step involves the study of the patient’s urine to determine how much discarded creatinine it contains. This type of creatinine clearance test requires the patient to collect his or her urine for an entire twenty-four hour period by urinating into a container. This container is then brought to the doctor and it is tested to determine how much creatinine it contains. If the creatinine level of the patient’s urine is found to be lower than normal for a person of similar height, weight, age, and physical fitness, then this may mean that some of the creatinine that should have been found in the urine is still present in the patient’s bloodstream. Assuming this is the case, the logical conclusion would be that the creatinine is still in the bloodstream because it was not properly filtered out by the kidneys. This, in turn, leads to the conclusion that the kidneys may not be functioning optimally.

The second step of a creatinine clearance test requires doctors to analyze the patient’s blood instead of his or her urine. By taking a blood sample from the patient and testing it to find out how much creatinine the blood contains, a doctor can then compare this value with data for persons of similar height, weight and age, to determine whether the patient’s levels of creatinine in the blood are higher than normal, lower than normal, or on target. Creatinine levels that are higher than normal can be a sign that the patient is having kidney problems, since the kidneys have not been able to adequately filter the blood. In the vast majority of cases, doctors will prefer to use the blood based form of the creatinine clearance test over the urine based form. This is purely for reasons of practicality, as it is much simpler for most patients to have a simple blood test than it would be for them to spend twenty-four hours collecting their urine and then bringing it to the doctor’s office for analysis.

Once the blood and urine have been analyzed, the results of this analysis are compared with one another so that a conclusion can be drawn as to whether or not the kidneys appear to be functioning at a normal level.

Overview

When the body metabolizes food into energy, a substance called creatine is produced. Creatinine is produced when the creatine phosphate found in muscle tissue breaks down and is released into the bloodstream. In healthy individuals this breakdown process happens at a consistent rate, and as a result the operation of the kidneys to clean the creatinine from the blood also happens at a predictable pace. This is why the creatinine clearance test is so useful: By identifying deviations from the expected levels of creatinine in a person’s blood or urine, the creatinine clearance test can serve as a way of finding out how efficiently the kidneys are working.

The main process used by the kidneys as they clear creatinine is known as glomerular filtration. A related process, proximal tubular secretion, also plays a role. The rate of glomerular filtration, sometimes referred to by the acronym GFR (glomerular filtration rate), is essentially a simple measurement of the rate at which fluid (blood) flows through, and is cleaned by, the kidneys. In addition to its use as a means of identifying patients who may be suffering from some form of kidney failure, the creatinine clearance test can also be used in conjunction with a diagnosis of congestive heart failure. This is because one of the consequences of congestive heart failure is that the heart is less able to pump blood throughout the body. Because blood is moving more slowly through the body in patients with congestive heart failure, it will move through the kidneys at a slower rate as well. As a result, it will take longer for the kidneys to filter the creatinine out of a given volume of blood, which will cause an increase in the amount of creatinine present in the bloodstream. Doctors monitoring patients suffering from congestive heart failure will use the creatinine clearance test to monitor heart function.

There are a number of different symptoms that may cause a doctor to perform a creatinine clearance test. Some of these include high blood pressure, decreased production of urine, urine that contains blood or that is unusually foamy or dark in coloration, swelling around the eyes or joints, and changes in frequency of urination or increased difficulty or pain associated with urination. In some cases, pain in the lower back—where the kidneys are located—is also cause for concern, particularly if it presents alongside one or more of the symptoms mentioned above.

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