How is it used?
The creatinine blood test is used along with a
BUN (blood urea nitrogen) test to assess kidney function. Both are frequently ordered as part of a
basic or
comprehensive metabolic panel (BMP or CMP), groups of tests that are performed to evaluate the function of the body’s major organs. BMP or CMP tests are used to screen healthy people during routine physical exams and to help evaluate
acutely or
chronically ill patients in the emergency room and/or hospital. If the creatinine and BUN tests are found to be abnormal or if you have an underlying disease, such as
diabetes, that is known to affect the kidneys, then these two tests may be used to monitor the progress of kidney dysfunction and the effectiveness of treatment. Blood creatinine and BUN tests may also be ordered to evaluate kidney function prior to some procedures, such as a CT (computed tomography) scan, that may require the use of drugs that can damage the kidneys.
A combination of blood and urine creatinine levels may be used to calculate a creatinine clearance. This test measures how effectively your kidneys are filtering small molecules like creatinine out of your blood.
Urine creatinine may also be used with a variety of other urine tests as a correction factor. Since it is produced and removed at a relatively constant rate, the amount of urine creatinine can be compared to the amount of another substance being measured. Examples of this are when creatinine is measured with protein to calculate a urine protein/creatinine ratio (UP/CR) and when it is measured with microalbumin to calculate microalbumin/creatinine ratio (also known as albumin/creatinine ratio, ACR). These tests are used to evaluate kidney function as well as to detect other urinary tract disorders.
Serum creatinine measurements (along with your age, weight, and gender) also are used to calculate the estimated glomerular filtration rate (eGFR), which is used as a screening test to look for evidence of kidney damage.
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When is it ordered?
Creatinine may be ordered routinely as part of a
comprehensive or
basic metabolic panel, during a health examination. It may be ordered when you have non-specific health complaints, when you are acutely ill, and/or when your doctor suspects your kidneys are not working properly. Some
signs and
symptoms of kidney dysfunction include:
- Fatigue, lack of concentration, poor appetite, or trouble sleeping
- 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
The creatinine blood test may be ordered, along with BUN test and microalbumin, at regular intervals when you have a known kidney disorder or have a disease that may affect kidney function or be exacerbated by dysfunction. Both BUN and creatinine may be ordered when a CT scan is planned, prior to and during certain drug therapies, and before and after to monitor the effectiveness of treatments.
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What does the test result mean?
Increased creatinine levels in the blood suggest diseases or conditions that 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, for example, by 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
Creatinine blood levels can also increase temporarily as a result of muscle injury and are generally slightly lower during pregnancy.
Low blood levels of creatinine are not common, but they are also not usually a cause for concern. They can be seen with conditions that result in decreased muscle mass.
Levels of 24-hour urine creatinine are evaluated with blood levels as part of a creatinine clearance test.
Random urine creatinine levels have no standard reference ranges. They are usually used with other tests to reference levels of other substances measured in the urine. Some examples include the microalbumin test and urine protein test.
NOTE: The result of your blood creatinine test is measured by your doctor against a reference range for the test to determine whether the result is “normal” (it is within the range of numbers), high (it is above the high end of the range), or low (it is below the low end of the range). Because there can be many variables that affect the determination of the reference range, the reference range for this test is specific to the lab where your test sample is analyzed. For this reason, the lab is required to report your results with an accompanying reference range. Typically, your doctor will have sufficient familiarity with the lab and your medical history to interpret the results appropriately
While there is no such thing as a “standard” reference range for blood creatinine, most labs will report a similar, though maybe not exactly the same, set of numbers as that included in medical textbooks or found elsewhere online. For this reason, we recommend that you talk with your doctor about your lab results. For general guidance only, we are providing the below from the classic medical text, Tietz Textbook of Clinical Chemistry and Molecular Diagnostics.
For more information on reference ranges, please read Reference Ranges and What They Mean.
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Is there anything else I should know?
Drugs such as aminoglycosides (vancomycin, gentamicin) can cause kidney damage and so creatinine is monitored. Other drugs, such as cephalosprins (cefoxitin), may increase creatinine concentration without reflecting kidney damage.
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