Creatinine

Creatinine is a byproduct of a chemical compound called creatine, which helps muscles get the energy that they need. As a waste product, creatinine is filtered out of the blood by the kidneys and removed from the body in urine.

A creatinine test measures the amount of this chemical in either the blood or urine. Creatinine levels can provide an indication of how well the kidneys are working.

Creatinine may be measured alone or included in a panel of tests that include other compounds found in the urine or blood.

Purpose of the test

The primary use of a creatinine test is to better understand how well the kidneys are working. A measurement of creatinine can be employed for screening, diagnosis, and/or monitoring of kidney problems.

Screening is trying to identify health problems before there are any signs or symptoms. In people who are at higher risk of kidney problems, a creatinine test may be used to try to detect a problem early.

Diagnosis involves testing to determine the underlying cause after symptoms have started. Creatinine may be measured to help diagnose symptoms that can be associated with kidney problems such as swelling in the feet, urinary changes, loss of appetite, and fatigue. A measurement of creatinine may also be included in a panel of tests for more general symptoms or when evaluating a patient in the emergency room.

Monitoring is how doctors track a patient’s condition over time. For people who have kidney disease, a creatinine test can provide information about how the disease may be progressing. Tests of kidney function can also be used to watch for potential side effects when a patient is prescribed medications that can affect the kidneys.

In many cases, screening, diagnosis, and monitoring of kidney problems involve more than just a creatinine measurement. For example, creatinine may be tested in order to calculate the estimated glomerular filtration rate (eGFR), which is an assessment of how effectively the kidneys are filtering the blood.

What does the test measure?

A creatinine test measures the total amount of the substance creatinine in the blood or the urine.

Creatinine is a waste byproduct of the normal metabolism of creatine, a compound that is involved in providing the muscles of the body the energy they need. In this process, some creatine breaks down into creatinine.

In order to clear this waste product from the body, the kidneys filter creatinine out of the blood and facilitate its removal from the body in urine.

A serum creatinine test measures the amount of creatinine in a sample of blood. A urine creatinine test measures the total amount of creatinine in all of the urine produced during a 24-hour period. These two values can also be compared in a creatinine clearance test.

Creatinine may be measured as part of other tests. For example, it may be included as one part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). These tests help assess the body’s energy use, electrolyte levels, and kidney function.

When should I get a creatinine test?

A creatinine test is most often prescribed when your active symptoms or overall health history reflect an elevated risk of kidney problems.

If you have symptoms that could be caused by an underlying problem that is affecting your kidneys, a creatinine test may be part of the diagnostic process.

If you don’t have symptoms, your doctor may suggest a creatinine test to screen for kidney problems if you have an increased risk of kidney disease. Some risk factors for kidney disease include:

• Diabetes
• A family history of kidney problems
• High blood pressure

If you take a medication that has the potential to disrupt kidney function, you may receive periodic creatinine testing to check for this side effect.

Creatinine testing can also be used to monitor your kidney health if you’ve had an abnormal kidney function test before or if you have already been diagnosed with kidney disease.

A creatinine measurement may be taken as part of a basic or comprehensive metabolic panel during a regular health checkup. Although there is little evidence showing the benefits of this kind of screening in otherwise healthy people, some doctors prescribe these panels during routine patient care.

How to get tested

Creatinine tests are normally ordered by a doctor. A blood sample taken from a vein in your arm in a medical office or hospital is used for a creatinine blood test. For a urine creatinine test, you use special containers to collect all of your urine over a 24-hour period.

Can I take the test at home?

Some options exist for collecting a sample for a creatinine test at home.

Urine creatinine tests are done with a 24-hour urine sample, so most doctor-prescribed urine creatinine tests require at-home collection.

Some at-home test kits include measurements of creatinine in several samples of dried urine. These samples are not analyzed at home but are sent to a lab where creatinine can be measured.

Some commercially available urine test strips change color based on the creatinine level. However, test samples taken over a longer period, such as a 24-hour urine collection, are generally considered to be more accurate than a one-time measurement for kidney function tests.

At-home blood test kits are available that include creatinine along with other measurements. These self-collection tests require mailing a blood sample to a laboratory for analysis.

How much does the test cost?

Several different factors can change the expected cost of a creatinine test. These factors include:

• Whether the test uses a sample of blood or urine
• Whether the test measures creatinine alone or with other substances
• Where testing is done
• Whether you have health insurance coverage

The total cost may include separate charges for office visits, phlebotomy fees, or laboratory analysis. Health insurance may cover these costs if testing is prescribed by a doctor, but you should talk with your insurance company to find out about any out-of-pocket costs, such as a deductible or copay, for creatinine testing.

At-home kidney tests that include creatinine are often available for around $100, but the exact cost can depend on the manufacturer and the specific measurements included in the test.

Creatinine can be measured in either blood or urine. Blood tests are normally done in a medical office or hospital and involve a routine procedure to withdraw a sample of blood from a vein in your arm.

For a urine test, you are provided containers or bags to collect all the urine you produce over a 24-hour period. Unless you are in the hospital, this requires sample collection at home that is then brought to the lab.

Although less common, some at-home test kits are available to test creatinine in blood or urine without having to go to a doctor’s office or lab.

Before the test

For a blood or urine creatinine test, it’s important to follow any pretest instructions provided by your doctor.

If you are being tested for creatinine alone, you usually do not have to fast beforehand. However, you may need to avoid eating meat for around 24 hours before the test because this can affect your creatinine levels. If creatinine is being tested in a panel, you may be told to fast for up to 12 hours before the test.

Some medications and supplements can alter creatinine levels, so your doctor may adjust your medication schedule before a creatinine test. You should review all prescriptions, over-the-counter drugs, and dietary supplements that you take with your doctor before having a creatinine test.

For a 24-hour urine collection, make sure to closely review the instructions with your doctor or nurse. While this collection can be inconvenient, accurately obtaining your urine samples over this full period is important to obtaining accurate results.

For at-home blood tests, make sure to look over the included instructions carefully and to go through each step of the required test preparations.

During the test

If you are having a serum creatinine test, a sample of blood will be taken from your arm. To increase blood flow in your veins, the nurse or technician will tie elastic around the upper part of your arm. They will use an antiseptic wipe to clean the skin near a vein and then insert a needle into the vein.

This type of blood test is usually over within a few minutes. While there may be a sting or some pain, it usually goes away quickly.

For a urine creatinine test, you will need to collect all the urine you produce over a 24-hour period. Most often, you are provided with special containers. On the first day of the test, you usually wake up and urinate in the toilet. After that, you collect all your urine that day as well as the first urination in the morning of the next day. You may also need to make notes about the times of the day when you urinate.

If you are taking an at-home blood test, you will obtain a drop of blood from your fingertip using a very small needle included in the test kit. You place that drop of blood on a strip of test paper and then package the sample to be mailed to a laboratory.

After the test

After a creatinine blood test, a bandage will be placed over the puncture site to stop it from bleeding. At that point, you can usually return to normal activities. Some pain or bruising is possible but is not typically long-lasting.

Once you have finished a 24-hour urine collection for a creatinine test, you should not have any side effects or other restrictions.

For a blood test done at home with a finger prick, you usually won’t have any lasting pain or other problems. A bandage can be used on your fingertip if it keeps bleeding after you have finished the test.

Receiving test results

The results from either a blood or urine creatinine test are generally available within a few business days after you take the test. You may receive a call or email from your doctor about the results, or you may get a test report in the mail or online.

For at-home test kits that require sending your blood sample to a lab, it may take a few extra days for results to become available. With these tests, you normally access the test report through a smartphone app or website.

Interpreting test results

Creatinine tests are generally used as a way of assessing kidney function. Under normal circumstances, creatinine levels are stable, reflecting typical muscle activity and the filtering and removal of creatinine from the bloodstream.

There is no single reference range for all creatinine tests or for all people. Measurements can vary between laboratories, so it is important to look carefully at your test report to see the listed reference range for the lab that conducted your test.

The reference range for creatinine also depends on whether it is measured in the blood or urine. The American Board of Internal Medicine lists the following reference ranges for blood and urine creatinine:

• Blood: 0.7 to 1.3 milligrams per deciliter (mg/dL) for males and 0.5 to 1.1 mg/dL for females.
• Urine: 15-25 milligrams per kilogram of body weight (mg/kg) per 24 hours.

When creatinine levels rise to abnormal levels, it may be an indication that the kidneys are not properly filtering the blood. The creatinine level may be used to calculate your estimated glomerular filtration rate, which is a way of assessing kidney function. Elevated creatinine can also be tied to dehydration, diseases that cause muscle problems, and some complications during pregnancy.

If creatinine levels are lower than expected, it may be a sign of malnutrition or conditions that provoke the loss of muscle mass.

A normal creatinine level does not guarantee kidney health. Moderate kidney impairment may not cause creatinine to rise to abnormal levels, so creatinine tests may not identify some cases of early kidney disease.

A number of individual factors, such as age, diet, and muscle mass, can affect creatinine levels, so it is important to remember that your test should be interpreted with the help of your doctor in the context of your specific situation. This interpretation will also depend on why the test was ordered and whether you have any symptoms.

When creatinine is measured in panel tests, it may be interpreted in conjunction with other test findings. For example, the amount of creatinine relative to blood urea nitrogen (BUN), another waste product, may provide useful information for the doctor about the cause of kidney issues.

Are test results accurate?

Measuring creatinine is a common laboratory procedure, and creatinine levels are generally reliable for medical decision-making. Initiatives in the U.S. have worked to calibrate creatinine tests and standardize methods among labs, reducing the likelihood of inaccuracies when using creatinine levels to evaluate kidney function.

Testing requires proper sample collection, and errors can occur, especially with 24-hour urine samples. In practice, some people find it challenging to collect all the urine produced during this time, which can affect test outcomes.

Creatinine testing is less dependable if you do not have stable kidney function. For this reason, it is less often used to assess kidney health in people who are hospitalized or have an acute illness.

In addition, it is important to be aware of individual factors that can alter creatinine levels and have an effect on test accuracy:

• Body size and muscle mass: Because creatinine is a byproduct of muscle activity, levels are influenced by a person’s total muscle mass. This may affect test accuracy in people who are amputees, bodybuilders, obese, malnourished, or otherwise have more or less muscle mass.
• Age: Older adults typically have reduced muscle mass, which often leads to lower creatinine levels.
• Sex: Reference ranges for creatinine are often lower for females, although this is believed to be tied to other factors like total muscle mass.
• Diet: Meat consumption can lead to a notable rise in creatinine consumption, which is why pretest instructions may restrict meat intake.
• Medications: Certain drugs, including some antibiotics and dietary supplements, can cause changes in creatinine levels.
• Pregnancy: Creatinine levels often fall during pregnancy, although levels may rise as a result of some types of complications in pregnancy.

Because there are numerous factors that can influence the accuracy and interpretation of a creatinine test, you should always discuss your test directly with your doctor to best understand its significance in your unique case.

Do I need follow-up tests?

If your creatinine test comes back with an abnormal result, you may need follow-up testing. Creatinine testing may be repeated and/or tested along with other measurements of kidney function.

Depending on the nature of your result and your overall health, you may have other laboratory tests of blood or urine, imaging tests, or a kidney biopsy. Sometimes a nephrologist, a specialist in kidney conditions, will be involved in your follow-up care.

Questions for your doctor about test results

To help understand your creatinine test result, you can bring up some of the following questions when discussing the test with your doctor:

• What was the reference range for the laboratory that did my test?
• What was my creatinine level? Was it normal or abnormal?
• Are there any individual factors that might affect the accuracy or meaning of my creatinine test?
• Should I have another kidney function test now or in the future?
• Are there any other follow-up tests that you recommend? Why or why not?

Creatinine is included in multiple kinds of tests, and it is one of many tests of kidney function. The following sections review some key similarities and differences between creatinine and several related tests.

How is a creatinine test different from an estimated glomerular filtration rate (eGFR) test?

The estimated glomerular filtration rate is an assessment of how effectively the kidney is filtering the blood. In many cases, eGFR is calculated with an equation that is based on a measurement of creatinine in the blood.

In most cases, a rise in creatinine levels reflects a decrease in the estimated glomerular filtration rate. However, the relationship is not exactly proportional, and both eGFR and creatinine measurements can be subject to individual variation.

How is a creatinine test different from a creatinine clearance test?

A creatinine test involves measuring the amount of creatinine in either the blood or urine. In contrast, a creatinine clearance test compares the levels of creatinine in the blood to those in the urine.

In this way, a creatinine clearance test utilizes a calculation of creatinine in the two samples to assess kidney function. Like creatinine alone, creatinine clearance may be used for estimating the glomerular filtration rate. However, in many cases, creatinine clearance overestimates how well the kidneys are filtering the blood.

How is a creatinine test different from a blood urea nitrogen (BUN) test?

Both creatinine and urea nitrogen are waste products that can be found in the blood and filtered by the kidneys. BUN is generated as a byproduct of protein breakdown.

BUN tests alone do not usually provide a clear assessment of kidney function. For that reason, BUN is often tested with creatinine and other compounds. In some situations, changes in the amount of BUN relative to creatinine can be an indication of certain types of kidney problems.

How is a creatinine test different from a basic or comprehensive metabolic panel test?

The basic metabolic panel and comprehensive metabolic panel are a collection of measurements. Creatinine is just one component of these two blood tests. These panels also provide information about the body’s energy use, electrolyte levels, and kidney function. The CMP also involves tests related to liver function.

Why is creatinine often measured in other types of urine tests?

Some urine tests measure creatinine even though creatinine and kidney function are not the focus of the test. For example, in urine drug testing, creatinine is frequently measured even though the test is looking for the presence of drugs of abuse.

The reason that creatinine is included in this kind of test is that it provides a way to make sure that the urine sample was not tampered with or diluted. Because creatinine levels in the urine are generally consistent in otherwise healthy people, the creatinine concentration can be used to determine whether a urine sample has been diluted.

In addition, the concentration of creatinine in one urine sample can be compared to the expected 24-hour creatinine concentration, and this ratio can be used to estimate the concentration of another substance in the urine. For some urine tests, this allows for more convenient measurement because it eliminates the need for a full 24-hour urine collection.

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