Creatinine Test

At a Glance

Also known as

 Creat

Formal Name

Creatinine

Related tests

BUN, eGFR, Creatinine Clearance, CMP, BMP

Why get tested?

To determine if your kidneys are functioning normally and to monitor treatment for kidney disease

When to get tested?

Routinely as part of a comprehensive or basic metabolic panel; if your doctor suspects that you are suffering from kidney dysfunction or if you are acutely or chronically ill with a condition that may affect your kidneys and/or be exacerbated by kidney dysfunction; at intervals to monitor treatment for kidney disease or kidney function while on certain medications

Sample required

A blood sample drawn from a vein in the arm and possibly a 24-hour urine sample

Test Sample

What is being tested?

This test measures the amount of creatinine in your blood and/or urine. Creatinine is a waste product produced in your muscles from the breakdown of a compound called creatine. Creatine is part of the cycle that produces energy needed to contract your muscles and it as well as creatinine are produced at a relatively constant rate. Almost all creatinine is excreted by the kidneys, so blood levels are a good measure of how well your kidneys are working. The quantity produced depends on the size of the person and their muscle mass. For this reason, creatinine concentrations will be slightly higher in men than in women and children.

How is the sample collected for testing?

A blood sample is drawn from a vein in the arm. You may be asked to collect a complete 24-hour urine sample in addition to having your blood drawn. Your doctor or the laboratory will give you a large container and instructions for properly collecting this sample. Typically, you start collecting urine after you wake up in the morning and empty your bladder and save all of the urine produced until the same time the following day.

The Test

How is it used?

The creatinine blood test is usually ordered 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 ordered on healthy people during routine physical exams and on 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 the patient has 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 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 sort of 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 the other substance (such as protein) being measured.

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.

When is it ordered?

Creatinine may be ordered routinely as part of a comprehensive or basic metabolic panel, when someone has non-specific health complaints, is acutely ill, and/or when a doctor suspects kidney dysfunction. The creatinine blood test may be ordered, along with the BUN test, at regular intervals when the patient has a known kidney disorder or has a disease that may affect kidney function or be exacerbated by dysfunction. Both may be ordered when a CT scan is planned, prior to and during certain drug therapies, and before and after dialysis to monitor the effectiveness of treatments.

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; or

• reduced blood flow to the kidney due to shock, dehydration, congestive heart failure, atherosclerosis, or complications of diabetes.

Creatinine can also increase temporarily as a result of muscle injury.

Low levels of creatinine are not common and are not usually a cause for concern. They can be seen with conditions that result in decreased muscle mass.

Creatinine levels are generally slightly lower during pregnancy.

Is there anything else I should know?

Drugs such as aminoglycosides (gentamicin) can cause kidney damage and so creatinine is monitored. Other drugs, such as cephalosprins (cefoxitin), may increase creatinine concentration without reflecting kidney damage.

Common Questions

1. Will exercise affect my creatinine levels?

   In general, moderate exercise will not affect your creatinine levels. As you continue to exercise and build muscle mass, your creatinine levels may increase slightly, but not to abnormal levels.

2. How does diet affect creatinine levels?

   In general, creatinine levels will not vary with a normal diet. Creatinine levels may be 10%-30% higher in people who eat a diet that is very high in meat.

3. What is creatine? If I take creatine, will my creatinine levels go up?

   Creatine is a compound that is made primarily in the liver and then transported to your muscles, where it is used as an energy source for muscle activity. Once in the muscle, some of the creatine is spontaneously converted to creatinine. The amount of both creatine and creatinine depend on muscle mass, so men usually have higher levels than women. Creatine is now available as a dietary supplement. If you take creatine, your creatinine levels may be higher than when you do not take the supplement. You should tell your doctor about all of the dietary supplements you are taking to help her evaluate your lab results.

4. Do creatinine levels change with age?

   Creatinine levels relate to both muscle mass and to kidney function. As you age, your muscle mass decreases but your kidneys tend to function less effectively. The net result is not much change in creatinine levels in the blood as you get older.

5. What is a BUN/Creatinine ratio?

   Occasionally, a doctor will look at the ratio between a person’s BUN and blood creatinine to help them determine what is causing these concentrations to be higher than normal. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as congestive heart failure or dehydration. It may also be seen with increased protein, from gastrointestinal bleeding, or increased protein in the diet. The ratio may be decreased with liver disease (due to decrease in the formation of urea) and malnutrition.