Creatinine

What is creatinine?

Creatinine is a waste product formed by the natural breakdown of creatine phosphate, which the muscles use to produce energy. It is produced in the body every day in a fairly steady amount that is proportional to muscle mass. Creatinine that enters the blood is filtered by the kidneys and excreted in the urine. Its blood level is therefore one of the most widely used indicators of how effectively the kidneys clear the blood.

The test is run on the serum portion of blood drawn from an arm vein, using enzymatic or colorimetric methods. The laboratory often also uses your creatinine result together with your age and sex to calculate the estimated glomerular filtration rate, or eGFR. Because muscle mass directly affects the result, the value may be higher in muscular people and lower in older people with little muscle mass. Since creatinine can stay within normal limits for a long time until a marked loss of kidney function has occurred, doctors consider the result together with other findings.

When is a creatinine test ordered?

Creatinine is one of the biochemistry tests most frequently ordered in routine check-ups. Because kidney diseases often cause no symptoms for a long time, this test helps detect problems early. Your doctor may order the test both for screening and to monitor a known kidney problem. The result also guides the dosing of certain medications that are eliminated through the kidneys. Creatinine is also often checked during an illness that causes fluid loss, such as diarrhea, vomiting or a high fever.

• Regular monitoring of conditions that can affect the kidneys, such as diabetes or high blood pressure
• Swelling in the legs, face or around the eyes, and reduced urine output
• Foamy urine, persistent fatigue, loss of appetite or nausea
• Use of medications that are eliminated by the kidneys or can affect the kidneys
• Assessment before imaging studies involving a contrast agent and before surgery
• Monitoring of chronic kidney disease and of the period after a kidney transplant

How do I prepare for a creatinine test?

Fasting is usually not required for a creatinine test; however, if other tests that require fasting will be run on the same sample, it is best to give blood in the morning on an empty stomach. Because cooked meat can temporarily raise creatinine levels, it is recommended that you do not eat meat in the 12 hours before the test. Heavy exercise can also raise the value by increasing muscle breakdown, so it is a good idea to rest the day before the test.

Creatine supplements used by athletes and certain medications can raise creatinine independently of kidney function. Tell your doctor about the medications and supplements you take, but do not stop any medication on your own. The blood sample is taken from an arm vein within a few minutes, and if you wish, a nurse can come to your home or workplace for this.

What does a creatinine result mean?

A high creatinine level may suggest that the kidneys are not filtering the blood adequately; this can be seen in acute kidney injury or in chronic kidney disease. Fluid loss, a urinary tract obstruction, intense exercise, eating a lot of meat and certain medications can also raise the value. Low creatinine, on the other hand, is mostly associated with low muscle mass and may be seen in older age, prolonged bed rest, malnutrition or pregnancy. How quickly the value changes also matters; a marked rise within a short time may require your doctor to assess you more quickly.

The change over time and the eGFR value often provide more information than a single measurement. Because of differences in muscle mass, men and women have separate reference ranges; ranges can also vary with the laboratory and with age. On this page you can see the laboratory's own reference range. What your creatinine result means should be determined by your doctor, who evaluates it together with your symptoms and other tests.