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Creatinine is made in the muscles as a breakdown product of muscle wear and tear and then filtered through the kidneys and put out into the urine. A serum creatinine test measures the level of creatinine in the blood.

The level of creatinine in the blood is a measure of overall kidney function. When the kidneys are not working well for any reason, they are not able to excrete creatinine into the urine. When this happens, the level of creatinine in the blood rises.

Who is a candidate for the test?

A candidate for the test is anyone whose kidneys a healthcare professional suspects may not be working well. The test is also included in some screening blood chemistry panels that are part of a general medical evaluation.

How is the test performed?

To perform a serum creatinine test, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube called a tourniquet is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.

A needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding. The blood sample is sent to the laboratory to determine the amount of creatinine circulating in the blood.

More recently, tests like the Cockgroft-Gault or the MDRD equation use formulas the incorporate the blood creatinine value with other numbers such as age and weight to get a more accurate estimate of kidney function.

What is involved in preparation for the test?

Usually, no preparation is needed for this test. A person should ask the healthcare professional for specific instructions.

What do the test results mean?

The normal level of creatinine varies slightly based on age, body size, and gender. The level also changes during pregnancy. However, the healthy range is usually between 0.5 and 1.4 milligrams per deciliter (mg/dL).

A decreased value for this test is rarely a concern. It can occur with decreased muscle mass, such as in elderly people, or in conditions such as muscular dystrophy, an inherited defect in muscles test. Pregnancy may also cause a lower value because the kidneys work harder in pregnancy .

A high value for this test can occur for many reasons. Some of these reasons include:

Decreased blood flow to the kidneys - because of the following conditions or events:

• severe dehydration or fluid losses
• massive blood loss
• heart failure
• blockage in the kidney arteries, called renal artery stenosis

Kidney damage - due to:

• high blood pressure
• diabetes
• an inherited condition called polycystic kidney disease
• a cancer called multiple myeloma
• autoimmune diseases, which are conditions in which a person's immune system attacks his or her own body (such as lupus)

Other conditions can also cause an elevated value for this test:

• blockage of urine flow, which occurs with enlargement of the prostate, also called benign prostatic hyperplasia
• taking certain medicines, such as captopril or non-steroidal anti- inflammatory drugs (NSAIDs)

The meaning of the results should be discussed with the healthcare professional.