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This test measures the amount of creatinine in a urine sample that may either be from a single sample of urine or from a 24 hour collection.

Creatinine is made in the muscles as a breakdown product of muscle wear and tear, then is filtered through the kidneys and put out into the urine. The amount of creatinine in the urine when combined with the value in blood (called creatinine clearance) can help measure of how well the kidneys are functioning.

Who is a candidate for the test?

The collection is performed on 2 types of people: as a measure of how well the kidneys are working (as a clearance) or as a measure of the adequacy of a urine collection that is used to check other products in the same sample (like calcium or uric acid in someone who has kidney stones).

How is the test performed?

Either a 24-hour urine sample or a single ("spot") sample may be done. For a spot sample, the person urinates and collects a small sample in a container. The container is sealed and given to the healthcare provider.

For a 24-hour urine sample, the person should follow the specific instructions of the healthcare professional.

In general, this or a similar schedule is followed:

• Day 1: The person urinates upon arising without collecting that sample (for example at 7 a.m.). After that, he or she collects all of his or her urine for the next 24 hours in a special container.
• Day 2: First thing in the morning (for example, again at 7 a.m.), the person urinates into the container again. Then the individual covers it and refrigerates it. The sample is brought to the healthcare professional, who will send it to the lab for analysis.

What is involved in preparation for the test?

The person should request specific instructions from the healthcare professional. If the test is done as part of a study for kidney stones, it may include a special diet.

What do the test results mean?

• spot collection, 25 to 400 milligrams/deciliter (mg/dL)
• 24-hour sample, 0 to 0.25 grams per 24 hours

The amount of creatinine excreted in the urine is usually a constant for a person at a given weight. If kidney damage occurs, the creatinine in the blood changes by increasing. So, in order to interpret a urine creatinine, calculations also using a blood creatinine are necessary. (see kidney function tests and creatinine clearance)