Creatinine and Creatinine Clearance Tests

Test Overview

Creatinine and creatinine clearance tests measure the level of the waste product creatinine (say "kree-AT-uh-neen") in your blood and urine. These tests tell how well your kidneys are working.

Another substance, creatine (say "KREE-uh-teen"), is formed when food is changed into energy through a process called metabolism. Creatine is broken down into creatinine. Your kidneys take creatinine out of your blood and pass it out of your body in urine.

If your kidneys are damaged and can't work as they should, the amount of creatinine in your urine goes down while its level in your blood goes up.

Three types of tests can be done.

Blood creatinine level

The blood creatinine level shows how well your kidneys are working. A high level may mean that your kidneys aren't working as they should. The amount of creatinine in the blood depends partly on the amount of muscle tissue you have. Men generally have higher creatinine levels than women.

Creatinine clearance

A creatinine clearance test measures how well creatinine is removed from your blood by your kidneys. This test gives better information than a blood creatinine test on how well your kidneys are working. The test is done on both a blood sample and on a sample of urine collected over 24 hours.

Blood urea nitrogen-to-creatinine ratio

This test measures the amount of urea in your blood. Urea is a waste product made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine.

The levels of blood creatinine and blood urea nitrogen (BUN) can be used to find the BUN-to-creatinine ratio. This ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.

Why It Is Done

These tests are done:

• To see if your kidneys are working normally.
• To find out if your kidney disease is changing.
• To see how well the kidneys work in people who take medicines that can cause kidney damage.
• To check for severe dehydration. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blocked urine flow from your kidney causes both BUN and creatinine levels to rise.

How To Prepare

You may be asked to:

• Not do any strenuous exercise for 2 days (48 hours) before having the tests.
• Not eat more than 8 ounces of meat, especially beef, or other protein for 24 hours before the blood creatinine test and during the creatinine clearance urine test.
• Drink plenty of fluids if you are asked to collect your urine for 24 hours. But don't drink coffee or tea. These are diuretics that cause your body to pass more urine than normal.

If you are asked to collect urine, your doctor will give you a large container that holds about 1 gallon. You will use the container to collect your urine for 24 hours.

Tell your doctor ALL the medicines, vitamins, supplements, and herbal remedies you take. Some may increase the risk of problems during your test. Your doctor will tell you if you should stop taking any of them before the test and how soon to do it..

How It Is Done

Blood tests

A health professional uses a needle to take a blood sample, usually from the arm.

Timed urine test

You collect your urine for a period of time, such as over 4 or 24 hours. Your doctor will give you a large container that holds about 1 gallon. You will use the container to collect your urine.

• When you first get up, you empty your bladder.

  But don't save this urine. Write down the time you began.

• For the set period of time, collect all your urine.

  Each time you urinate during this time period, collect your urine in a small, clean container. Then pour the urine into the large container. Don't touch the inside of either container with your fingers.

• Don't get toilet paper, pubic hair, stool (feces), menstrual blood, or anything else in the urine sample.
• Keep the collected urine in the refrigerator for the collection time.
• Empty your bladder for the last time at or just before the end of the collection period.

  Add this urine to the large container. Then write down the time.

How long the test takes

The urine test will take 24 hours. The blood test will take a few minutes.

How It Feels

Blood test

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Urine test

This test usually doesn't cause any pain or discomfort.

Risks

Risks of a blood test

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Urine test

There are no known risks from having this test.

Results

These tests give information about how well your kidneys are working. The creatinine clearance value is found from the amounts of creatinine in the urine and blood and from the amount of urine you pass in 24 hours. This value is the amount of blood cleared of creatinine per minute, based on your body size.

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

Blood creatinine, creatinine clearance, and BUN-to-creatinine ratios can be measured.

High values

• High creatinine blood levels can be caused by:
  • Serious kidney damage or chronic kidney disease. Kidney damage can be caused by a life-threatening infection, shock, cancer, or low blood flow to the kidneys.
  • Dehydration.
  • Muscle injury and conditions. These include crush injuries, burns, rhabdomyolysis, muscular dystrophy, polymyositis, and strenuous exercises.
  • Shock. This is low blood pressure with many causes including severe bleeding and very severe infection.
• High creatinine clearance levels can be caused by carbon monoxide poisoning and pregnancy.
• High BUN-to-creatinine ratios occur with sudden (acute) kidney problems. This may be caused by shock or severe dehydration. A very high BUN-to-creatinine ratio may be caused by bleeding in the digestive tract.

Low values

• Low blood creatinine levels can mean lower muscle mass caused by a disease, such as muscular dystrophy, or by aging. Low levels can also mean some types of severe liver disease or a diet very low in protein. Pregnancy can also cause low levels.
• Low creatinine clearance levels can mean you have chronic kidney disease or serious kidney damage. Kidney damage can be from conditions such as a life-threatening infection, shock, cancer, low blood flow to the kidneys, or urinary tract blockage. Other conditions, such as heart failure and dehydration, can also cause low clearance levels.
• Low BUN-to-creatinine ratios may be linked with a diet low in protein, a severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, or the use of certain medicines.