Creatinine (Blood)

Serum creatinine, blood creatinine

This is a blood test that measures how well your kidneys work. Filtering and clearing waste products out of your blood are important kidney functions.

Creatinine is a normal waste product that builds up in your blood from using your muscles. Your body makes creatinine at a constant rate all the time, and healthy kidneys remove almost all of this creatinine. By comparing the amount of creatinine in your blood with a standard normal amount, your healthcare provider can get a good idea of how well your kidneys are working.

You may need this test as part of your regular medical checkup. It's often included in routine blood tests to check your overall health.

You may need this test if you have signs or symptoms of kidney disease. Your risk for kidney disease is higher if you are an older adult, have high blood pressure, have a family history of kidney disease, or have diabetes. You may also be at increased risk if you are Black, Hispanic, Asian, Pacific Islander, or American Indian. Signs and symptoms of kidney disease include:

• Frequent tiredness

• Swelling in your feet or ankles

• Poor appetite

• Puffiness around your eyes

• Dry, itchy skin

• Muscle cramps

• Frequent urination

• Painful urination

• Blood or protein in your urine

If you are being treated for kidney disease, you may also need this test to see how well your treatment is working.

Your healthcare provider may use your blood creatinine level, along with your age, sex, and other factors, to calculate your glomerular filtration rate (GFR). The GFR is considered the best measure of kidney function.

Blood urea nitrogen (BUN) is another blood test that's often done with a creatinine test. BUN is a waste product that comes from the digestive process. Healthcare providers also measure it to see how your kidneys are functioning.

You may also have a test that measures the amount of creatinine in your urine.

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

A normal level of creatinine depends on how much muscle mass you have. A normal level for a man is higher than it is for a woman. Children have lower levels than both men and women. Creatinine is measured in milligrams per deciliter (mg/dL). Here are the normal values by gender:

• 0.7 to 1.3 mg/dL for adult males

• 0.5 to 1.1 mg/dL for adult females

If your creatinine is high, it may mean you have:

• Kidney disease

• Blockage in your urinary system

• Muscle disease

• Congestive heart failure

• Diabetes

• Dehydration

• Overactive thyroid gland

• Shock

If your creatinine is low, it may mean you have:

• Muscle loss

• Severe liver disease

• Not enough protein in your diet

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore.

Some factors that could interfere with your creatinine test include:

• Being pregnant

• Eating a lot of meat recently

• Taking large doses of vitamin C

• Taking certain medicines, especially antibiotics

You don't need to prepare for this test. Tell your healthcare provider if you are pregnant. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.