A creatinine blood test measures the level of creatinine in the blood. Creatinine is a waste product that forms when creatine, which is found in your muscle, breaks down. This test tells your doctor how well your kidneys are working.
Each kidney has millions of small blood-filtering units called nephrons. The nephrons constantly filter blood through a very tiny cluster of blood vessels known as glomeruli. These structures filter waste products, excess water, and other impurities out of the blood. The toxins are stored in the bladder and then removed during urination.
Creatinine is one of the substances that your kidneys normally eliminate from the body. Doctors measure the level of creatinine in the blood to check kidney function. High levels of creatinine may indicate that your kidney is damaged and not working properly.
Creatinine blood tests are usually performed along with several other laboratory tests, including a blood urea nitrogen (BUN) test and a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). These tests are done during routine physical exams to help diagnose certain diseases and to check for any problems with your kidney function.
Your doctor may order a creatinine blood test to assess your creatinine levels if you show signs of kidney disease. These symptoms include:
- fatigue and trouble sleeping
- a loss of appetite
- swelling in the face, wrists, ankles, or abdomen
- lower back pain near the kidneys
- changes in urine output and frequency
- high blood pressure
Kidney problems can be related to different diseases or conditions, including:
- glomerulonephritis, which is an inflammation of the glomeruli due to damage
- pyelonephritis, which is a bacterial infection of the kidneys
- prostate disease, such as an enlarged prostate
- blockage of the urinary tract, which may be due to kidney stones
- decreased blood flow to the kidneys, which may be caused by congestive heart failure, diabetes, or dehydration
- the death of kidney cells as a result of drug abuse
- streptococcal infections, such as poststreptococcal glomerulonephritis
Aminoglycoside medications, such as gentamicin (Garamycin, Gentasol), can also cause kidney damage in some people. If you’re taking this type of medication, your doctor may order regular creatinine blood tests to make sure your kidneys remain healthy.
A creatinine blood test doesn’t require much preparation. Fasting isn’t necessary. You can and should eat and drink the same as you do normally to get an accurate result.
However, it’s important to tell your doctor about any prescription or over-the-counter (OTC) medications you’re currently taking. Some drugs may increase your creatinine levels without causing kidney damage and interfere with your test results. Let your doctor know if you take:
- cimetidine (Tagamet, Tagamet HB)
- nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin (Bayer) or ibuprofen (Advil, Midol)
- chemotherapy drugs
- cephalosporin antibiotics, such as cephalexin (Keflex) and cefuroxime (Ceftin)
Your doctor may ask you to stop taking your medication or to adjust your dosage before the test. They’ll also take this into consideration when interpreting your test results.
The creatinine blood test is a simple test that requires the removal of a small sample of blood.
A healthcare provider first asks you to pull up your sleeves so that your arm is exposed. They sterilize the injection site with an antiseptic and then tie a band around your arm. This makes the veins swell with blood, allowing them to find a vein more easily.
Once they find a vein, they insert a needle into it to collect the blood. In most cases, a vein on the inside of the elbow is used. You might feel a slight prick when the needle is inserted, but the test itself isn’t painful. After the healthcare provider removes the needle, they put a bandage over the puncture wound.
A creatinine blood test is a low-risk procedure. However, there are some minor risks, including:
- fainting at the sight of blood
- dizziness or vertigo
- soreness or redness at the puncture site
Once enough blood is drawn, the sample is sent to a laboratory for analysis. Your doctor will give you the results within a few days of testing.
Creatinine is measured in milligrams per deciliter of blood (mg/dL). People who are more muscular tend to have higher creatinine levels. Results may also vary depending on age and gender.
In general, however, normal creatinine levels range from 0.9 to 1.3 mg/dL in men and 0.6 to 1.1 mg/dL in women who are 18 to 60 years old. Normal levels are roughly the same for people over 60.
High serum creatinine levels in the blood indicate that the kidneys aren’t functioning properly.
Your serum creatinine levels may be slightly elevated or higher than normal due to:
- a blocked urinary tract
- a high-protein diet
- kidney problems, such as kidney damage or infection
- reduced blood flow to the kidneys due to shock, congestive heart failure, or complications of diabetes
If your creatinine is truly elevated and it’s from an acute or chronic kidney injury, the level won’t decrease until the problem is resolved. If it was temporarily or falsely elevated due to dehydration, a very high-protein diet, or supplement usage, then reversal of those conditions will lower the level. Also, a person receiving dialysis will have lower levels after a treatment.
It’s uncommon to have low levels of creatinine, but this can occur as a result of certain conditions that cause decreased muscle mass. They’re usually not any cause for concern.
It’s important to note that normal and abnormal ranges can vary among labs because some use unique measurements or test different samples. You should always meet with your doctor to discuss your test results in more detail. They’ll be able to tell you if more testing is necessary and if any treatment will be required.