What Is a Creatinine Test?
A creatinine test, also called a serum creatinine test, measures how well your kidneys are working. Creatinine is a waste product from the normal breakdown of muscle tissue. As your body makes it, it's filtered through your kidneys and expelled in urine. Doctors use your creatinine levels to estimate how well your kidneys are filtering the blood, which is called the glomerular filtration rate (GFR).
Normal creatinine levels
Creatinine levels vary depending on your muscle mass and size. Women and people assigned female at birth (AFAB) typically have a lower normal range of creatinine levels than men and people assigned male at birth (AMAB).
- Women/AFAB: 0.6 to 1.1 milligrams/deciliter (53 to 97.2 micromoles/liter)
- Men/AMAB: 0.7 to 1.3 milligrams/deciliter (61.9 to 114.9 micromoles/liter)
Higher than normal levels of creatinine in your blood may indicate problems with your urinary tract or kidneys. Lower levels may indicate conditions that lead to lowered muscle mass.
A GFR score under 60 may mean kidney disease. A creatinine clearance test measures GFR by examining creatinine levels in your urine over 24 hours. Lower results in your age range might indicate poorly functioning kidneys or decreased blood flow to them.
Creatinine and Kidney Function
All theblood in your body flows through your kidneys hundreds of times each day.
The rate at which the kidneys filter blood is called GFR. The kidneys push the liquid part of blood through tiny filters called glomeruli, which are tiny bundles of blood vessels and other cells inside nephrons. Most of the fluid is then reabsorbed back into the blood. Fluid and waste products that the kidneys don't reabsorb are sent out of your body in your urine. Your doctor can’t measure the GFR itself, so that's where serum creatinine comes in.
Your kidneys filter creatinine from the blood into the urine and reabsorb almost none of it. The serum creatinine is then used to estimate the GFR (eGFR) based on a formula.
Why Creatinine Tests Are Done
Doctors use serum creatinine and eGFR to check how well your kidneys work. As kidney function gets worse, GFR also goes down. Your doctor may also order a creatinine test if you have symptoms of kidney disease, including:
- Nausea
- Throwing up
- Loss of appetite
- Tiredness and weakness
- Trouble sleeping
- Changes in how much you pee
- Swollen feet and ankles
Frequently, the test is part of routine laboratory panels, often performed during routine physical exams. Kidney function is also usually checked if you are taking certain medications or if you have conditions such as heart disease, liver disease, diabetes, hypertension, or cancer.
How Is a Creatinine Test Done?
There are two main ways doctors use creatinine tests to measure kidney function:
Urine tests
Your GFR can be estimated by measuring the creatinine level in your pee sample. You may be asked for a sample at your doctor's office. Alternatively, you may be asked to collect your urine over a 24-hour period in a container provided by your health care team for a creatinine clearance test. This method is inconvenient and not commonly used anymore, but it may still be necessary to diagnose some kidney conditions.
eGFR blood test
Doctors can estimate GFR using a single blood sample of creatinine, which they enter into a formula. Different formulas take into account your age and sex, and older formulas may use race/ethnicity. The higher the blood creatinine level, the lower the estimated GFR.
For practical reasons, the blood test method for GFR is used far more often than the 24-hour urine collection test. But urine collections may still be useful in people who have large muscle mass or a significant decrease in muscle mass.
You may need to fast overnight for the eGFR blood test. For both types of tests, you may have to refrain from eating meat and stop taking any creatinine supplements, if you use them, before the test.
Creatinine Levels: What Do They Mean?
A low GFR indicates kidney disease. The decline in kidney function can be either acute (sudden, often reversible) or chronic (long-term and permanent). Repeated GFR measurements over time can tell if your kidney disease is acute or chronic. Kidney function and GFR go down with age. Fortunately, your kidneys have a huge reserve capacity. Most people can lose 30%-40% of their renal function without major problems.
Stages of chronic kidney disease
Doctors figure out the severity of chronic kidney disease with a staging system that uses GFR:
- Stage I: GFR 90 or greater (normal kidney function)
- Stage II: GFR 60-89
- Stage IIIa: GFR 45-59 (mild kidney disease)
- Stage IIIb GFR 30-44 (moderate kidney disease)
- Stage IV: GFR 15-29 (advanced kidney disease)
- Stage V: GFR less than 15 (close to or at kidney failure, often requiring dialysis or kidney transplantation)
Causes of abnormal creatinine levels
People over age 60 may have an apparently normal creatinine blood level but still have a low GFR. The opposite can also be true, where the person may have a mildly low GFR without having kidney disease. The 24-hour urine collection method or another blood test called cystatin C can more accurately identify who has kidney disease and who does not.
Abnormal creatinine test results don’t always mean kidney disease. Sometimes, they can be signs of:
- Hard exercise
- A diet high in red meat
- Side effects of certain medicines
- Increased muscle mass
- Heart failure
- Bladder issues
- Dehydration
What to Do About a Low GFR
If you have a low GFR, your doctor will design an action plan with you to address the problem.
The main causes of chronic kidney disease are high blood pressure and diabetes. If you have these conditions, the first step is to get them under control with improved diet, exercise, and medications. Otherwise, you may need more testing to identify the cause of kidney disease.
If the cause of the kidney disease is unclear, or if the kidney disease is advanced, your doctor will likely refer you to a kidney specialist called a nephrologist.
Checking the GFR regularly allows you and your doctor to monitor any decline in kidney function over time. Your doctor may need to make changes in your medications to adjust for any changes in kidney function.
Because over-the-counter medications (especially those for mild aches, pains, and headaches), herbs, and supplements can all affect your kidneys, don’t take any of these without first discussing them with your doctor.
Most people don’t need dialysis until GFR falls very low (such as below 10). But because kidney function naturally declines with age, it's important to take action early to keep as much as you can.
Takeaways
Creatinine is a waste product your body makes when it uses your muscles for energy. Your kidneys help filter this waste. A blood or urine creatinine test measures how well your kidneys work by measuring their glomerular filtration rate (GFR). Abnormal creatinine test results can sometimes (but not always) mean your kidneys are damaged or diseased.
Creatinine FAQs
What do creatinine and creatinine clearance blood tests measure?
These tests measure your kidney function by examining how much creatinine is in your blood or urine. Abnormal creatine levels may indicate kidney disease.
Why are creatinine tests important for kidney function?
The function of your kidneys is to separate waste from your blood. Creatinine is a type of waste product your body makes when your muscles produce energy. The levels of creatinine in your blood or urine indicate how well your kidneys are functioning.
How is a creatinine clearance test performed?
A creatinine clearance test is one of several that doctors use to measure how well your kidneys are working. With this test, your urine is collected over a 24-hour period instead of once at a doctor's office.
What can abnormal creatinine levels indicate?
High creatinine clearance levels usually signal your kidneys are not working well, but sometimes abnormal results can indicate other health problems such as heart failure, bladder problems, and dehydration. It may also mean that you're eating too much red meat.