Creatinine

You do not need to do anything special before a creatinine blood test. A healthcare professional will take a small blood sample from a vein in your arm. 

If your doctor has also requested a urine creatinine test, they will let you know what type of sample is needed. This may be a single urine sample (called a spot sample) or, less commonly, a 24-hour urine collection. For a 24-hour collection, you will need to collect all your urine over a full day. Your doctor or nurse will give you instructions. 

Eating meat just before your test may temporarily raise your creatinine level. Ideally, meat should be avoided for at least 12 hours before the test. Tell your doctor if you take creatine supplements, as these can also affect the result. 

What the test measures 

Creatinine is a waste product made naturally in your muscles. It comes from the normal breakdown of a substance called creatine, which your muscles use for energy. Your body produces creatinine at a steady rate every day. 

Healthy kidneys filter creatinine out of the blood and remove it in your urine. If your kidneys are not working well, creatinine builds up in the blood. This is why a blood creatinine level is a useful measure of kidney function. 

Your creatinine level depends on your muscle mass. People with more muscle tend to have higher creatinine levels. For this reason, men usually have slightly higher levels than women, and levels are lower in children and older people who have less muscle mass. 

What your results mean 

Your result will be compared with a reference range shown on your lab report. This range varies depending on your age, sex and the laboratory that analysed your sample. Your doctor will interpret your result alongside your symptoms, medical history and other test results. 

If your creatinine is high 

A raised creatinine level suggests your kidneys may not be filtering waste from your blood as efficiently as they should. The higher the level, the more likely it is that kidney function is reduced. 

Common causes of a raised creatinine include: 

• Dehydration: not drinking enough fluids reduces blood flow to the kidneys. This is a common and temporary cause of raised creatinine. 
• Reduced blood flow to the kidneys: this can happen with heart failure, low blood pressure or severe illness. 
• Kidney disease: conditions that damage the kidneys include glomerulonephritis (inflammation of kidney filtering units), pyelonephritis (kidney infection) and acute tubular necrosis (damage to kidney tubule cells). 
• Urinary tract blockage: kidney stones, prostate problems or other blockages can prevent the kidneys from draining properly. 
• Diabetes and high blood pressure: over time, these can damage blood vessels in the kidneys. 
• Muscle injury: a condition called rhabdomyolysis (rapid breakdown of muscle tissue) releases large amounts of creatinine and other substances into the blood. This can happen after a major injury, extreme exercise or certain medicines, and can damage the kidneys if severe. 

A single raised result does not always mean serious kidney disease. Your doctor will look at whether the level is mildly or significantly raised, how quickly it has changed, and what other tests show. They will work with you to find the cause. 

If your creatinine is low 

A low creatinine level is less common and is not usually a cause for concern on its own. 

It can occur in people with reduced muscle mass, such as older adults, or people who follow a very low-protein diet. It may occasionally be seen in advanced liver disease. Creatinine levels are also naturally slightly lower during pregnancy. 

If you have had a urine creatinine test 

A urine creatinine result on its own does not have a standard reference range. It is most useful when compared with another substance in the same urine sample. For example protein, to calculate the protein-to-creatinine ratio (UPCR), or albumin, to calculate the albumin-to-creatinine ratio (ACR). These help detect early signs of kidney damage. Your doctor will explain what your result means in this context. 

If your creatinine is within the normal range, no further action is usually needed. Your doctor may repeat the test at a future appointment as part of routine monitoring. 

If your creatinine is raised, your doctor will consider the likely cause. They may repeat the test after a few days, particularly if dehydration or a temporary illness could be responsible. If the level remains raised, they may refer you to a kidney specialist (nephrologist) or arrange further tests. 

If you are already being treated for kidney disease, your doctor will use your creatinine and eGFR results to decide whether to continue, change or adjust your treatment. 

• Muscle mass: people with more muscle tend to have naturally higher creatinine levels. Athletes and people who do a lot of strength training may have results at the higher end of the normal range. 
• Diet: eating meat can temporarily raise creatinine by 10–30%. Ideally, meat should be avoided for at least 12 hours before taking a blood sample for creatinine. 
• Creatine supplements: taking creatine supplements can raise creatinine levels. Tell your doctor if you take them. 
• Medicines: some medicines can interfere with the creatinine test or affect kidney function. Your doctor will take this into account. 
• Age: creatinine levels tend to stay fairly stable with age. Although muscle mass falls as you get older, kidney function also declines, so the two effects roughly balance each other out. 
• Pregnancy: creatinine levels are naturally slightly lower during pregnancy.

Your doctor will often request creatinine alongside other tests to build a fuller picture of your kidney health. 

The eGFR is calculated directly from your creatinine result and is usually reported at the same time. It gives a more complete picture of how well your kidneys are filtering your blood. 

Other tests that may be requested include urea (another waste product filtered by the kidneys), electrolytes such as sodium, potassium, and calcium. A urine albumin-to-creatinine ratio (ACR) or urine protein-to-creatinine ratio (UPCR) may also be measured to check for signs of early kidney damage. 

In some cases your doctor may request a creatinine clearance test or a cystatin C test. These give additional information about kidney function.

Kidney disease means the kidneys are not working as well as they should. It can be short-term (acute) or long-term (chronic). Many people with early kidney disease have no symptoms. Early detection and treatment can slow its progress.