Creatinine is a waste product that is found in your blood. Almost all of it is filtered from your blood by your kidneys and passed into your urine to be removed. Measuring creatinine is a good way to find out if your kidneys are working properly and is usually ordered along with other tests for checking kidney function. It is also used to monitor treatment for kidney disease.

Measuring levels of creatinine is often part of a routine blood test if you have non-specific health complaints that your doctor suspects could be to do with your kidneys. The test is also used at regular intervals to monitor treatment for kidney disease.

Creatinine results from the normal wear and tear of your muscles.  It is a by-product of a chemical called creatine that is found in muscle cells and is used to produce the energy needed to contract muscles. Creatine breaks down as a matter of course and becomes creatinine. 

As a waste product creatinine is filtered by your kidneys into the urine and then out of the body through urination.

How your kidneys work

The kidneys are a pair of bean-shaped organs located at the bottom of the rib cage either side of the spine.  

Within them are about a million tiny blood filtering units called nephrons. In each nephron blood is continually filtered through a microscopic cluster of looping blood vessels, called the glomerulus.

The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. 

Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Kidneys can be damaged by a range of health problems with the most common causes being diabetes and high blood pressure. When your kidneys are damaged waste products and fluid are less easily filtered and removed and then build up in your body.

Measuring creatinine
The quantity of creatine you produce depends on your gender, size, and age as well as your muscle mass. For this reason, creatinine concentrations will be slightly higher in men than in women and children.

The eGFR

Results from creatinine tests are used as part of a calculation to estimate the amount of blood filtered per minute by the kidneys. The eGFR (estimated glomerular filtration rate) uses the results of your creatinine blood test together with your age, body size and gender to come up with a number. It is the best way to measure how effectively your kidneys are filtering small molecules like creatinine out of your blood. 

Creatinine and urea, together with the eGFR, are the first line tests used to check how well the kidneys filter waste products from your blood. Your doctor may also order electrolyte tests such as sodium,  potassium and bicarbonate or calcium to help understand how your kidneys are functioning.

Urine creatinine

Urine creatinine is also used with other urine tests as a way to improve their usefulness and assist your doctor with their interpretation. The concentration of urine varies throughout the day with more or less liquid being released in addition to the body's waste products. Since it is produced and removed at a relatively constant rate the amount of creatinine in urine can be compared to the amount of any other substance that is being measured. 

Examples of this are when creatinine is measured with protein to calculate a urine protein/creatinine ratio (UP/CR) and when it is measured with albumin to calculate microalbumin/creatinine ratio (also known as urine albumin/creatinine ratio, UACR). These tests which are corrected for creatinine are a more accurate way of measuring renal function or detecting other urinary tract disorders than just measuring protein or albumin alone. 

Sample

Blood and urine.

Occasionally, in certain circumstances you may also be asked to collect a complete 24-hour urine sample in addition to having your blood taken (see creatinine urine). If this is required your doctor or the laboratory will give you a large container and instructions for properly collecting this sample. You will normally be asked to collect urine as soon as you wake up in the morning until the same time the following day.

Any preparation?
None.

Reading your test report

Your results will be presented along with those of your other tests on the same form.  You will see separate columns or lines for each of these tests.

Increased creatinine levels suggest diseases that reduce kidney function. These can include:

• glomerulonephritis (swelling of the kidney's blood vessels)
• diabetic kidney disease (the filters clog up with a proteinaceous substance)
• pyelonephritis (pus-forming infection of the kidneys)
• acute kidney injury (death of cells in the kidney's small tubes)
• urinary tract obstruction (e.g. prostate disease or kidney stones)
• reduced blood flow to the kidney due to shock, dehydration, congestive heart failure, atherosclerosis, or complications of diabetes.

Reduced levels of creatinine are not common but are not usually a cause for concern. They can be low due to decreased muscle mass (such as in the elderly), and occasionally in advanced liver disease.
 

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people. 
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Creatinine reference intervals

The reference values for this test are common reference intervals which means that all laboratories in Australia should be using this range.

Adult
Male        60-110 µmol/L
Female    45- 90 µmol/L
 

Paediatrics
0 day to <1 week          22-93 µmol/L
1 week - <4 weeks         17-50 µmol/L
4 weeks - <2 years         11-36 µmol/L
2 years - <6 years           20-44 µmol/L
6 years - <12 years         27-58  µmol/L
 

Males
12 years -<15 years        35 - 83 µmol/L
15 years - <19 years       50 - 100 µmol/L
19 years - 60 years       60 - 110 µmol/L
 

Females
12 years to <15 years     35 - 74 µmol/L
15 years to <19 years     38 - 82 µmol/L
19 years to 60 years     45 - 90 µmol/L

eGFR results

• Your eGFR is reported in millilitres per minute per 1.73m2 (mL/min/1.73m2).
• A value over 60 mL/min/1.73m2 indicates your kidney function is normal.
• A value below 60 mL/min/1.73m2 suggests some loss of kidney function. Your doctor will most likely want to confirm this result by repeating the blood test.

The choice of tests your doctor makes will be based on your medical history and symptoms.   It is important that you tell themeverything you think might help. 

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely. 

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

Eating large amounts of meat can cause temporary increases in blood creatinine levels. Taking creatine supplements can also increase creatinine.

A few medications interfere with the creatinine test and you will need to talk to your doctor about these before you have the test.

Some medications can cause some impairment in kidney function and you may need to have your creatinine levels monitored if you are taking one of these.

Generally, moderate exercise will not affect your creatinine levels. As you continue to exercise and build muscle mass your creatinine levels may increase slightly but not to abnormal levels.

Creatinine levels relate to both muscle mass and to kidney function. As you age, your muscle mass decreases but your kidneys tend to function less effectively.

The net result is that there is not much change in creatinine levels in the blood as you get older.

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access  your results whenever and wherever needed.

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