• Creatinine is a waste product filtered from your blood by your kidneys.
• If your kidneys are damaged, they allow too much creatinine to pass through.
• 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.
• Results from creatinine tests are used to estimate the amount of blood filtered per minute by the kidneys. This is the eGFR (estimated glomerular filtration rate).
• Creatinine is also often measured along with albumin, a protein in your blood.
• A microalbumin-creatinine ratio test compares the amount of albumin to the amount of creatinine.
• This is an accurate way to detect the early stages of kidney damage.

Creatinine is a waste product that is found in your blood. It comes from the digestion of protein in your food and the normal breakdown of muscle tissue. 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 general health concerns 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 which is used to produce the energy needed to make muscles flex. Creatine breaks down naturally over time and becomes creatinine. This process happens at a relatively constant rate, depending on muscle mass.
The more muscle someone has, the more creatinine they produce.

Creatinine is filtered by your kidneys into the urine, along with other substances your body no longer needs, and then leaves your body when you urinate.

People with greater muscle mass will naturally produce more creatinine which means a body builder can be expected to have higher creatinine levels than normal. 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.

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 water and small molecules to leak out and be filtered from the body by the kidneys, but retains blood cells and larger molecules.

Attached to each glomerulus is a tiny tube (tubule). These tubules collect 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 amount of creatine you produce depends on your gender, size and age as well as your muscle mass. For this reason, the amount of creatinine in your blood 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 – called the eGFR (estimated glomerular filtration rate). The eGFR 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 well 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 to improve their usefulness and assist your doctor with their interpretation. The amount of waste products in urine varies throughout the day. Since creatinine is produced and removed at a relatively constant rate the amount 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 kidney function or detecting other urinary tract issues than just measuring protein or albumin alone.

Sample
Blood and urine.

Urine: Sometimes, your doctor may ask you 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?

A few medications interfere with the creatinine test, and you need to talk to your doctor about these before you have the test. Your doctor may advice you not to eat large amounts of meat and not take creatinine supplements before your test, as these can both cause short-term increases in blood creatinine levels. Moderate exercise will normally not affect your creatinine levels.

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.

Reference intervals - comparing your results to the healthy population

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

• Reference intervals are the range of results expected in healthy people.
• They are used to provide a benchmark for interpreting a patient's test results.
• When compared against them, your results may be flagged high or low if they sit outside this range.
• Some reference intervals are harmonised or standardised, which means all labs in Australia use them.
• Others are not because for these tests, labs are using different instruments and chemical processes to analyse samples.
• Always compare your lab results to the reference interval provided on the same report.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation.

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything 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?

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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