• Cystatin C is a waste product that’s made by almost all cells in the body and released into the blood at a steady rate.
• It is filtered from your blood by your kidneys and then broken down and reabsorbed by the body
• There should be very low levels of cystatin C in your blood if your kidneys are working normally.
• Measuring levels of cystatin C in your blood can help show how well your kidneys are working in some people.

Cystatin C is a waste product continuously made by almost all your body’s cells as part of daily life. After it enters the bloodstream, the body does not reuse it and it is filtered out of your blood by your kidneys.

You have a pair of kidneys located just at the bottom of your ribcage on either side of your spine. Inside are about a million tiny blood filtering units called the glomerulus. These filter your blood and remove wastes and excess fluid from your body, which are disposed of in your urine. If the kidneys are not working properly, waste products can build up.

Cystatin C is broken down by your kidneys and there should be only very low levels in your blood if your kidneys are working properly.

Measuring levels of cystatin C in your blood can show how well your kidneys are working.

Cystatin C testing is used as an alternative to creatinine and creatinine clearance testing which have been used for many decades to assess kidney function. Creatinine is another substance filtered by your kidneys. It comes 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 make muscles flex. Creatine breaks down naturally over time and becomes creatinine. The more muscle someone has, the more creatinine they produce.

Unlike creatinine, cystatin C levels are not affected by muscle mass. This makes the test especially useful in people with low muscle mass such as children, the elderly and those who are frail as well as people who have large muscle mass such as body builders.

Like creatinine, the results of the cystatin C test can be used as part of a calculation called the glomerular filtration rate (GFR). The GFR is an estimate of how much blood your kidneys filter each minute.

The eGFR is a calculation that uses blood creatinine or cystatin C (or both) together with your age and gender. It is considered the most accurate estimate of kidney function.

When your doctor orders a cystatin C test, the laboratory will most likely automatically calculate:

• eGFRcys — estimated kidney filtration rate using cystatin C alone, and
• eGFRcr cys — a combined estimate using both creatinine and cystatin C.

Cystatin C is a newer, more refined test that has been shown to improve accuracy, especially when used together with creatinine. However, cystatin C is not used routinely because it requires sophisticated testing techniques and it is performed by only a few laboratories. It is not rebated by Medicare and is billed privately. Having the test can involve an out-of-pocket fee.

Nevertheless, under certain conditions a specialist doctor (such as a renal physician or endocrinologist) may request the test:

• In people who have lower muscle mass including the elderly, children (creatinine testing is unreliable in children because muscle mass and growth vary widely)
• In people who have higher muscle mass including body builders and those with severe obesity
• In transgender people who are taking hormone treatment
• To confirm a diagnosis when creatinine and eGFR test results are uncertain or borderline
• When someone has a health condition that affects creatinine levels, including liver disease, malnutrition and long-term corticosteroid use (steroids can cause a slight increase in cystatin C levels)
• To help in making careful decisions about management and treatment in someone who is seriously ill.

Sample

Blood.

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.

Cystatin C results are used with creatinine results to give a more accurate estimate of kidney filtration than either test alone.

Both are used to calculate the amount of blood filtered by your kidneys per minute. This is called the eGFR or the estimated glomerular filtration rate. eGFR is the main screening and monitoring test for kidney function. For more on how the eGFR is calculated see eGFR.

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.

Reference intervals for cystatin C

Reference intervals for cystatin C vary slightly between laboratories in Australia because they use different measurement instruments. Always check your specific laboratory’s ranges.

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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