Cystatin C
At a Glance
Why Get Tested?
Currently not routinely available but in the future, may serve as an alternative to creatinine to monitor kidney function in those with known or suspected kidney diseases
When to Get Tested?
It may prove useful if you have symptoms of kidney dysfunction and/or have a disease or condition that is known to affect kidney function
Sample Required?
A blood sample drawn from a vein in your arm
The Test Sample
What is being tested?
Cystatin C is a small protein that is produced by nucleated cells (those with chromosomes and genetic material at their centre, which is most of the body's cells). It is produced at a constant rate and is found in a variety of body fluids such as serum, spinal fluid and breast milk.
Cystatin C is filtered out of the blood by the glomerulus - tiny blood vessels in the kidneys that allow water and dissolved substances and wastes to pass through their walls while retaining blood cells and larger proteins. What passes through the glomerulus forms a filtrate fluid. The rate at which this fluid is formed is called the glomerular filtration rate (GFR). The kidney reabsorbs Cystatin C, glucose and many other substances from the filtrate, while allowing waste products to be carried with the fluid to the bladder and eventually out of the body as urine.
All the reabsorbed cystatin C is completely broken down and none reappears in the blood so this makes it very useful for measuring the glomerular filtration rate.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
However, currently cystatin C is not routinely tested in Australia.
The Test
How is it used?
Cystatin C may be used as an alternative to creatinine and creatinine clearance to screen for and monitor kidney dysfunction in those with known or suspected kidney diseases. It may be especially useful in detecting early decreases in kidney function (as measured by GFR) and in those for which creatinine measurement is a problem: for instance in those who have liver cirrhosis, are very obese, malnourished or have a very small muscle mass (creatinine is produced in the muscles). It may also have a role along with other tests, such as urine microalbumin to monitor kidney function over time in those with diseases such as diabetes.
When is it requested?
When available cystatin C may be requested when you have a known or suspected disease that affects or potentially affects kidney function and GFR. Your doctor requests cystatin C to check for early kidney dysfunction and/or to monitor known impairment over time. It may be requested by itself or be done along with other tests such as urine microalbumin.
Early kidney impairment may not cause elevated creatinine levels or symptoms. If your doctor is not satisfied with your creatinine clearance test or feels that your GFR would be better estimated by another test, cystatin C may prove to be a useful alternative.
What does the test result mean?
An elevated serum cystatin C indicates a low GFR and kidney dysfunction. Since cystatin C is produced throughout the body at a constant rate and removed by glomerular filtration (and subsequently broken down in another part of the kidney), it should remain at a steady state in the blood if the kidneys are working efficiently and the GFR is normal.
Cystatin C levels are not affected by muscle mass and age like creatinine but can be affected by other factors in some patients. In Australia, the cystatin C test is rarely used because it is expansive to perform. Instead the eGFR test is widely used. This is a calculation that compensates for the effects of age and muscle mass on serum creatinine levels.
Is there anything else I should know?
Currently cystatin C is not routinely available in Australia.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
