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 disease and/or have a disease or condition that is known to affect kidney function
Sample Required?
A blood sample taken from a vein in your arm
Test Preparation Needed?
None
The Test Sample
What is being tested?
Cystatin C is a small protein that is produced by nucleated cells. It is produced and destroyed at a constant rate and is found in a variety of body fluids such as blood, spinal fluid, and breast milk.
Cystatin C is filtered out of the blood by the glomeruli which are clusters of 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 walls of the glomeruli 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 other substances . The remaining fluid and wastes are carried to the bladder and excreted as urine. The reabsorbed cystatin C is broken down and is not returned to the bloodstream. Decreases in kidney function lead to decreases in the GFR, and to increases in cystatin C and waste products in the blood.
Because cystatin C levels fluctuate with changes in GFR there has been interest in the cystatin C test being used as one method of evaluating kidney function. Tests currently used include creatinine, a byproduct of muscle metabolism that is measured in the blood and urine, urea or eGFR which is an estimate of the GFR. Unlike creatinine, cystatin C is not significantly affected by muscle mass, gender, age, or race. When the kidneys are functioning normally, concentrations of cystatin C in the blood are stable, but as kidney function deteriorates, the concentrations begin to rise. This increase occurs as the GFR falls and is often detectable before there is a measurable decrease in the GFR. While there are growing data and literature supporting the use of cystatin C, there is still a degree of uncertainty about when and how it should be used.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. However it is not currently used routinely in the UK.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
Cystatin C may be used as an alternative to creatinine and creatinine clearance to look for and monitor kidney dysfunction in those with known or suspected kidney diseases. It may be especially useful in those cases where creatinine measurement is not appropriate: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 be useful in the early detection of kidney disease, when other test results may still be normal and an affected person may have few, if any, symptoms. Researchers are exploring other uses of cystatin C, and the reasons for doctors requesting it may change over time. In addition to kidney dysfunction, it has been associated with an increased risk of cardiovascular disease and heart failure in older adults.
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 reduces the glomerular filtration rate (GFR). Your doctor may request cystatin C to check for early kidney disease 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 hence kidney dysfunction. Since cystatin C is produced throughout the body at a constant rate and removed and broken down by the kidneys, it should remain at a steady level in the blood if the kidneys are working efficiently and the GFR is normal.
Concentrations of cystatin C are not affected by gender, age or race and cystatin C is not affected by most drugs, infections, diet or inflammation. Recent studies suggest that increased levels of cystatin C may also indicate an increased risk of heart disease, heart failure, stroke, and mortality.
Is there anything else I should know?
Cystatin C is not currently routinely available.
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.
