The Test Sample
What is being tested?
Cystatin C is a relatively small protein that is produced throughout the body by all cells that contain a nucleus and is found in a variety of body fluids, including the blood. It is produced, filtered from the blood by the kidneys, and broken down at a constant rate. This test measures the amount of cystatin C in blood to help evaluate kidney function.
Cystatin C is filtered out of the blood by the glomeruli, clusters of tiny blood vessels in the kidneys that allow water, 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. From this fluid, the kidneys reabsorb 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 then broken down and is not returned to the blood.
The rate at which the fluid is filtered is called the glomerular filtration rate (GFR). A decline in kidney function leads to decreases in the GFR and to increases in cystatin C and waste products such as creatinine in the blood.
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.
Because cystatin C levels fluctuate with changes in GFR, there has been interest in the cystatin C test 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, blood urea nitrogen (BUN), and eGFR (an estimate of the GFR usually calculated from the blood creatinine level). Unlike creatinine, cystatin C is not significantly affected by muscle mass (hence, sex or age), race, or diet, which has led to the idea that it could be a more reliable marker of kidney function and potentially used to generate a more precise estimate of 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. However, testing is becoming increasingly more available and steps are being taken toward standardizing the calibration of cystatin C results.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
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.