What is Cystatin C?

1 Answer

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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. 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 other measures of kidney function, such as creatinine and urea in the blood. The increases in these levels occur because the kidneys are not able to properly filter the blood at a normal rate, causing their accumulation in the blood. On the other hand, improvement in kidney function is expected to lead to increases in GFR, which would cause cystatin C, creatinine, and urea to decline as a result of the kidneys being able to effectively clear them from the blood. 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. To test Cystatin C, blood sample is taken from the arm vein, no fasting is required.
A high level of cystatin C in the blood corresponds to a decreased glomerular filtration rate (GFR) and hence to 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. Recent studies suggest that increased levels of cystatin C may also indicate an increased risk of heart disease, heart failure, and mortality.
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