Cystatin

What is being tested?
Cystatin C is an inhibitor of cysteine proteases, a small molecule produced by nucleated cells (those with chromosomes and genetic material inside them, which are the most common cells in the body). It is produced in a constant amount and is found in various body fluids such as serum, spinal fluid and breast milk.

Cystatin C is filtered by the glomeruli, tiny blood vessels in the kidneys that let water, substances dissolved in it and waste products pass through while holding back large molecules and blood cells. What passes through the walls of the glomeruli forms the filtrate. The body reabsorbs cystatin C, glucose and other substances from the filtrate, while carrying waste products to the bladder and out of the body as urine. The amount of filtrate produced is called the glomerular filtration rate (GFR).

GFR can be determined in several ways and is used to assess how well the kidneys are working. In the past the most common method for determining GFR was creatinine clearance, a test that compares the creatinine in the blood with the creatinine in a 24-hour urine collection. This procedure was widely used, but was not as accurate as it should have been, and it is being replaced by others. The American Kidney Foundation recommends using serum creatinine together with other information such as sex, age, weight and (in one formula) race in one of several formulas to calculate an estimated GFR (eGFR). Cystatin C rises when GFR falls; high levels indicate a low GFR. Although data and literature supporting the use of cystatin C are growing, there are still many points of disagreement. The clinical usefulness of this parameter has yet to be established

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
Cystatin C can be used as an alternative to creatinine and creatinine clearance to assess and monitor kidney dysfunction in people suspected of having kidney disease. It may be useful for detecting an early decline in kidney function (as measured by eGFR) and in people in whom creatinine measurement is problematic: people with cirrhosis, obesity or malnutrition, or with reduced muscle mass (creatinine is produced in muscle).

When is it requested?
Cystatin C is not yet universally used or accepted, but it may be requested when there is, or is suspected to be, a disease that affects or may affect kidney function and GFR. The doctor requests cystatin C when looking for early signs of kidney dysfunction or to monitor known diseases.

Early damage may not cause raised creatinine or symptoms. If the doctor is not satisfied with creatinine clearance or thinks GFR needs to be estimated more accurately, they may request cystatin C as an alternative.

What does the test result mean?
The doctor will request this test to see whether cystatin C is raised. High serum cystatin C corresponds to a low GFR and kidney dysfunction. Since cystatin C is produced at a constant rate in the body and removed by glomerular filtration (and then broken down elsewhere in the kidney), it should remain in a steady state if the kidneys are working efficiently and GFR is normal. Cystatin C may be raised in malignant or rheumatic diseases, and some medicines can affect its measurement: corticosteroids can raise its level, while ciclosporin can lower it. Its measurement is not normally affected by most medicines, infections, diet, inflammation or multiple tumours. Sex, age (except in the very young or very old) and race do not seem to interfere either.

Is there anything else I should know?
Since the cystatin C test has only recently become commercially available and its use is still debated, it is available in a limited number of laboratories. If your doctor decides to request this test, your blood sample will probably need to be sent to a specialist laboratory, which will take a few days.