What is being tested?
Cystatin C is a cysteine proteinase inhibitor, a small molecule that is produced by nucleated cells (those with genetic material at their center, which is most of the body’s 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, dissolved substances and wastes to pass through their walls while retaining blood cells and larger proteins. What passes through the glomerulus walls forms a filtrate fluid. The body then reabsorbs Cystatin C, glucose, and some other substances from the filtrate, while allowing the remaining substances to be carried with the fluid to the bladder and eventually out of the body as urine. The Cystatin C that is reabsorbed when the glomerular filtrate is formed is then broken down and is not returned to the blood. When the rate at which the fluid filtrate is formed is reduced, indicating decreased kidney function, blood levels of substances removed by them (such as Cystatin C) increase and are an indication of how well a patient’s kidneys are functioning.

There has been interest in the Cystatin C test as one method of evaluating kidney function. It is not affected by muscle mass, gender, age, or race unlike creatinine. 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 detectible 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.