Cystatine C

AsseyMethod: Colorimetry
Abbrevation: Cystatine C
Sector: Biochemistry
SampleType: S
S.Vol: 0.5
Transport: at 2-8˚c, -20˚c
Storage: 2 days at 2-8˚c for longer time at -20˚
Test Name: Cystatine C
Normal Range: Male:0.7-1.6 Female: 0.61-1.22

This test is related to
Why Get Tested?

To assess your kidney function if you have known or suspected kidney disease, but only in specific circumstances when traditional kidney function tests are misleading. It is used to calculate how well your kidneys are able to filter waste products from the blood.

When To Get Tested?

When your doctor suspects that you may have decreased kidney function but blood creatinine measurements are not reliable because of circumstances specific to you. It may be measured at intervals over a longer time to monitor your kidney function.

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

None

What is being tested?

In healthy people, blood concentrations of cystatin C remain relatively constant throughout life. If kidney function declines, the blood concentration rises.

Cystatin C is a small protein that is produced by all cells that contain a nucleus (i.e. the majority of cells in the body). In healthy people, 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 kidneys reabsorb cystatin C, glucose and other substances from this fluid. The remaining fluid and wastes are carried to the bladder and excreted as urine. The reabsorbed cystatin C is broken down in the kidney cells and is not returned to the blood.

The rate at which the filtrate fluid is formed is called the Glomerular Filtration Rate (GFR), which is an important determinant of kidney function. A decline in kidney function leads to a decrease in the GFR, so less cystatin C is cleared from the blood and the concentration rises.

Rarely, high blood concentrations of cystatin C are the result of increased production by cells rather than a kidney problem, for example in people with an overactive thyroid, with some cancers or in people taking steroid medication.

Because cystatin C concentrations fluctuate with changes in GFR, the cystatin C test can be used to evaluate kidney function and estimate the GFR. More commonly used blood tests include creatinine, a by-product of muscle metabolism, and urea, a by-product of protein breakdown. Urea is the least reliable of these tests as it is affected by diet and many other diseases, not just kidney disorders.  Creatinine is the most widely used but is affected by muscle mass, how much meat you eat, age, race and gender. Cystatin C is much less affected by these. In people with very high or low muscle bulk, estimates of GFR based on measurements of creatinine concentration will not be reliable, in contrast to those based on cystatin C.

When the kidneys are functioning normally, the concentration of cystatin C in the blood is stable, but as kidney function deteriorates, the concentration begins to rise, often before that of creatinine. This increase occurs as the GFR falls and is usually detectable before a person has any symptoms of kidney disease.

Creatinine and cystatin C can be used alone or in combination to calculate an estimated GFR. The calculations use race, age and gender as well as the concentrations measured by the blood test. This calculated estimate of GFR is a more accurate measure of kidney function than the blood concentrations of cystatin C or creatinine alone.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in your arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.