Q&A
Frequently asked questions.
Questions and Answers about Cystatin C GFR
Q. Isn’t the GFR determined by serum creatinine measurement good enough?
A. Creatinine values are dependent on variables including age and body mass and you must correct for this in calculating GFR. Cystatin C GFR gives a direct and accurate measurement of GFR independent of age and muscle mass. [3]
Q. What is the “creatinine-blind area”? Doesn’t Cystatin C GFR have a blind area?
A. Serum creatinine values are not a direct measure of GFR. GFR can be reduced by as much as 50% while the serum creatinine value is still within the normal range(4) - this is the “creatinine-blind area”. Cystatin C value gives an immediate and true measurement of GFR. Cystatin C doesn’t have a blind area so Cystatin GFR is more accurate. Click here to read more about the creatinine blind area.
Q. Serum creatinine is the only relevant test for determining GFR our clinical laboratory offers. How can we get access to the Gentian Cystatin GFR test?
A. Contact our sales division for introductory offers in your area: sales @ gentian.no
Q. Our clinical test laboratory says that Cystatin C tests are too expensive – is this true?
A. This has been the case with existing tests. Gentian Cystatin C test uses new production techniques and patented technology giving higher precision. [6] The Gentian immunoassay is both better and cheaper than existing Cystatin C tests.
Q. Is the Gentian Cystatin C test affordable? It’s still more expensive than the creatinine test – right?
A. It is a little more expensive than the creatinine test, but much less expensive than the creatinine clearance test, which also involves 24 hours urine collection. And the total cost of a test is frequently more than the test alone. For example, the containers and work in a 24-hour urine collection cost far more than the Gentian Cystatin C test itself! In addition there is minimal discomfort for the patient. The Cystatin GFR does not involve any urine collection.
Q. What do you mean by minimal patient invasion? You still have to take a blood sample – right?
A. Blood samples are an unavoidable part of patient investigation. The Gentian Cystatin C analysis can be carried out on one routine blood sample, which can be done together with other standard tests in clinical analysers. Patients do not have to be invaded specifically for the Gentian Cystatin C test; it’s just a tick-off on a standard blood analysis. More importantly, Gentian Cystatin C can replace a range of unpleasant tests, including urine collection or radioactive measurements with repeated blood sampling. Especially for paediatricians, Cystatin GFR is the preferred test method.
Q. How does Cystatin GFR compare to gold standards for precise determination of GFR?
A. Gold standard methods include the use of intravenous injections of radio labelled substances such as Cr-51-EDTA and Tc-99m-DTPA, or iodinated agents such as iohexol. GFR determined as Cystatin C GFR shows a direct relationship between the reciprocal of cystatin C concentration in serum/plasma and GFR. Correlation is very good, and is not hampered by collection errors or errors introduced by multiple measurements of blood or urine samples.
Q. Different producers use different equations for calculating Cystatin GFR. Please explain this, surely it means that Cystatin GFR varies according to the test used?
A. The equation varies because the tests are not identical. If the test is calibrated correctly then the Cystatin GFR will be the same for the tests used.
Q. I can’t calculate Cystatin GFR using that equation every time I need a GFR. How can I get a direct measurement of GFR?
A. The Gentian ruler allows you to convert Cystatin C measurement directly to GFR. Just read the Cystatin C level given by the clinical analyser in mg/L and the corresponding GFR is shown directly on the ruler. There is a reciprocal relationship between Cystatin C levels and GFR.
Click here to read about and download the Gentian ruler.
Q. Why is there no international reference material available for cystatin C?
A. The IFCC (International Federation of Clinical Chemistry) has established a working group on standardisation of cystatin C. The group comprises authorities, lead by professor Anders Grubb, and representatives from producers of the Cystatin C tests.
Click here to read that the Gentian calibrator is already standardized against the coming world calibrator.
Q. Creatinine measurement doesn’t always give an early indication of impending renal problems. Why does Cystatin C measurement give a so much better measurement of GFR?
A. Creatinine levels are affected by many factors; gender, age and muscle mass being the most important, while even a protein-rich diet can affect creatinine levels. Serum cystatin C level is independent of these factors as it is produced at a constant rate by all nucleated cells in the body. Kidney dysfunction will cause a variation in serum cystatin C and can be used to give an immediate and accurate measurement of GFR.
