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Cystatin C is a folded protein structure composed of a chain of 120 amino acids. Cystatin C is a potent cysteine proteinase inhibitor produced at a constant rate by all nucleated cells. Cystatin C is filtered effectively by the glomerulus and fully reabsorbed and broken down by the renal tubules for recycling. A raised level of serum cystatin C correlates closely with GFR and is a good indicator of reduced kidney function. Cystatin C is primarily a marker of kidney function and GFR. However, newer research suggests that cystatin C is a useful marker for a number of other conditions, read more about those situations in the cystatin c clinical use section.
Precision studies are carried out as part of application validation and is instrument specific. See instrument specific application notes for more information
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.
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 – 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
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This has been the case with existing tests. Gentian cystatin C test uses new production techniques and patented technology giving higher precision. The Gentian immunoassay is both better and cheaper than existing cystatin C tests.
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.
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..
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..