The Cystatin C test system is a device intended for the in vitro quantitative determination of Cys C in serum or plasma.
Glomerular Filtration Rate (GFR) is a direct marker of renal functions, which starts to decline early in the course of renal diseases. Accurate determination of GFR is required for monitoring the progress of renal diseases when deciding on therapy to avoid impairing the organ function. GFR is determined by measuring the clearance of exogenous substances such as inulin, iohexol and so on, which can freely filter through the glomerular membrane and re-enter the circulation. However, their routine measurement is limited for technical, economical and organizational reasons. Determination of creatinine clearance is the most widely used method for non-invasive estimation of GFR in current practice. Serum creatinine is usually considered moderately specific but of poor sensitivity, as significant increases are only observed if GFR is reduced to 50% or less (creatinine blind range).Creatinine evaluation is influenced by a muscle mass, body surface and flood intake, so it must consider about the age, gender, height and body composition. Creatinine clearance leads to significant overestimation on GFR in case of patients with highly decreased GFR due to tubular secretion. The collection of 24 hr urine is time-consuming and creates additional sources of errors.
Cystatin C is a base proteinase inhibitor with a low molecular mass of 13Kd, and it is produced at a constant rate in all nucleated cells and appears in human plasma and serum. Cystatin C is freely filtered through the glomerulus, is not secreted by the tubule or eliminated via any extra-renal route, and is almost completely absorbed and catabolized by proximal tubular cells.
Cystatin C is not influenced by acute phase reaction (vs. Beta2-microgloblin), and not influenced by endogenous or analytical factor (vs. creatinine or creatinine clearance). These advantages makes Cystatin C an excellent non-invasive indicator for GFR.
Clinical applications of Cystatin C are for monitoring GFR in children and elderly patients, for assessment of renal transplantation status, for monitoring GFR in nephrotoxic drug therapy, for monitoring GFR in acute and chronic kidney diseases including a diabetic nephropathy.
Sample is reacted with a buffer and anti-Cys C coated latex. The formation of the antibody-antigen complex during the reaction results in an increase in turbidity, the extent of which is measured as the amount of light absorbed at 570 nm. By constructing a standard curve from the absorbance of the standards, Cystatin C concentration of sample can be determined.
The method is linear up to a concentration of 0.05-8.0 mg/L. Sample above this concentration should be diluted 1:1 with 0.9% NaCL and repeat assay. Multiply the result by 2.
The following analytes concentrations were not found to affect the assay:
Hemoglobin: up to 500 mg/dl
Bilirubin: up to 20 mg/dl
Introllipid: up to 2500 mg/dl
Ascorbic acid: up to 50 mg/dl
RF: up to 500 IU/ml
The minimum detectable level of Cys C with an acceptable level of precision has been determined as 0.05 mg/L.
The CV of the test should be ≤ 5%.