Glomular Filtration Rate (GFR) is the metric normally used to assess kidney function. It can be determined either by measuring the concentration of endogenous markers such as urea nitrogen or creatinine in serum, or by measuring the clearance rate of exogenous indicators such as inulin, iohexol or 51Cr-EDTA or 125I-iothalamate. None of these methods are ideal. The tests involving exogenous markers are expensive and time consuming, and the need to administer the marker requires these to be scheduled tests – making them largely incompatible with the routine monitoring needs of some patients. Although widely used, urea nitrogen and creatinine also have limitations as markers in some situations. For example, there are non-GFR factors that influence the overall levels of creatinine in serum, including things like muscle mass that cannot adequately be accounted for by simple adjustments for age, sex, race or gender. Cystatin C is a small protein with a basic isoelectric point that has emerged as an alternative marker for kidney function. The justification for the use of cystatin C as a marker for renal function follows the same basic logic as that for creatinine. Since cystatin C is not secreted and does not return to the blood stream but rather is reabsorbed by tubular epithelial cells and subsequently degraded, it avoids some of the non-renal effectors such as muscle mass, age or gender that complicate the use of other endogenous markers.
A recent paper by Longkumer et al. compared serum creatinine and serum cystatin C in patients with known chronic kidney disease. Measurement of serum cystatin C was done using the Arbor Assays Human Cystatin C EIA kit (K012-H1), while measurement of serum creatinine was done using a colorimetric method based on the Jaffe-reaction. In both cases, inverse correlation between the serum marker and GFR was good. Serum creatinine levels were elevated over the established normal range in 67 of 69 patients assessed (97.1%). Serum cystatin C was elevated over the established normal range in all 69 patients (100%).
Although a small study, this work suggests that serum cystatin C is at least as good a marker for kidney function as serum creatinine, and may be even slightly better for patients of this type. Since cystatin C can be reliably and reproducibly measured in serum samples without the need to adjust for body size etc., it provides an excellent alternative to older methods for the routine long term monitoring required for many patients with chronic kidney disease.
Cystatin C Serum Linearity
Arbor Assays Cystatin C Human EIA kit is designed to measure human cystatin C levels in a variety of biological matrices and produces results in just two hours.
Typical Cystatin C Standard Curve