Evaluation of adsorption of urine cystatin C to the polymer materials on the microplate by an antigen capture enzyme-linked immunosorbent assay

BackgroundCystatin C is a low molecular weight protein of 13 kDa with an isoelectric point of 9.3. Its adsorption on the urine sampling containers may cause the underestimation of cystatin C levels. We newly developed an antigen capture enzyme-linked immunosorbent assay (ELISA) of sandwich method for measurement of adsorbed level.MethodsWe used a polystyrene microplates with 3 different polymers. These include high hydrophobic, low hydrophobic, and hydrophilic materials. Using the same microplate, the absorbed protein was measured by an antigen Capture ELISA, and calibration was conducted by an ordinary ELISA.ResultsIn normal urine the concentrations of absorbed cystatin C levels to the 3 materials at day 1 were 0.50, 0.32–0.84 μg/l (median, interquartile range), 0.28, 0.21–0.37 μg/l, and < 0.08, < 0.08–0.09 μg/l in high hydrophobic, low hydrophobic, and high hydrophilic material, respectively. The absorption rate was 6%, 3%, and 1%, respectively. The adsorption is dependent on urine pH. It changes reciprocally with urine protein concentration. In pathologic urine, the absolute absorption level was < 0.08 μg/l on the median, and the adsorption ratio (absorption level/urine level) was much less than 0.5% of that in normal urine.ConclusionIn the clinical setting, the absorption of cystatin C to sample containers is negligible since the rate of adsorption is low both in normal and pathologic urine. The material with high hydrophilic surface processing may be used for other proteins when interaction of the proteins with surface material affects the value to clinical decision.