Integrated microfluidic immunoassays for point-of-care diagnostic measurement of human serum cystatin C in chronic kidney disease

Cystatin C (CYS-C) is considered superior to creatinine as a serum biomarker of estimated glomerular filtration rate (eGFR) for chronic kidney disease (CKD) assessment. It is minimally influenced by non-filtration-specific factors such as muscle mass, age and gender. However, currently CYS-C testing is limited to specialized diagnostic labs and no point-of-care (PoC) tools are clinically available. To address this gap, we leveraged the enabling power of microfluidic devices for PoC tests of CYS-C. Among the microfluidic devices, PDMS-based wet chips and paper-based dry chips have been widely used for disease biomarker measurements with their respective advantages and limitations. Therefore, in this study, we developed a PDMS-based microfluidic immunoturbidity chip allowing side-scattering optical measurement and a paper-based microfluidic lateral flow immunoassay chip plus a novel electric field-assisted antibody loading protocol for quantitative CYS-C tests. We demonstrated that both chips meet the clinical requirements of the serum CYS-C detection range and limit. Importantly, each type of microfluidic chip is integrated with a custom-developed portable reader as PoC test prototypes. In validation studies using serum samples from CKD patients, both chip tests showed a similar agreement level with the traditional well plate-based immunoturbidity assay test results. As an integrated PoC test system, the microfluidic CYS-C paper chip test showed higher accuracy in determining the eGFR range based on several clinically relevant grouping criteria compared to the turbidity chip test. Collectively, these integrated microfluidic assays offer practical solutions for decentralized PoC diagnostic tests of CKD with competitive advantages over existing methods.