Development and Analytical Validation of a Smartphone‐Based Quantitative Lateral Flow Immunoassay for Serum Cystatin‐C

ABSTRACT Background Cystatin‐C is an important renal function biomarker, but conventional quantification requires centralized laboratory analyzers, limiting timely testing in primary care and resource‐limited settings. Methods We developed and validated a quantitative smartphone‐based lateral flow immunoassay (SP‐LFIA) comprising a colorimetric strip, a Redmi Note 12R smartphone, a fixed‐geometry macro‐lens module with integrated LED illumination, and an Android application. The application directly converted background‐corrected test/control‐line (T/C) ratios into concentrations using a batch‐specific third‐order polynomial calibration model. Analytical validation followed relevant Clinical and Laboratory Standards Institute guidelines. Results The assay showed a quantitative range of 0.32–8.00 mg/L and a limit of detection of 0.15 mg/L. Intra‐ and inter‐assay coefficients of variation were below 10%. No significant interference was observed from bilirubin, triglycerides, hemoglobin, rheumatoid factor, gentamicin sulfate, norvancomycin hydrochloride, or cisplatin at the tested concentrations. Accelerated testing supported strip performance after storage at 50°C for 28 days. In 120 clinical serum samples, SP‐LFIA agreed closely with the commercial particle‐enhanced turbidimetric immunoassay reference method (R 2  = 0.9946), with a mean bias of −0.01 mg/L. Conclusion The smartphone‐based LFIA provides a reliable, rapid, low‐cost, and practical approach to quantitative Cystatin‐C testing in point‐of‐care settings where serum preparation is available.