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

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 (R2 = 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.