A review of portable molecular diagnostics: Paper and smartphone synergy for pathogen detection

Molecular diagnostics (MDx) enables sensitive and specific pathogen detection, but conventional laboratory-based methods require specialized instruments, trained personnel, and centralized infrastructure, limiting their accessibility in resource-limited settings. Paper-based MDx platforms offer low-cost, portable, and user-friendly alternatives for decentralized point-of-care testing (POCT); however, many systems still rely on subjective visual readout, partial workflow integration, and limited standardization. This review summarizes representative paper-based molecular diagnostic platforms for pathogen detection and critically examines how smartphone integration can support signal acquisition, quantitative or semi-quantitative analysis, data processing, and field deployability when imaging conditions and calibration strategies are appropriately controlled. We discuss paper-compatible nucleic acid amplification and molecular detection strategies, including loop-mediated isothermal amplification (LAMP), recombinase polymerase amplification (RPA), CRISPR-assisted assays, and amplification-free strand-displacement systems. Emphasis is placed on colorimetric and fluorescence-based readouts, smartphone-assisted image acquisition, app-based interpretation, and optical setup design. Representative examples demonstrate sensitive and rapid detection of pathogens such as SARS-CoV-2, MRSA, HIV-1, and Escherichia coli across diverse human-derived, food, environmental, and agricultural sample matrices. Rather than solely reporting analytical sensitivity, we compare platforms in terms of detection modality, smartphone role, data-processing strategy, workflow integration, real-sample validation, and practical deployment constraints. Remaining challenges include incomplete sample-to-answer integration, sample preparation bottlenecks, smartphone-to-smartphone variability, calibration robustness, reagent stability, and limited field validation. Overall, smartphone-integrated paper-based MDx provides a promising route toward accessible POCT, but future progress will depend on system-level co-optimization of paper architecture, molecular assay chemistry, optical acquisition, software analysis, and user-centered workflow design.