Simple one-pot assay for visual and smartphone-based quantification of human albumin in urine and biological fluids

This study introduces a simple and effective one-pot assay for the quantification of human albumin (HA) using H8 aptamer-modified magnetic beads (H8Apt-MB-HA), applicable across various biological fluids, including urine, serum, and sweat. Upon binding to HA, the aptamer-bead complexes exhibit concentration-dependent aggregation: well-dispersed at low HA levels and clustered at higher concentrations, with rapid sedimentation under a magnetic field in U-shaped well plates. These aggregation patterns are visually discernible, enabling rapid qualitative screening without the need for complex instrumentation. For quantitative analysis, digital images captured by a smartphone or a plate scanner (Cytation™ 5) are processed using ImageJ software, allowing for accurate signal quantification. The top and bottom-view assay achieves a detection limit of 5.9 and 6.5 mg L^-1 and demonstrates robust performance across a urinary pH range of 5.0-7.0. Minimal cross-reactivity with other biological components confirms the aptamer's high specificity. Clinical validation with patient urine samples shows excellent correlation with standard immunoturbidimetric methods, underscoring the assay's potential for point-of-care testing. This approach supports immediate interpretation and clinical decision making, especially valuable in resource-limited settings and for routine monitoring of kidney-related disorders.