Highly Sensitive Serum Protein Analysis Using Magnetic Bead-Based Proximity Extension Assay

Many protein biomarkers are present in biofluids at a very low level but may play critical roles in important biological processes. The fact that these low-abundance proteins remain largely unexplored underscores the importance of developing new tools for highly sensitive protein detection. Although digital enzyme-linked immunosorbent assay (ELISA) has demonstrated ultrahigh sensitivity compared with conventional ELISA, the requirement of specialized instruments limits the accessibility and prevents the widespread implementation. On the other hand, proximity ligation assays (PLA) and proximity extension assays (PEA) show sensitive and specific protein detection using regular laboratory setups, but their sensitivity needs to be further improved to match digital ELISA. To achieve highly sensitive protein detection with minimal accessibility limitation, we develop a magnetic bead-based PEA (magPEA), which posts triple epitope recognition requirement and enables extensive washing for improved sensitivity and enhanced specificity. We demonstrate that the incorporation of magnetic beads into PEA workflow facilitates orders of magnitude sensitivity improvement compared with conventional ELISA, homogeneous PEA, and solid-phase PLA and achieves limits of detection close to that of digital ELISA when using IL-6, IL-8, and GM-CSF as validation. Our magPEA provides a simple approach for highly sensitive protein detection that can be readily implemented to other laboratories and will thus ultimately accelerate the study of the low abundance protein biomarkers in the future.