Fishing plasma-circulating autoantibodies with magnetic pseudo-platelets for rapid optomagnetic quantification

Accurate detection of anti-platelet autoantibodies is crucial for the diagnosis of immune thrombocytopenia (ITP). Conventional assays, mostly detecting platelet-surface-bound antibodies rather than free circulating ones in plasma, are labor-intensive, merely semi-quantitative, and further constrained by low platelet counts in ITP patients. Herein, magnetic pseudo-platelets were fabricated to fish plasma-circulating autoantibodies, and an optomagnetic sensing system was established based on antibody-induced particle agglutination. Two detection formats were systematically compared. For the homogeneous format, a 5-min real-time immuno-agglutination assay was employed. The heterogeneous format utilized an additional 5-min secondary signal amplification with Protein G-coated magnetic nanoparticles. Limits of detection of 6.1 ng/mL and 0.83 ng/mL were achieved in buffer for the homogeneous and heterogeneous formats, respectively. In 10% plasma, the heterogeneous format exhibited nonspecific binding of endogenous IgG, yet still delivered a detection limit of 5.3 ng/mL, superior to that of the homogeneous format (130 ng/mL). When benchmarked against flow cytometry (the gold standard for platelet-bound antibody detection), our magnetic pseudo-platelet-based biosensor showed excellent discrimination between ITP patients and healthy controls (n = 43), providing complementary diagnostic information with equivalent accuracy. This method is independent of platelet count, rapid (∼11 min), and suitable for point-of-care testing, thus establishing a versatile platform for fast screening of blood protein biomarkers.