Flow Induced Dispersion Analysis (FIDA) can measure both drug activity and immunogenicity in patient serum from a single assay. The researchers used FIDA on a Fida 1 instrument to analyse serum from 40 arthritis patients treated with adalimumab, tracking the apparent size (hydrodynamic radius) of fluorescently labelled TNF-α, the drug's target, across a serum dilution series. Because active adalimumab binds TNF-α and enlarges the complex, this absolute, calibration-free size readout revealed how much active drug was present and whether neutralizing or non-neutralizing anti-drug antibodies had formed. FIDA classified patients into normal responders, neutralizing-antibody positive and non-neutralizing-antibody positive groups, detecting more immunogenicity than a comparison ELISA. The method works under biologically relevant conditions without immobilization, and could support companion diagnostics and treatment optimization for biopharmaceuticals with soluble targets.