The present disclosure relates to a method for detection of an antibody or antibody fragment in a biological sample from a subject. In some embodiments, the methods comprise immobilizing a first binding agent on a surface plasmon resonance (SPR) biosensor; adding a ligand that binds to the first binding agent under conditions such that a complex of the ligand and the first binding agent is formed; adding an aliquot of the biological sample under conditions such that the antibody and/or antibody fragment binds to the ligand that is complexed to the first binding agent; and detecting the presence of the antibody and/or antibody fragment as a change in signal obtained from SPR. In some embodiments, the antibody is an antibody therapeutic such as certolizumab pegol. Also disclosed are systems and kits for detecting an antibody in a biological sample from a subject using SPR.