An immunoassay kit including a monoclonal antibody coupled to a labeling agent (labeled mAb) for detecting tumor pyruvate kinase M2 (tM2-PK) in a biological sample. The monoclonal antibody includes a heavy chain variable region (HCVR) including heavy chain complementarity-determining regions (CDRs) (SEQ ID NOs: 3, 4, and 5) and a light chain variable region (LCVR) including light CDRs (SEQ ID NOs: 7, 8, and 9).