The therapeutic monoclonal antibody drug Trastuzumab is widely used for treating metastastatic breast cancer patients with overexpression of HER2 on the tumor. Quantitative targeted proteomics based approaches utilize LC-MS technologies and are evolving as a complementary technique to standard ligand-binding based assays. In the current review, a surrogate peptide based quantitative proteomics assessment by selecting specific signature peptides from the complementary determining region of Trastuzumab is discussed. Double stable isotope labelling method utilizing two surrogate peptides to evaluate accurate quantification of the target analyte peptide of Trastuzumab is discussed. This mini-review highlights the strength of the double stable isotope label approach for the quantitative evaluation of monoclonal antibody biologics in a human biological matrix.