Method for quantifying therapeutic antibodies

A method for quantifying a therapeutic antibody in a sample of a human individual comprising:

• a) adding to a test sample which may contain therapeutic antibodies to be quantified a known amount of two or more labeled forms of said therapeutic antibodies, wherein the two or more labeled forms of said therapeutic antibodies are whole antibodies, whereby a pre-proteolysis sample is provided,
• b) subjecting the pre-proteolysis sample to an enzyme proteolysis, so as to provide a proteolysis sample comprising (i) proteolysis labeled peptides derived from the labeled whole therapeutic antibodies and (ii) proteolysis peptides derived from the therapeutic antibody contained in the test sample,
• c) determining by mass spectrometric analysis the ratio between (i) one or more selected proteolysis labeled peptides and (ii) one or more corresponding proteolysis peptides derived from the said therapeutic antibody,
• d) calculating from the ratio determined at c) the amount of the said therapeutic antibody in the test sample.