VTSEGAGLQLQK- 13 C 6 , 15 N 2 is the 13 C and 15 N labeled VTSEGAGLQLQK. VTSEGAGLQLQK is an amino acid sequence in the C-terminal region of recombinant human alpha-acid glucosidase (rhGAA). VTSEGAGLQLQK can be used to bind anti-drug antibodies (ADA) in plasma and quantitatively analyze the therapeutic effect.