In a first aspect, there is provided an isolated polypeptide substrate for a disintegrin-like and metallopeptidase with thrombospondin type-1 motif, 13 (ADAMTS13) that is from 45 to 70 amino acids in length and has an amino acid sequence that is substantially similar to part of the von Willebrand factor A2 domain sequence set forth in SEQ ID NO: 2, with one or more of the following modifications: (i) the amino acid corresponding to position 1599 of SEQ ID NO: 2 is mutated from Q to K; (ii) the amino acid corresponding to position 1610 of SEQ ID NO: 2 is mutated from N to C; and (iii) the amino acids corresponding to Q1624 to R1641 of SEQ ID NO: 2 are deleted. In another aspect, there is provided an ADAMTS13 polypeptide substrate that is from 50 to 75 amino acids in length and has an amino acid sequence that is substantially similar to part of the von Willebrand factor A2 domain sequence set forth in SEQ ID NO: 2, with one or more of the following modifications: (i) the amino acid corresponding to position 1599 of SEQ ID NO: 2 is mutated from Q to K; (ii) the amino acid corresponding to position 1610 of SEQ ID NO: 2 is mutated from N to C; (iii) the amino acid corresponding to position 1629 of SEQ ID NO: 2 is mutated from G to E; and (iv) the amino acids corresponding to G1631 to R1641 of SEQ ID NO: 2 are deleted.