ADAMTS13 and thrombotic thrombocytopenic purpura: TMAs are now often categorized into 3 groups as follows: (1) thrombotic thrombocytopenic purpura (TTP) due to a deficiency in a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) activity, (2) typical hemolytic uremic syndrome (HUS) due to Shiga toxin (STX)-producing Escherichia coli (STEC) infections, and (3) atypical HUS (aHUS), a disorder that is categorized further into primary and secondary aHUS (associated with miscellaneous conditions as listed in Table 1).1, -3