Fictional numbers of thiopurine metabolites illustrate that despite different values for TGTP, the total number of 6-TGNs can be similar between wildtype and NUDT15 variant patients using conventional 6-TGN assays. Toxic thiopurine levels can be detected using DNA-TG in NUDT15 variants, but not using 6-TGNs. 6-MMP, 6-methylmercaptopurine; TGMP, thioinosine monophosphate;TGDP, thioguanosine diphosphate; TGTP, thioguanosine triphosphate.