Use and interpretation of thiopurine metabolites
| TGN (pmol/8×108 RBCs) | MeMP (pmol/8×108 RBCs) | Interpretation | Treatment change to consider |
| Undetectable | Undetectable | Poor/variable compliance | Patient education Rarely poor absorption |
| Low (<235) | Low/normal (<5700) | Subtherapeutic dosing | Uptitrate dose then repeat metabolites |
| Low (<235) | High (>5700 or MeMP: TGN >11) | Thiopurine hypermethylator (occurs in up to 20% of patients with normal TPMT) | Reduce dose to 25–33% + start allopurinol 100 mg/day, then repeat metabolites |
| Therapeutic (235–450) | Normal (<5700) | Therapeutic (if responding) Thiopurine resistant (if not responding) | If responding, continue current dose If not responding, change drug category |
| Therapeutic (235–450) | High (>5700) | Possible supratherapeutic dosing | Attempt dose reduction and repeat in 4 weeks. If non-responding and TGN low end of normal range, consider low dose azathioprine with allopurinol as above |
| High (>450) | High (>5700) | Supratherapeutic dosing | Reduce dose then repeat metabolites |
Adapted from Goel et al.1286
MeMP, methylmercaptopurine nucleotides; RBC, red blood cells; TGN, 6-thioguanine nucleotides; TPMT, thiopurine methyltransferase.