Table 10

Use and interpretation of thiopurine metabolites

TGN (pmol/8×108 RBCs)MeMP (pmol/8×108 RBCs)InterpretationTreatment change to consider
UndetectableUndetectablePoor/variable compliancePatient education
Rarely poor absorption
Low (<235)Low/normal (<5700)Subtherapeutic dosingUptitrate 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 dosingAttempt 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 dosingReduce dose then repeat metabolites
  • Adapted from Goel et al.1286

  • MeMP, methylmercaptopurine nucleotides; RBC, red blood cells; TGN, 6-thioguanine nucleotides; TPMT, thiopurine methyltransferase.