Table 2.

Thiopurine monitoring based on metabolites.

6-TGN	6-MMP	Cause	Consequences	Recommendation
Normal or high	Low	Therapeutic dose Refractoriness if absence of response	Control of disease activity No control of disease activity	Continue therapy Change therapy
High	High	Overdose	Myelotoxicity Hepatotoxicity	Reduce dose
		Refractoriness if absence of response	No control of disease activity	Change therapy
High	Low	Low TPMT activity	Risk of myelotoxicity	Reduce dose
		Refractoriness if absence of response	No control of disease activity	Change therapy
		Response	Control of disease activity	Continue monitoring
Low	High	Hypermethylators	No control of disease activity Hepatotoxicity	Reduce dose (25–50%) and add allopurinol
Low	Low	Underdose No compliance	No control of disease activity	Increase dose Assess adherence

6-TGN levels: low (<230–235 pmol/8 × 10⁸), high (> 450 pmol/8 × 10⁸), normal (230–450 pmol/8 × 10⁸).

6-MMP levels: high (>5,700 pmol/8 × 10⁸), low (<5,700 pmol/8 × 10⁸).