Metabolite Interpreter • Therapeutic Drug Monitoring
Enter the laboratory thiopurine metabolite values to visualize the pharmacological status and dosing optimization matrix.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Monitoring patients on Azathioprine (AZA) or 6-Mercaptopurine (6-MP) for IBD
Investigating therapy failure (non-response) or "loss of response"
Screening for potential hepatotoxicity or myelosuppression risk
To guide dose adjustment when combined with Allopurinol
The Metabolite Balance
Thiopurine dosing aims to maximize the "Therapeutic" 6-TGN (which incorporates into DNA to stop T-cell proliferation) while minimizing the "Toxic" 6-MMP (associated with liver injury).
Section 2
Formula & Logic
Therapeutic Ranges (pmol/8x10⁸ RBC)
6-TGN Lower Limit
235 pmol (Minimum for efficacy)
6-TGN Optimal Range
235–450 pmol
6-TGN Toxicity Limit
> 450 pmol (Increased leukopenia risk)
6-MMP Toxicity Limit
> 5,700 pmol (Increased hepatotoxicity risk)
Phenotype Interpretation
01
Low 6-TGN / Low 6-MMP: Under-dosed or non-compliance.
02
Low 6-TGN / High 6-MMP: "Shunter" pattern (TPMT preference for 6-MMP). High risk of liver failure/no response.
03
High 6-TGN / High 6-MMP: Over-dosed.
04
Low 6-TGN / Normal 6-MMP: Escalation required.
Section 3
Pearls/Pitfalls
The Allopurinol "Rescue"
For "Shunters" (Low TGN/High MMP), adding low-dose Allopurinol (100mg) inhibits the Xanthine Oxidase pathway, forcing the thiopurine down the 6-TGN path. CRITICAL: When adding allopurinol, the thiopurine dose MUST be reduced to 25%–33% of the original dose to avoid fatal myelosuppression.
Steady State Requirement
Metabolites should only be checked after at least 4 weeks of a stable dose, as this is the time required for red blood cell turnover to reflect steady-state levels.
Clinical Pearls
6-TGN levels > 450 correlate with improved mucosal healing but significantly higher risk of neutropenia
6-MMP > 5700 is associated with a 3-fold increase in the risk of hepatotoxicity (Elevated ALT/AST)
Patients with NUDT15 or TPMT variants (Poor/Intermediate metabolizers) will have naturally high 6-TGN even on very low doses
Section 4
Next Steps
Clinical Action
01
Shunter (6-MMP/6-TGN ratio > 20): Consider switching to Allopurinol + Low-dose AZA strategy.
Thiopurines have been used since the 1960s, but "Metabolite-guided therapy" emerged in the 1990s as a way to salvage patients who clinical failed standard weight-based dosing. It represents one of the first successful applications of precision medicine in gastroenterology.
