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Thiopurine Metabolites

Test ID: 


CPT code:



Thiopurine Metabolites
6MP Metabolites
6TG Metabolites
Azathioprine Metabolites
Mercaptopurine Metabolites

Clinical Use:

This assay measures the red-cell concentration of 6-MMP (formed from 6-MP by thiopurine methyltransferase) and also measures the concentration of the resulting 6-TG after removal of the mono-, di-, or tri-phosphates from the various 6-thioguanine nucleotides. Once thiopurine therapy has been undertaken and an equilibrated drug level is achieved (usually three to six months), literature suggests that the measurement of thiopurine metabolites is warranted in the following situations:
1. Unresponsive patients to rule out (a) patient noncompliance (low 6-MMP and low 6-TG in those not taking medications), (b) those with diversion of metabolites away from 6-TG production (low 6-TG and normal or increased 6-MMP), (c) those with refractory to treatment (adequate 6-TG but lack of clinical response), and (d), those with excessive 6-MMP (or increased 6-MMP:6-TG ratio with adequate 6-TG) leading to increased hepatotoxicity.
2. Clinically responsive patients to look for excessive levels of either 6-MMP or 6-TG to avoid toxicity.

Test Information:

This test should only be performed for patients currently on thiopurine therapy. This therapy includes administration of azathioprine or mercaptopurine. This test may not be useful in patients with autoimmune hepatitis.

Specimen Type:

Whole blood

Requested Volume: 

3 mL

Minimum Volume: 

1 mL  (Note: This volume does not allow for repeat testing.)

Container Type: 

Lavender-top (EDTA) tube


Collect blood in lavender-top (EDTA) tube only. Gently invert to mix. Submit whole blood refrigerated. Do not separate specimen. Do not freeze.

Storage Instructions:


Stability Requirements:

stable refrigerated for 14 days. Stable at room temperature for one day. Unstable frozen.

Rejection Criteria

Frozen specimen; specimen not EDTA whole blood; gross hemolysis

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