Ordering Recommendation

Use this phenotyping test to optimize dosing of thiopurine drugs. This test can identify thiopurine metabolite (6-TGN and 6-MMPN) concentrations that may contribute to risk of toxicity. 

For enzyme phenotyping prior to thiopurine treatment, refer to Thiopurine Methyltransferase, RBC (0092066). For pharmacogenetic testing (prior to or during treatment), refer to TPMT and NUDT15 (3001535).

New York DOH Approval Status

This test is New York state approved.

Specimen Required

Patient Preparation

Trough collection (within 1 hour prior to the next dose).


Lavender (EDTA) or pink (K2EDTA).

Specimen Preparation

Transport 5 mL whole blood. (Min: 2.5 mL)

Storage/Transport Temperature


Unacceptable Conditions

Gel separator tubes. Hemolyzed or lipemic specimens. Frozen specimens.


Ambient: 24 hours; Refrigerated: 7 days; Frozen: Unacceptable


Quantitative Liquid Chromatography-Tandem Mass Spectrometry


Tue, Thu, Sat


1-5 days

Reference Interval

Interpretive Data

Thiopurine drug therapy is used to treat autoimmune diseases, inflammatory bowel disease, acute lymphoblastic leukemia, and to prevent rejection after solid organ transplant. Thiopurine drugs are metabolized to active 6-thioguanine nucleotides, which are regulated by thiopurine methyltransferase (TPMT) and nudix hydrolase 15 (NUDT15). Certain variants in the TPMT and/or NUDT15 genes can be associated with an accumulation of cytotoxic metabolites that increase the risk of drug-related toxicity with standard doses of thiopurine drugs. Thiopurine metabolites concentrations are used to assess therapeutic and toxic concentrations of thiopurine drugs.

INTERPRETIVE INFORMATION: Thiopurine metabolites
Therapeutic range (optimal dosing) 235-450 pmol 6-TGN/8x10^8 red blood cells
Suboptimal dosing <235 pmol 6-TGN/8x10^8 red blood cells
Increased risk for toxicity >450 pmol 6-TGN/8x10^8 red blood cells may increase the risk for myelotoxicity and leukopenia
Increased risk for toxicity >5700 pmol 6-MMPN/8x10^8 red blood cells may increase the risk for hepatotoxicity

Compliance Category

Laboratory Developed Test (LDT)


Hotline History


CPT Codes

80299 (Alt code: G0480)


Component Test Code* Component Chart Name LOINC
3016507 6-Thioguanine RBC 32660-3
3016508 6-Methylmercaptopurine RBC 32654-6
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.


  • 6 Methyl Mercaptopurine
  • 6-Mercaptopurine
  • 6-Methylmercaptopurine
  • 6-MMP
  • 6-MMPN
  • 6-MP
  • 6-TG
  • 6-TGN
  • 6-Thioguanine
  • 6MMP
  • 6MMPN
  • 6MP
  • 6TG
  • 6TGN
  • AZA
  • Imuran Metabolite
  • Mercaptopurine
  • Methylmercaptopurine
  • Purine antimetabolites
  • Purinethol
  • THIO
  • Thioguanine
  • Thiopurine
  • Thiopurine Metabolites
  • TPMT
Thiopurine Metabolites in Red Blood Cells