Thiopurine Methyltransferase (TPMT), Enzyme Activity, Erythrocytes

CPT: 84433
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  • TPMT Biochemical Assay, Myelotoxicity

Special Instructions

For New York state patients: Test orders for TPMT Enzyme Activity must include an attestation that the provider has the patient's informed consent for genetic testing. See sample physician office consent form (Informed Consent for TPMT Enzyme Activity Test) in Related Documents.

Expected Turnaround Time

7 - 12 days

Specimen Requirements


Whole blood


8 mL adults; 5 mL pediatric

Minimum Volume

5 mL


Green-top (sodium heparin) tube or lavender-top (EDTA) tube

Storage Instructions


Stability Requirements




14 days



Freeze/thaw cycles


Causes for Rejection

Frozen specimen; gross hemolysis; clotted specimen

Test Details


Determination of TPMT levels that may be associated with toxicity of anticancer and anti-inflammatory drugs


For patients having an intrinsic low level of TPMT, recent RBC transfusion can variably increase their assayed enzymatic activity depending on the amount and circulating half-life of the transfused red blood cells.

This test was developed and its performance characteristics determined by Labcorp. It has not been cleared or approved by the Food and Drug Administration.


Enzymatic endpoint/liquid chromatography/tandem mass spectrometry (LC/MS-MS).

Reference Interval

• Normal: 15.1−26.4 units/mL RBC

• Heterozygous for low TPMT variant: 6.3−15.0 units/mL RBC

• Homozygous for low TPMT variant: <6.3 units/mL RBC


Relling MV, Gardner EE, Sandborn WJ, et al. Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing. Clin Pharmacol Ther. 2011 Mar; 89(3):387-391. 21270794


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
510750 Thiopurine Methyltransferase 79713-4 510751 TPMT Activity Units/mL RBC 21563-2
510750 Thiopurine Methyltransferase 79713-4 510752 Interpretation: 79713-4
510750 Thiopurine Methyltransferase 79713-4 510754 Methodology 49549-9

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