Preferred Sample Type

Thiopurine S-Methyltransferase (TPMT)

Suitable Specimen Types

  • EDTA Whole Blood
4 mL whole blood (paediatric 2 mL)

Specimen Transport

For internal users, transport via usual mechanisms. For external laboratories, store at 4ºC prior to transport, and send by first class post.

Sample Processing in Laboratory

DO NOT CENTRIFUGE. Store specimens at 2-8ºC.

Sample Preparation

Store whole blood at 2-8ºC on receipt in laboratory.

Turnaround Time

14 days

Sample Stability

Store whole blood at 2-8ºC (stable for 2 weeks). For longer stability, store aliquots suitable for analysis frozen (discuss with Clinical Scientist in Specialist Testing).

Thiopurine S-Methyltransferase (TPMT)

General Information

Thiopurine drugs, such as azathioprine and mercaptopurine, are widely used as immunosuppressants in gastroenterology, dermatology and post-transplant etc. Thiopurine drugs are metabolised by two major pathways — by the enzyme xanthine oxidase and thiopurine S-Methyl-transferase (TPMT). Individual patients vary greatly in susceptibility to the toxic effects of these drugs, due to genetic differences in drug metabolism. TPMT is absent in approximately 1 in 300 people, and around 11% of the population have low activity. Patients who fall into these categories are at risk of severe side-effects. The measurement of TPMT levels at the start of thiopurine therapy allows the dose of thiopurine drugs to be adjusted appropriately for successful treatment.

 

In addition to TPMT activity detemination, genetic analysis is also carried out. To date more than 20 single-nucleotide polymorphisms (SNP) have been associated with decreased TPMT activity. The most common defects leading to inactivated alleles are G238C mutation in exon 5 (TPMT*2), point mutations in A719G in exon 10 and G460A in exon 7 (TPMT*3A), and an isolated mutation in position A719G (TPMT*3C). These cause rapid degradation of the protein by an ATP dependent proteosome mediated pathway, resulting in loss of catalytic activity of the enzyme.

 

 

 

 

Patient Preparation

Patient should not have had recent blood transfusions (both phenotype and genotype results affected).

 

TPMT activity should be determined BEFORE starting on thiopurine drugs

 

 

 

 

Notes

Drugs such as aziathioprine have been reported to induce TPMT activity, and sulfasalazines can inhibit TPMT activity.

 

Renal impairment can cause false elevations in TPMT activity

 

 

 

 

Reference Range

TPMT phenotype/activity reference ranges:

<6 nmol/ghb/hr = Deficient

6-34 nmol/gHb/hr = Low

35-79 nmol/gHb/hr = Normal

>79 nmol/gHb/hr = High

 

TPMT genotype classifications:

WT = wild-type, normal result

H3 = TPMT 1*/3* - heterozygous for TPMT, this would indicate a reduced activity.  Advise reduce dose of azothioprine.

H2 = TPMT 1*/2* - heterozygous for TPMT, this would indicate a reduced activity.  Advise reduce dose of azothioprine
 
H03 = TPMT 3*/3* - homozygous for TPMT, this would indicated deficient TPMT activity.  Advise stop treatment with azothioprine.
 
H02 = TPMT 2*/2* - homozygous for TPMT, this would indicated deficient TPMT activity.  Advise stop treatment with azothioprine.

 

 

 

 

 

 

 

Specifications

  • EQA Status: NEQAS (Pilot)

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