Pathology tests database

Sample and container requirements

EDTA Whole Blood

Minimum volume required

4 mL whole blood (paediatric 2 mL)

Special requirements

Do not centrifuge.

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.

Sample viability

Turnaround time

14 days

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.

Clinical guidance

Please refer to LabTestsOnlineUK to learn more about TPMT. https://labtestsonline.org.uk/tests/tpmt

Notes

Drugs such as azathioprine have been reported to induce TPMT activity, and sulfasalazines can inhibit TPMT activity. Renal impairment can cause false elevations in TPMT activity.