Thiopurine Methyltransferase

Sample type:

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

Uses:

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

Precautions:

• For measuring TPMT enzyme activity (TPMT phenotyping), the test must be performed prior to taking a thiopurine drug since it may affect results

Interfering Factors:

• 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.

Pre analytical errors:

1. Inappropriate specimen received.

2. Unlabeled specimen or name discrepancy between specimen and request label;

3. Specimen received after prolonged delay (usually more than 72 hours);

4. Specimen received in expired transport container.

Corrective action:

1. Appropriate specimen received.

2. Labeled specimen and no name discrepancy between specimen and request label.

3. Specimen received without delay.

4. Specimen received in valid transport container.

Post analytical errors:

1. Failure in reporting.

2. Erroneous validation of analytical data.

3. Improper data entry.

Corrective action:

1. Write correct report.

2. All analytical data should be valid.

3. Proper data entry.

Reference range:

• 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.