TPMT enzyme activity
What this test measures
TPMT is an enzyme that breaks down azathioprine and mercaptopurine. About one person in three hundred makes almost none and one in ten makes less than usual, which changes how these medicines are handled.
Why doctors request it
Measured once before starting azathioprine or mercaptopurine for bowel, skin, joint or transplant conditions.
What commonly pushes it up
- There is no problem with a high value, though it may mean the medicine is broken down quickly
What commonly pushes it down
- An inherited low-activity variant
- A recent blood transfusion, which gives a false normal in someone who is deficient
Everyday reasons for a value outside the range
- Units and ranges differ between the few labs that run it
- A transfusion in the previous three months invalidates it
About the sample
Once before treatment; no fasting needed.
Upload a photo or PDF and see every line against the range your lab printed, explained in plain English. First report free.
Read my report freeThis is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.