TPMT (Thiopurine Methyl Transferase)
Thiopurine drugs such as azathioprine, 6-mercaptopurine and
thioguanine are converted in vivo to form cytotoxic thionucleotides
which provide the therapeutic effect (white cell
TPMT provides the major catabolic pathway for these drugs. If
the enzyme is absent in the patient there will be an 'over-dosing'
effect, frequently resulting in severe bone marrow depression, also
Complete deficiency of TPMT occurs in 1:300 Caucasians (it is
also known in patients of African origin). Partial deficiency
occurs in more than 1:10
Prospective measurement of TPMT provides a rational basis for
deciding on levels of thiopurine drug therapy.
Request on ICE
Analysed by referral laboratory if specific criteria met.
Investigation will normally only be undertaken if patient is
about to commence therapy. Repeat requests will not be undertaken
unless agreed with the Consultant Biochemist
Recent blood transfusions will confuse the enzyme
TPMT is inducible so the analysis should be done before commencing
If the patient is already receiving azathioprine,
this must be stated on the request form.
Whole blood (EDTA)
5-10 ml (paediatric 2-5 ml)
Causes for Rejection
Unlabelled or inadequately labelled sample.
Not meeting specific criteria for analysis.
Whole blood TPMT
20 - 67
68 - 150