Intraindividual variability in red blood cell thiopurine methyltransferase activity

Objective. Long-term (13 weeks) and circa dian (24 hours) intraindividual variability in red blood cell (RBC) thiopurine methyltransferase (TPMT) activity in healthy subjects was studied. Methods: RBC TPMT activity was measured radiochemically. Results: The variability in RBC TPMT activity was low and was only slightly higher than the imprecision of the TPMT assay. Mean long-term intraindividual variability in RBC TPMT activity was 6.5% (CV) (n = 46). Mean intraindividual circadian variability in RBC TPMT activity was 6.4% (CV) (n = 18). Conclusions: In contrast to what has been observed in children with acute lymphoblastic leukaemia, the intraindividual variability in RBC TPMT activity in healthy subjects was low. The reported changes in baseline RBC TPMT activity in patients are probably therefore due to drugs, disease, assay variation or other, unidentified factors.