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Objective. More than 11% of the Caucasian population are heterozygous or homozygous carriers of thiopurine S-methyltransferase (TPMT) mutants and are at risk for toxic side effects when treated with thiopurine drugs. Therefore, screening for TPMT polymorphisms in a patient prior to prescribing these agents is recommended. The goal of this study was to determine a cut-off concentration of the TPMT activity assay beyond which genotyping of the TPMT gene should be performed. Methods. The TPMT activity of 240 unrelated Caucasian subjects was measured using high-performance liquid chromatography. Genotyping for the most frequent allelic variants, TPMT*2, *3A, *3B, *3C and *7 was performed by LightCycler technology and sequencing. Results. The inter-individual TPMT activity showed a range from 23 nmol MTG/g*Hb*h-1 to 97 nmol MTG/g*Hb*h-1 with a median of 56 nmol MTG/g*Hb*h-1. Using a cut-off concentration of 45.5 nmol MTG/g*Hb*h-1, a test sensitivity of 100% and a specificity of 89% were reached for heterozygous carriers of a TPMT mutation. We identified 1 carrier of TPMT*2, 14 carriers of TPMT*3A and 3 carriers of TPMT*3C, resulting in a TPMT heterozygosity prevalence of 7.5%. Conclusions. This study defines the cut-off value for the TPMT phenotyping assay at 45.5 nmol/g*Hb*h-1, beyond which additional genotyping elucidates the individual risk for drug therapy. Using this cut-off concentration, the number of genotyping assays could be reduced by about 60%. [ABSTRACT FROM AUTHOR]