The thiopurine S-methyltransferase (TPMT) genetic polymorphism has a significant clinical impact on the toxicity of thiopurine drugs. It has been proposed that the identification of patients who are at high risk for developing toxicity on the basis of genotyping could be used to individualize drug treatment. In the present study, phenotype-genotype correlation of 1214 healthy blood donors was investigated to determine the accuracy of genotyping for correct prediction of different TPMT phenotypes. In addition, the influence of gender, age, nicotine and caffeine intake was examined. TPMT red blood cell activity was measured in all samples and genotype was determined for the TPMT alleles *2 and *3. Discordant cases between phenotype and genotype were systematically sequenced. A clearly defined trimodal frequency distribution of TPMT activity was found with 0.6% deficient, 9.9% intermediate and 89.5% normal to high methylators. The frequencies of the mutant alleles were 4.4% (*3A), 0.4% (*3C) and 0.2% (*2). All seven TPMT deficient subjects were homozygous or compound heterozygous carriers for these alleles. In 17 individuals with intermediate TPMT activity discordant to TPMT genotype, four novel variants were identified leading to amino acid changes (K119T, Q42E, R163H, G71R). Taking these new variants into consideration, the overall concordance rate between TPMT genetics and phenotypes was 98.4%. Specificity, sensitivity and the positive and negative predictive power of the genotyping test were estimated to be higher than 90%. Thus, the results of this study provide a solid basis to predict TPMT phenotype in a Northern European Caucasian population by molecular diagnostics.

Prediction of responsiveness after clopidogrel loading dose may substantially be improved by adding CYP2C19*2 genotype to nongenetic risk factors.

A 28 year-old heretofore healthy woman was transferred to our hospital with a two-month history of recurring episodes of bleeding. Administration of vitamin K and prothrombin complex concentrates in the transferring hospital had only temporarily corrected both the markedly elevated international normalized ratio (INR) and the prolonged activated partial thromboplastin time (aPTT). The patient's medical and family history revealed no reason for these abnormalities. Our laboratory analyses revealed a sustained deficiency of vitamin K-dependent clotting factors. Presence of an acquired inhibitor of clotting factors was excluded. Thus we suspected, intoxication with an anticoagulant rodenticide. Liquid chromatography-mass spectrometry (LC-MS/MS) revealed pharmacologically active concentrations of flocoumafen, a rodenticide belonging to the superwarfarin family, in the patient's serum. While the long elimination half-life of superwarfarins is well described in rodents, information on pharmacokinetics in humans is not yet available. Therefore, patient management was not limited to prolonged administration of vitamin K, but also included repeated measurements of flocoumafen serum levels. During follow-up visits, clotting tests remained normal and flocoumafen levels gradually decreased, reaching the limit of quantification after 48 days. Based on the repeated measurements of flocoumafen serum levels, a half-life of 6.7 days was estimated in our patient, which is in clear contrast to the 220 days reported in rodents. Thus, monitoring flocoumafen serum concentrations in affected patients may provide a rational basis for the duration of vitamin K substitution and adequate follow-up intervals.

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