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Many methods to detect CYP2D6 poor metabolizers have been validated. Some of them are based on phenotyping (metabolism of dextromethorphan or debrisoquine) and some others on genotyping. Up to now, CYP2D6 pharmacogenetics has been restricted to the field of research, in spite of poor metabolizer profile concerns 5 to 10 % of caucasian population. Nevertheless, the polymorphism of CYP2D6 is responsible for the metabolism of many drugs, particularly of two opioids involved in pain management: codeine and tramadol, their metabolites representing the most effective part of the drug effect. So prescribing codeine or tramadol in a patient poor metabolizer for the CYP2D6 is likely to be ineffective in pain management. O-demethyl-tramadol, the metabolite of tramadol via CYP2D6, is important to consider because its analgesic effect is 2 to 4 times more potent than tramadol. The investigators propose to phenotype CYP2D6 in post-operative patients treated by tramadol by monitoring seric concentrations of O-demethyl tramadol and tramadol to make a ratio in comparison with genotype, and to find a threshold to determine poor metabolizers. As already described, genotyping CYP2D6 will use a rapid detection method of the alleles implicated in poor metabolizer status (CYP2D6*3, *4, *5 et *6) in a Caucasian population. Sampling will be executed at two times (H24 and H48 after surgery) and only with blood (three EDTA tubes) during the post-operative monitoring of the patients. This study is likely to include 320 post-operative patients treated with intravenous tramadol during one year in three university hospitals centers (CHU of Caen, Creteil and Rouen). The first aim of this study is the validation of monitoring seric concentrations of O-demethyl-tramadol and tramadol to make the ratio in order to detect CYP2D6 poor metabolizers in therapeutic situation, comparing the result with genotyping. The finding of a poor metabolizer status in a patient will make the choice of analgesic drugs easier, avoiding tramadol and codeine. The final objective of this research is to be able to determine the CYP2D6 phenotype in a patient treated by tramadol without a good analgesia. By a single take of blood and a rapid response, this method should be liked to improve pain management. Furthermore, CYP2D6 phenotyping is interesting for the patient because many other drugs depend on this way of metabolism.