We 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 comparision with genotype, and to find a threeshold 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