Mintjens N, Brummans R, Soetens F, et al. - Given that variants of butyrylcholinesterase are often related to prolonged response to suxamethonium or mivacurium, and that phenotyping and ascertainment of genotype can characterize butyrylcholinesterase, but there is a possibility of inappropriate timing of blood sampling interfering with phenotyping methods, therefore, researchers compared butyrylcholinesterase activity ([BChE]) as well as phenotype in an early (T1) and late (T2) phase and also evaluated the phenotype/genotype correlation. In patients (n = 20) with prolonged neuromuscular block induced by mivacurium or suxamethonium, the results were analyzed. At T1 and T2, [BChE] was found to be different in these patients. In this study, anaesthesia was shown to interfere with [BChE], but not with phenotyping. Blood collected during or immediately post-recovery of mivacurium or suxamethonium can be used to conduct phenotyping to screen for clinically relevant variants of butyrylcholinesterase. However, further verification by ascertainment of genotype is required for accurate diagnosis of butyrylcholinesterase deficiency.