['To investigate the importance of blood sampling conditions for multiple electrode platelet aggregometry (MEA) in cardiac surgery patients.', 'Eighty-one patients undergoing first time CABG surgery were included in three prospective, observational studies. MEA was used to analyze platelet aggregability after addition of adenosine-diphosphate (ADP) or thrombin activating peptide 6 (TRAP). In substudy 1, hirudin and citrate tubes were compared. In substudy 2, samples from peripheral vein, central venous catheter, and radial artery were compared and in substudy 3, the effect of surgery was investigated by analyzing pre- and postoperative samples.', 'Platelet aggregability values were 30% higher in hirudin tubes than in citrate tubes. There was a significant correlation between hirudin and citrate tubes in TRAP-induced aggregability (r = 0.84, p < 0.001) but not in ADP-induced aggregability (r = 0.25, p = 0.13). The blood sampling site did not influence platelet aggregability. Surgery reduced ADP-induced aggregability by 31% (p < 0.001) and TRAP-induced aggregability by 30% (p < 0.001) with large intraindividual variations.'] Do sampling conditions influence multiple electrode platelet aggregometry in cardiac surgery patients?