“…11,14,15,20,26,33,[35][36][37][38] Of the two prospective observational studies, one left the choice for aprotinin to the discretion of the attending anaesthesiologist 16 and the other used a timed-based allocation method, 18 causing a high risk for selection bias. Discrete factors, such as physician bias regarding patient selection, 10,12,[21][22][23]25,31,39,42 choice of aprotinin use, 34,41 heterogeneous control group (TXA or no drug) with two different CPB circuits, 10 lack of standardised transfusion protocols 12,13,16,21,25,29,30,32,40,43 and lack of full blinding of surgical personnel and outcome assessors, 11,15,16,18,20,36 may account for some of the heterogeneity and conflicting effects observed in the observational and retrospective studies. Two studies 15,20 only applied blinding during surgery, resulting in an important risk of detection bias.…”