The before/after study design in transfusion medicine: methodologic considerations.

To ascertain the effectiveness of an intervention, a randomized controlled trial (RCT) is considered the gold standard. An obstacle to conducting an RCT only rarely discussed is the universal implementation of an intervention. Universal implementation clearly precludes the feasibility of conducting an RCT. Thus, the most attractive alternative study design in such instances becomes the before/after study. This article describes important methodologic considerations in undertaking a before/after evaluation. The methodologic considerations to be discussed are (1) threats to internal validity, (2) precision and (3) generalizability. Two before/after studies evaluating the potential effectiveness of universal leukoreduction serve as examples. Because of the universal application of many transfusion interventions, one has to consider carefully the methodologic rigor as to which of these interventions are evaluated. We have outlined the major methodologic issues one must consider when undertaking a before/after study design. When properly conceived, conducted, and analyzed, such a before/after study design can yield informative associations.