Prevalence of Survivor Bias in Observational Studies on Fresh Frozen Plasma:Erythrocyte Ratios in Trauma Requiring Massive Transfusion

Observational studies on transfusion in trauma comparing high versus low plasma: erythrocyte ratio were prone to survivor bias because plasma administration typically started later than erythrocytes. Therefore, early deaths were categorized in the low plasma: erythrocyte group, whereas early survivors had a higher chance of receiving a higher ratio. When early deaths were excluded, however, a bias against higher ratio can be created. Survivor bias could be reduced by performing before-and-after studies or treating the plasma: erythrocyte ratio as a time-dependent covariate. We reviewed 26 studies on blood ratios in trauma. Fifteen of the studies were survivor bias-unlikely or biased against higher ratio; among them, 10 showed an association between higher ratio and improved survival, and five did not. Eleven studies that were judged survivor bias-prone favoring higher ratio suggested that a higher ratio was superior. Without randomized controlled trials controlling for survivor bias, the current available evidence supporting higher plasma: erythrocyte resuscitation is inconclusive.