Correlation of transfusion volume to change in hematocrit

While it has generally been accepted that a 3% change in hematocrit is equivalent to a 1-"unit" loss of blood, few studies have been published to actually document this. A more complete understanding of this correlation would be helpful in estimating blood loss from changes in hematocrit in patients sustaining hemorrhage as well as in those receiving transfusions. In this retrospective study, we analyzed the post-transfusion alterations in hematocrit from 61 independent transfusions in 48 different pelvic fracture patients (age range, 16-62 years). Transfusion volumes were correlated with changes in hematocrit over 24-hr periods between the 5th and 12th hospital days, a time when there was no ongoing hemorrhage in these patients. The average increase in hematocrit per liter of packed red blood cells transfused was 6.4% +/- 4.1%. If 1 "unit" of packed red blood cells is approximately 300 mL, this becomes a change of hematocrit of 1.9% +/- 1.2% per "unit" of blood. The accepted correlation of about 1 "unit" of blood loss per 3% change in hematocrit would be valid for a 500-cc unit, but a typical unit of packed red blood cells is typically 300 cc. In addition, the variability is substantial as indicated by our standard deviation. Limitations of this study include hematocrit changes due to fluid resuscitation, dehydration, age, persistent hemorrhage, and the retrospective nature of the evaluation.