A 56-year-old woman is scheduled for an extensive surgical procedure. She weighs 50 kg and her hematocrit prior to surgery is 37%. You determine that an intraoperative hematocrit of 21% would necessitate transfusion of packed red blood cells. What is this patient’s MAXIMUM allowable blood loss?

  • (A) 700 mL
  • (B) 1,400 mL
  • (C) 2,800 mL

Several formulas exist for estimating perioperative blood loss. These are typically based on estimated blood volume (EBV) and the patient’s hemoglobin or hematocrit values both before and after surgery. Some authors have demonstrated a poor correlation between the commonly used formulas, while others have suggested that, while inaccuracies exist, protocol-driven transfusion practices based on these estimates are useful in the perioperative blood management of patients undergoing surgical procedures. Considering the type of surgical procedure and estimating the amount of blood loss that would trigger transfusion for a particular patient may allow the perioperative care team to better plan and prepare for a patient’s perioperative management.

EBV can be determined based on age, sex, and body weight. This is particularly important in pediatric patients. There is some thought that, for obese adults, the ideal body weight may derive a more accurate estimate. The Table lists typical values used to estimate blood volume in different patient populations.


A patient’s perioperative hemoglobin and hematocrit can be influenced by a myriad of factors. The commonly used calculations and objective measurements do not consider the patient’s individual medical history or currently existing volume status. They can, however, provide a rough estimate. Preexisting anemia, whether due to volume overload, hemoglobinopathy, or chronic disease, can have an effect on the hemoglobin content and hematocrit of a patient and thus render some calculations less representative.

A common formula for calculating a patient’s maximum allowable blood loss (MABL) during surgery takes into consideration the provider’s predetermined trigger for transfusion:

MABL=EBV×Starting HCT−target HCTStarting HCT

where EBV indicates the estimated blood volume, starting HCT is the patient’s hematocrit before surgery, and target HCT is the lowest hematocrit that the provider would consider acceptable prior to transfusion of red blood cells. The provider’s trigger for transfusion can be influenced by many factors, including patient comorbidity, institutional transfusion practice, type of surgical procedure, and provider bias.

For the patient in the scenario, the MABL would be determined as follows:

MABL=50 kg×65 mL/kg×37%−21%37%=1,405 mL

where the EBV is 3,250 mL (50 kg × 65 mL/kg), the starting HCT is 37%, and the target HCT is 21%, for a MABL of 1,405 mL.