Table 1. Estimated and calculated blood loss for pelvic fractures treated with ORIF versus percutaneous fixation.
Courtesy of Michelle M. Lawson, MD


Published 12/31/2023
Terry Stanton

Study Indicates Blood Loss Is Underestimated in Percutaneously Treated Pelvic Fractures

Percutaneous pelvic fixation is often considered to be a type of surgery that is associated with low blood loss, yet a study presented at the AAOS 2023 Annual Meeting reported that orthopaedic surgeons may significantly underestimate the amount of blood loss that occurs during percutaneous treatment of pelvic ring injuries.

Specifically, the study, presented by Michelle M. Lawson, MD, of Oregon Health and Science University, found that surgeons estimated blood loss (EBL) at 92 cc, whereas the calculated blood loss (based on hemoglobin levels, transfusions, and blood volume) was as much as 627 cc. Additionally, the researchers found that surgeons underestimated blood loss in pelvic fractures treated with open reduction–internal fixation (ORIF), and that ORIF had significantly higher blood loss when compared with percutaneously managed fractures.

“Expected blood loss often factors into operative planning of pelvis fractures,” the authors wrote in the study’s introduction. “Against the perception of low blood loss,” they commented, “we suspect that the quantity of blood lost through screw entry incisions cannot account for all changes in anemia status in patients with complex pelvic ring injuries.” The investigators sought to compare intraoperative blood loss between pelvic fractures treated with ORIF and pelvic fractures treated with percutaneous fixation.

The authors observed that commonly used clinician estimates of blood loss are routinely unreliable. “In the pre-hospital setting, volume-based estimates of traumatic blood loss exhibit poor accuracy,” they wrote. “Anesthesia estimates as well are not accurate, and unfortunately training does not appear to improve accuracy.” Surgeons, they contended, “are no better at visual blood loss estimates,” and they appear to commonly underestimate blood loss by as much as 30 percent—a misperception that may potentially worsen as volumes increase above 500 cc.

One potential reason behind surgeon underestimation of blood loss is what is known as hidden blood loss (HBL), the authors explained. HBL is defined as the calculated blood loss minus the EBL and is thought to be due to blood penetrating local tissues, staying in dead spaces, or getting lost in hemolysis. One report found that in total knee arthroplasty, HBL made up 50 percent of total blood loss. This same pattern has been seen in surgeries for lumbar fusion, hip fracture, and total hip replacement. Factors that appear to increase HBL in hip fracture fixation include older patient age, general anesthesia (compared to intraspinal), osteoporosis, and use of anticoagulation. Although not previously studied, it stands to reason that pelvis surgery would also have some HBL.

For this study, the authors retrospectively reviewed 335 adult patients undergoing operative fixation of pelvis fractures at a level 1 trauma center between 2008 and 2018. The researchers calculated blood loss for standardization by using a hemoglobin balance method that accounts for estimated total blood volume (using height, weight, and gender as described by Nadler), as well as perioperative changes in hemoglobin and intraoperative transfusions. The researchers also assessed the surgeon-recorded EBL. Means were compared with t tests for statistical analysis.

Pelvic fractures treated with open surgery had significantly greater perioperative blood loss than those treated with percutaneous fixation; however, there was significant discordance between surgeon EBL and calculated EBL. Average surgeon-reported EBL for open surgeries was 92 cc and only 76.8 cc for percutaneous surgeries (P <0.0001). mean perioperative blood loss according to the hemoglobin balance equation was 1,852 cc for open surgeries and 627 cc for percutaneous surgeries (>P = 0.0001) (Table 1). Surgeon-reported EBL may significantly underestimate intraoperative blood loss for both open and percutaneous surgeries when compared against the hemoglobin balance method (P <0.0001).>

Dr. Lawson said that she and her colleagues were “surprised by the magnitude of difference between the surgeon’s estimated blood loss and the calculated blood loss,” which in percutaneously treated pelvic fractures was 7 times higher than the estimated blood loss. “Our data suggest we may be routinely underestimating blood loss in percutaneously treated pelvic fractures. This hidden blood loss may have important clinical impacts on patient hemodynamics, and we should be taking this into account when planning surgeries and postoperative care.”

She also noted, “Polytrauma patients are a unique population who experience hematologic-related consequences of their traumatic events. Underestimating blood loss in this population may have more dramatic impacts.” Dr. Lawson suggested that further areas of exploration may include investigations into the nature of HBL in pelvic fractures, as well as methods of minimizing blood loss.

Addressing the study’s limitations, she commented: “As a retrospective study, there are inherent limitations in the conclusions we can make about direct causation. In addition, hemoglobin labs were collected at different time points preoperatively and postoperatively, which may skew the calculated blood loss.”

Dr. Lawson’s coauthors of “The Tip of the Iceberg: Are We Underestimating Blood Loss in Percutaneously Treated Pelvic Fractures?” are Natalie L. Zusman, MD; Natasha McKibben, BS; Jason Agtarap, BS; Karalynn Lancaster, BS; Darin M. Friess, MD; and Zachary M. Working, MD.

Terry Stanton is the senior medical writer for AAOS Now.


  1. Frank M, Schmucker U, Stengel D, et al: Proper estimation of blood loss on scene of trauma: tool or tale? J Trauma 2010;69(5):1191-5.
  2. Rothermel LD, Lipman JM: Estimation of blood loss is inaccurate and unreliable. Surgery 2016;160(4):946-53.
  3. Sehat KR, Evans R, Newman JH: How much blood is really lost in total knee arthroplasty? Correct blood loss management should take hidden loss into account. The Knee 2000;7(3):151-5.
  4. Donovan RL, Lostis E, Jones I, Whitehouse MR: Estimation of blood volume and blood loss in primary total hip and knee replacement: An analysis of formulae for perioperative calculations and their ability to predict length of stay and blood transfusion requirements. J Orthop 2021;24:227-32.