Abstract
Background: Perioperative blood loss in lumbar spine surgery is closely linked to postoperative recovery and complications, yet hidden blood loss (HBL) is often underestimated. This study aimed to compare total and hidden blood loss between biportal endoscopic spine surgery (BESS) and open surgery and to identify perioperative risk factors influencing HBL. Methods: This retrospective study included 261 patients who underwent lumbar decompression or fusion between January 2020 and February 2024. Patients were divided by surgical approach, procedure type and number of levels. Total blood loss (TBL), visible blood loss (VBL), and HBL were calculated from hematocrit changes using established formulas. Linear mixed and multivariate regression models were used to assess intergroup differences and identify independent predictors. Results: BESS demonstrated significantly reduced intraoperative bleeding and drainage volumes, shorter hospital stay, and earlier ambulation than traditional open surgery. Postoperative hematocrit decline was smaller in BESS group in multi-level decompression and fusion indicating a smaller postoperative decline in the BESS group. Although BESS reduced visible bleeding, HBL remained a major portion of TBL in both groups. Multivariate analysis identified longer operative time, diabetes mellitus, multi-level surgery, and fusion procedures as independent predictors of increased TBL and HBL, while smoking and dorsal extensor thickness were associated with increased loss only in traditional open surgery. Conclusions: BESS effectively reduces visible perioperative bleeding and enhances early recovery, but HBL remains clinically significant. Surgical extent and metabolic factors, rather than approach alone, determine overall bleeding, emphasizing the importance of surgical proficiency and meticulous hemostasis.