title: The further exploration of hidden blood loss in posterior lumbar fusion surgery.
abstract: Hidden blood loss (HBL) plays an important role in lumbar fusion surgery. However, there was huge computation error when calculating hidden loss in previous studies because they regarded the volume of drainage as postoperative blood loss. We should not ignore the fact that the composition of drainage varies from person to person and also with time-lapse after operation. The purpose of this study was to demonstrate a more accurate HBL calculation formula by comparing it with previous formula and to address the importance of HBL in patients undergoing posterior lumbar fusion surgeries.
  
  
  
  For each patient, the HBL was calculated by previous formula and our recommendable formula. We compared the result of HBL calculated by two different methods. At the same time, we explored the HBL in patients undergoing posterior lumbar fusion surgeries with various levels.
  
  
  
  In our study, the average total blood loss was 771ml. The mean intraoperative blood loss was 268ml, and the total volume of post-operative drainage is 276ml. The mean hidden loss of 90 patients calculated with previous method was 227ml and 29.4% of total loss. When taking change of drainage HCT into account, the mean hidden loss calculated with our recommendable method was 362.8ml and 47% of total loss. The results were significantly different (P<0.01). There was no significant difference in the percentage of the HBL between patients with single or multiple surgical levels.
  
  
  
  There is often a substantial unmeasured blood loss in lumbar fusion surgeries. However, the component of drainage changed radically with time, we should take the true blood contained in drainage into considerations when calculating the HBL.
  
  
  
  Our research is a case-control study and the level of proof is III.
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: xuderong19880001@163.com.
authors.forename: D
authors.initials: D
authors.lastname: Xu
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: renzhinan@163.com.
authors.forename: Z
authors.initials: Z
authors.lastname: Ren
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: chenxin@163.com.
authors.forename: X
authors.initials: X
authors.lastname: Chen
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: zhuangqianyu@163.com.
authors.forename: Q
authors.initials: Q
authors.lastname: Zhuang
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: huishangyi@163.com.
authors.forename: S
authors.initials: S
authors.lastname: Hui
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: shenglin@163.com.
authors.forename: L
authors.initials: L
authors.lastname: Sheng
authors.affiliation: Department of Orthorpaedic Surgery, Peking Union Medical College Hospital, Beijing, China. Electronic address: lishugangpumc@163.com.
authors.forename: S
authors.initials: S
authors.lastname: Li
mesh_terms: D016063:Blood Loss, Surgical; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods; D013130:Spinal Stenosis / Q000601:surgery*
keywords: Hidden blood loss; Lumbar fusion surgery; Recommendable calculation method