Objective Identification of the risk factors for extraordinary hidden blood loss (HBL) could clarify the underlying causes and provide more appropriate management. This study aims to identify the predictors of HBL in spinal surgery.Methods Medical records were retrospectively retrieved to collect the data of patients who undergoing posterior thoracic and lumbar fusion surgery or scoliosis surgery. Demographic information, perioperative visible blood loss volume, as well as laboratory results were recorded. The patients receiving fusion surgery or scoliosis surgery were further divided into the HBL positive subgroup and the HBL negative subgroup. Differences in the variables between the groups were then analyzed. Binary logistic regression analysis was performed to determine independent risk factors associated with HBL.Results For patients undergoing posterior spinal surgery, the independent risk factors associated with HBL were autologous transfusion (for fusion surgery P=0.011, OR: 2.627, 95%CI: 1.574-2.782; for scoliosis surgery P<0.001, OR: 2.268, 95%CI: 2.143-2.504) and allogeneic transfusion (for fusion surgeryP<0.001, OR: 6.487, 95%CI: 2.349-17.915; for scoliosis surgery P<0.001, OR: 3.636, 95%CI: 2.389-5.231).Conclusions Intraoperative blood transfusion might be an early-warning indicator for perioperative HBL.