Predictors and Early Outcomes of Hidden Blood Loss Following Surgery for Spinal Metastases: A Retrospective Study Focusing on Tomita Type 1–5 Lesions

<b>Background</b>: Hidden blood loss (HBL) following surgery for spinal metastases constitutes a major portion of total blood loss (TBL), yet its predictors and impact on early recovery remain unclear. This study aimed to identify independent predictors of HBL in patients with Tomita type 1-5 lesions and to assess its association with early clinical outcomes. <b>Methods</b>: In this retrospective study of 230 patients undergoing posterior tumor resection with cement augmentation and fixation, HBL was calculated using the Gross equation. Predictors were identified via univariate and multivariate linear regression. The impact of HBL on postoperative length of stay, change in Karnofsky Performance Status (ΔKPS), moderate-to-severe anemia, and complications was evaluated using adjusted regression models. Additionally, receiver operating characteristic curve analysis was performed to explore the predictive value of HBL for adverse events. <b>Results</b>: Mean HBL was 449.87 ± 284.86 mL (37.1% of total loss). Independent predictors included higher body mass index (BMI), longer surgery, extensive vertebral involvement (Tomita 4-5), and preoperative hypertension (all <i>p</i> < 0.05). Higher HBL independently predicted longer hospital stay (β = 0.023, <i>p</i> < 0.001), worse ΔKPS (β = -0.012, <i>p</i> < 0.001), increased anemia risk (OR = 1.002, <i>p</i> < 0.001), and more complications (OR = 1.003, <i>p</i> < 0.001). Receiver operating characteristic curve analysis suggested that a HBL >382.5 mL was associated with an increased risk of complications requiring intervention, and a HBL >344.0 mL was associated with an increased risk of postoperative moderate-to-severe anemia. <b>Conclusions</b>: HBL is influenced by both patient-related and surgery-related factors. Greater HBL negatively affects early recovery by prolonging hospitalization, impeding functional recovery, and increasing complication risks. The findings provide a preliminary basis for integrating HBL monitoring into Enhanced Recovery After Surgery (ERAS) pathways. Proactive perioperative blood management is recommended for high-risk patients to improve prognosis.