Abstract:[Objective] To investigate the factors related to hidden blood loss in endoscopic lumbar interbody fusion. [Methods] A retrospective study was conducted in 120 patients who underwent endoscopic lumbar interbody fusion from September 2018 to June 2022. Of them, 37 patients underwent endoscopic transforaminal lumbar interbody fusion (E-TLIF), 30 patients underwent endoscopic posterior lumbar interbody fusion (E-PLIF), and 53 patients were treated with unilateral biportal endoscopic posterior lumbar interbody fusion (UBEPLIF). The univariate comparison, univariate correlation and multiple linear stepwise regression were conducted to research the factors related to the hidden blood loss. [Results] All patients had surgical procedures accordingly performed successfully with no significant differences in operation time, preoperative Hb, RBC, Hct and coagulation parameters among the three groups (P>0.05). The Hb, RBC and Hct of all the three groups significantly decreased after surgery (P<0.05), which were not significantly different between E-TLIF group and E-PLIF group (P>0.05), whereas the UBE-PLIF group was significantly lower than the E-TLIF group in terms of Hb [(111.7±12.7) g/L vs (123.9±16.0) g/L, P<0.05], RBC [(4.0±0.7) 1012 vs (4.4±0.6) 1012, P<0.05] and Hct [(34.7±5.0)% vs (36.8±4.7)%, P<0.05]. In addition, the UBE-PLIF group proved significantly greater than the E-TLIF and E-PLIF groups in terms of intraoperative blood loss [(161.8±77.9) ml vs (63.8±45.1) ml vs (56.3±30.1) ml, P<0.05], total blood loss [(553.8±459.7) ml vs (257.2±283.1) ml vs (262.5±302.3) ml, P<0.05] and hidden blood loss [(392.1± 419.2) ml vs (193.4±269.9) ml vs (145.5±205.2) ml, P<0.05], whereas there was no significant differences in the above indexes between the E-TLIF group and the E-PLIF group (P>0.05). As consequence of univariate correlation, the surgical method (UBE-PLIF) and preoperative Hct were significantly positively correlated with hidden blood loss (P<0.05). Regarding to multiple linear regression analysis, the UBE-PLIF and preoperative Hct were independent risk factors for hidden blood loss (P<0.05). [Conclusion] Endoscopic lumbar interbody fusion also has the possibility of large amounts of hidden blood loss perioperatively, especially the UBE-PLIF.