Abstract: BACKGROUND: Transforaminal lumbar interbody fusion is one of the main surgical methods for treating degenerative lumbar diseases such as lumbar disc herniation, lumbar spinal stenosis, and lumbar spondylolisthesis. Hidden blood loss refers to the concealed loss of blood volume in patients, which is often overlooked by people.
OBJECTIVE: To evaluate the perioperative blood loss during transforaminal lumbar interbody fusion using a new method, calculate the hidden blood loss based on the new method, and analyze its risk factors.
METHODS: The medical records of 93 patients with lumbar degenerative diseases (lumbar spinal stenosis, lumbar disc herniation, and lumbar spondylolisthesis) who were hospitalized in the Department of Spine Surgery of Second Affiliated Hospital of Soochow University from October 2023 to October 2024 were retrospectively analyzed. The general data of patients were collected, such as age, gender, height, body mass, body mass index, and whether they had hypertension and diabetes; surgical data, such as the number of surgical segments, operation time, and American Society of Anesthesiologists anesthesia grade; laboratory tests, such as prothrombin time, activated partial thromboplastin time, international normalized ratio, platelet count, fibrinogen, and D-dimer level. Pearson or Spearman correlation analysis was used to explore the correlation between patient characteristics and postoperative hidden blood loss, and multivariate linear regression analysis was utilized to determine the independent risk factors for postoperative hidden blood loss. 
RESULTS AND CONCLUSION: (1) The average hidden blood loss calculated by the new method for transforaminal lumbar interbody fusion was (284.24±352.76) mL, accounting for 58.6% of the total blood loss; while the average hidden blood loss calculated by the traditional surgical method was (165.77±339.89) mL, accounting for 34.15% of the total blood loss. The difference between the two was significant (P < 0.05). (2) In the univariate analysis, hidden blood loss was significantly correlated with the number of segments (r=0.213, P=0.040) and operation time (r=0.210, P=0.043) and negatively correlated with platelet count (r=-0.324, P=0.018). (3) In the multivariate linear regression analysis, decreased platelet count was an independent risk factor for hidden blood loss (P=0.016). (4) It is concluded that the new method was more accurate in estimating hidden blood loss, and hidden blood loss was an important part of the total blood loss during the perioperative period. Increased number of segments, prolonged operation time, and decreased platelet count were risk factors for hidden blood loss in transforaminal lumbar interbody fusion, among which decreased platelet count was an independent risk factor.

Key words: hidden blood loss, new method, transforaminal lumbar interbody fusion, risk factor, perioperative blood loss