The number of elderly patients with degenerative spinal deformity (DSD) is increasing, and posterior thoracolumbar fusion surgery is an effective treatment option, but there are often postoperative major complications, which may hinder the benefit for elderly patients. Currently, there is no consensus on the best risk assessment technique for predicting major complications in elderly patients undergoing long-segment fusion surgery.This study constructs a risk assessment model using the Modified 5-Item Frailty Index (mFI-5) and serum albumin and evaluates its predictive value.This is a retrospective analysis of a prospectively established database of DSD.Consecutive patients (aged 65 and older) who underwent open posterior thoracolumbar fusion surgery for DSD between April 2018 and December 2023 were included.Outcome measures included postoperative major complications, length of hospital stay [LOS], readmission and reoperation within 30 days, discharge disposition, physiological function recovery.The study reviewed consecutive patients who underwent open posterior thoracolumbar fusion surgery for DSD. Patients were divided into three groups based on the presence or absence of frailty or frailty combined with malnutrition. Spearman ρ analysis was used to assess the correlation between mFI-5 and serum albumin levels. Univariate analyses and multivariate logistic regression were conducted to explore the relationship between frailty and malnutrition defined by mFI-5 and serum albumin and major postoperative complications. Finally, the Receiver Operating Characteristic (ROC) curve was used to evaluate the predictive value of this model for major complications.Compared to the Normal group (n=59), both the Frailty group (n=121) and the Frailty and Malnutrition group (n=50) had higher rates of major complications (21.5% vs. 8.5%, p=.035; 28% vs. 8.5%, p=.002). Multivariate logistic regression showed that frailty and malnutrition status, higher ASA score, and more bleeding were independent predictors of major postoperative complications. The ROC curve demonstrated that frailty combined with malnutrition defined by mFI-5 and serum albumin had a larger area under the curve compared to mFI-5 or serum albumin alone (AUC: 0.676; 95% CI: 1.101-14.129; p<.001).Compared to considering frailty or malnutrition alone, the combined assessment of frailty and malnutrition using mFI-5 and serum albumin is valuable in predicting major complications in elderly patients undergoing posterior thoracolumbar fusion surgery.Copyright © 2024 Elsevier Inc. All rights reserved.