Purpose To compare the utility of the frailty phenotype (FP), the FRAIL scale, and 5-item modified Frailty Index (mFI-5) in predicting postoperative adverse events after enhanced recovery after lumbar fusion surgery in older patients. Methods This study prospectively included older patients (> 75 years) who underwent transforaminal lumbar interbody fusion from June 2019 to August 2021. Frailty status was evaluated using FP, the FRAIL scale, and mFI-5. The study investigated the associations between these three frailty tools and total adverse events, complications, and secondary outcomes. Multivariable logistic regression analysis was performed to identify predictors of total adverse events, complications, and secondary outcomes. Results Correlation analysis demonstrated that frailty assessed by the FP was significantly associated with an increased incidence of complications (55.7% vs. 41.5%, p =.028) and prolonged hospital stays (29.5% vs. 16.9%, p =.021). Frailty identified using the FRAIL scale was significantly linked to prolonged hospital stays (33.7% vs. 16.2%, p =.002). Additionally, frailty assessed by the mFI-5 was associated with higher rates of 30-day readmission (18.5% vs. 6.3%, p =.003) and 30-day reoperation (7.4% vs. 0.6%, p =.007). Multivariate logistic regression revealed that an FP score of >= 3 was an independent risk factor for total adverse events, complications, and secondary outcomes, while a FRAIL score of >= 3 was an independent risk factor for secondary outcomes. Conclusion Frailty as defined by the FP is an independent risk factor for total adverse events, complications, and secondary outcomes. Frailty as defined by the FRAIL scale is an independent risk factor for secondary outcomes. The mFI-5 has not been demonstrated to be an independent risk factor for any postoperative adverse events.