Spine pathologies are diverse and require varying levels of surgical care and in some cases necessitate interhospital transfer (IHT). In prior literature, 12–30% of patients admitted for spine pathology were brought in via IHT [1], [2], [3]. IHT to tertiary care centers are driven by a number of factors, including but not limited to lack of full-time spine surgeon coverage or appropriate expertise, bed availability, complexity of trauma, insurance coverage issues, and/or patient preference [2], [4], [5], [6]. Acceptance of IHT patients from institutions lacking the required resources or expertise is required by law under the U.S. Emergency Medical Treatment and Active Labor Act (EMTALA) [7]. While EMTALA assures nondiscriminatory access to surgical care, some IHTs are considered a misuse or misallocation of resources that pose risks to certain patients and deplete resources needed for the most serious surgical emergencies [1], [5], [6]. The decision to transfer a patient is complex and is thus preceded by professional discussion between the sending and accepting providers. During such discussions, it is critical to consider patient/family wishes, safety of transport, and benefit/risk ratio of tertiary care interventions. Given this complexity, IHTs are a popular topic in health outcomes, quality improvement and cost effectiveness research.

In recent years, frailty, a measure of physiological reserve, in lieu of chronological age alone, has proven itself a robust preoperative predictor of surgical outcomes [8], [9], [10], [11], [12]. Frailty indices are straight-forward to calculate and may support IHT-related decision making for spine surgery patients. However, prior literature assessing the utility of frailty in relation to postoperative outcomes of IHT for spine surgery is scarce [2]. The present study analyzed a nationally representative sample of patients receiving spine surgery using data extracted from the American College of Surgeons prospectively collected registry, National Surgical Quality Improvement Project (ACS-NSQIP). The specific aims of the study were to investigate the effect of IHT on postoperative outcomes and analyze the effect of baseline frailty status [as measured by the modified frailty index-5 (mFI-5)] vs. chronological age on postoperative outcomes in IHT patients receiving spine surgery.