We agree that a major limitation of the NSQIP database in investigating this study question is the ambiguity regarding the definition of “outpatient surgery.” As the NSQIP database collects data from many unique institutions and providers, there is inherent heterogeneity introduced into the data pool, including heterogeneity in how each institution defines outpatient surgery. Nevertheless, many other studies have similarly relied on the NSQIP documentation of surgery setting to compare inpatient and outpatient data [2-5]. A potentially insightful analysis would be to perform a similar analysis with “outpatient” defined as length of hospital stay=0 as some previous studies have done [6,7].

As you have noted, our study lacks analysis of some important outcomes to consider when evaluating safety and efficacy of surgical procedures. This is another limitation inherent to the NSQIP database, which does not record spine-specific outcomes data, radiographic measures, or patient-reported outcome measures. For this reason, analysis of the construct stability based on follow-up radiographic evaluation was not possible. In addition, the study also lacks analysis of patient-reported outcomes. Such variables are critical elements in evaluating surgical outcomes, but investigation of these items cannot be performed utilizing the NSQIP database. Regarding the lack of overall cost-benefit measures, our study aimed to evaluate the outcome variables which may provide insight regarding the safety of outpatient PCDF, including readmission, reoperation, morbidity, and complications, and cost analysis was not a main goal of this study. Nevertheless, we agree that quantification of the potential differences in healthcare costs between inpatient and outpatient PCDF would be valuable and should be pursued in a future study.

Furthermore, we believe this study is valuable for providing an exploratory analysis of the differences in short-term outcomes between inpatient and outpatient PCDF. While there certainly exist variables which are not captured by the database, utilization of the NSQIP database allowed for a broad analysis including a large, multicenter study sample. We also agree that another limitation of the NSQIP database is that not every institution in the United States is involved and that there is likely a bias toward larger academic medical centers within the database. Despite this, the database currently includes over 600 hospitals across the country, contributing significant generalizability.

We would like to express our gratitude once again for your letter highlighting these important considerations when interpreting these data. As spine surgery continues to trend toward the outpatient setting, these will be critical factors to consider in all future studies, especially those utilizing large national databases with similar inherent limitations.