title: Definitional Differences of 'Outpatient' Versus 'Inpatient' THA and TKA Can Affect Study Outcomes.
abstract: BACKGROUND
  There has been great interest in performing outpatient THA and TKA. Studies have compared such procedures done as outpatients versus inpatients. However, stated "outpatient" status as defined by large national databases such as the National Surgical Quality Improvement Program (NSQIP) may not be a consistent entity, and the actual lengths of stay of those patients categorized as outpatients in NSQIP have not been specifically ascertained and may in fact include some patients who are "observed" for one or more nights. Current regulations in the United States allow these "observed" patients to stay more than one night at the hospital under observation status despite being coded as outpatients. Determining the degree to which this is the case, and what, exactly, "outpatient" means in the NSQIP, may influence the way clinicians read studies from that source and the way hospital systems and policymakers use those data.
  
  
  OBJECTIVE
  The purposes of this study were (1) to utilize the NSQIP database to characterize the differences in definition of "inpatient" and "outpatient" (stated status versus actual length of stay [LOS], measured in days) for THA and TKA; and (2) to study the effect of defining populations using different definitions.
  
  
  METHODS
  Patients who underwent THA and TKA in the 2005 to 2014 NSQIP database were identified. Outpatient procedures were defined as either hospital LOS = 0 days in NSQIP or being termed "outpatient" by the hospital. The actual hospital LOS of "outpatients" was characterized. "Outpatients" were considered to have stayed overnight if they had a LOS of 1 day or longer. The effects of the different definitions on 30-day outcomes were evaluated using multivariate analysis while controlling for potential confounding factors.
  
  
  RESULTS
  Of 72,651 patients undergoing THA, 529 were identified as "outpatients" but only 63 of these (12%) had a LOS = 0. Of 117,454 patients undergoing TKA, 890 were identified as "outpatients" but only 95 of these (11%) had a LOS = 0. After controlling for potential confounding factors such as gender, body mass index, functional status before surgery, comorbidities, and smoking status, we found "inpatient" THA to be associated with increased risk of any adverse event (relative risk, 2.643, p = 0.002), serious adverse event (relative risk, 2.455, p = 0.011), and readmission (relative risk, 2.775, p = 0.010) compared with "outpatient" THA. However, for the same procedure and controlling for the same factors, patients who had LOS > 0 were not associated with any increased risk compared with patients who had LOS = 0. A similar trend was also found in the TKA cohort.
  
  
  CONCLUSIONS
  Future THA, TKA, or other investigations on this topic should consistently quantify the term "outpatient" because different definitions, stated status or actual LOS, may lead to different assignments of risk factors for postoperative complications. Accurate data regarding risk factors for complications after total joint arthroplasty are crucial for efforts to reduce length of hospital stay and minimize complications.
  
  
  METHODS
  Level III, therapeutic study.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA.
authors.forename: Patawut
authors.initials: P
authors.lastname: Bovonratwet
authors.affiliation: Department of Orthopaedic Surgery, Hospital of the University of Pennsylvania, Philadelphia, PA, USA.
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Webb
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA.
authors.forename: Nathaniel T
authors.initials: NT
authors.lastname: Ondeck
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA.
authors.forename: Adam M
authors.initials: AM
authors.lastname: Lukasiewicz
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA.
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Cui
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA.
authors.forename: Ryan P
authors.initials: RP
authors.lastname: McLynn
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 47 College Street, New Haven, CT, 06520, USA. jonathan.grauer@yale.edu.
authors.forename: Jonathan N
authors.initials: JN
authors.lastname: Grauer
mesh_terms: D000556:Ambulatory Surgical Procedures / Q000009:adverse effects / Q000145:classification; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000145:classification; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000145:classification; D057225:Data Mining; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007297:Inpatients / Q000145:classification; D053208:Kaplan-Meier Estimate; D007902:Length of Stay; D008297:Male; D015999:Multivariate Analysis; D010343:Patient Admission; D017064:Process Assessment, Health Care; D057216:Propensity Score; D012189:Retrospective Studies; D012307:Risk Factors; D009626:Terminology as Topic; D013997:Time Factors; D016896:Treatment Outcome; D014481:United States