title: Observation stays in administrative claims databases: underestimation of hospitalized cases.
abstract: OBJECTIVE
  Recent policy changes in the USA have led to an increasing number of patients being placed into observation units rather than admitted directly to the hospital. Studies of administrative data that use inpatient diagnosis codes to identify cohorts, outcomes, or covariates may be affected by this change in practice. To understand the potential impact of observation stays on research using administrative healthcare data, we examine the trends of observation stays, short (≤2 days) inpatient admissions, and all inpatient admissions.
  
  
  METHODS
  We examined a large administrative claims database of commercially insured individuals in the USA between 2002 and 2011. Observation stays were defined on the basis of the procedure codes reimbursable by Medicare or commercial insurers. We report monthly rates of observation stays and short inpatient admissions overall and by patient demographics.
  
  
  RESULTS
  We identified 5 355 752 observation stays from 2002 to 2011. Over the course of study, the rate of observation stays increased, whereas the rate of short inpatient stays declined. The most common reason for observation stays was nonspecific chest pain, also the third most common reason for short inpatient stays. The increasing trend of observation stays related to circulatory diseases mirrors the decreasing trend of short inpatient stays.
  
  
  CONCLUSIONS
  The use of observation stays has increased in patients with commercial insurance. Failure to account for observation stays may lead to an under-ascertainment of hospitalizations in contemporary administrative healthcare data from the USA.
authors.affiliation: Division of Pharmaceutical Outcomes and Policy, UNC Eshelman School of Pharmacy, University of North Carolina, Chapel Hill, NC, USA.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Overman
authors.forename: Janet K
authors.initials: JK
authors.lastname: Freburger
authors.forename: Magdalene M
authors.initials: MM
authors.lastname: Assimon
authors.forename: Xiaojuan
authors.initials: X
authors.lastname: Li
authors.forename: M Alan
authors.initials: MA
authors.lastname: Brookhart
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D016208:Databases, Factual / Q000706:statistics & numerical data*; D005260:Female; D006757:Hospital Units / Q000706:statistics & numerical data; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D007297:Inpatients / Q000706:statistics & numerical data; D007902:Length of Stay / Q000706:statistics & numerical data*; D008297:Male; D006278:Medicare; D008875:Middle Aged; D013997:Time Factors; D014481:United States; D055815:Young Adult
keywords: administrative claims; observation stays; pharmacoepidemiology; research methods