title: Does case mix matter for substance abuse treatment? A comparison of observed and case mix-adjusted readmission rates for inpatient substance abuse treatment in the Department of Veterans Affairs.
abstract: OBJECTIVE
  To develop a case mix model for inpatient substance abuse treatment to assess the effect of case mix on readmission across Veterans Affairs Medical Centers (VAMCs).
  
  
  METHODS
  The computerized patient records from the 116 VAMCs with inpatient substance abuse treatment programs between 1987 and 1992.
  
  
  METHODS
  Logistic regression was used on patient data to model the effect of demographic, psychiatric, medical, and substance abuse factors on readmission to VAMCs for substance abuse treatment within six months of discharge. The model predictions were aggregated for each VAMC to produce an expected number of readmissions. The observed number of readmissions for each VAMC was divided by its expected number to create a measure of facility performance. Confidence intervals and rankings were used to examine how case mix adjustment changed relative performance among VAMCs.
  
  
  METHODS
  Ward where care was provided and ICD-9-CM diagnosis codes were used to identify patients receiving treatment for substance abuse (N = 313,886).
  
  
  RESULTS
  The case mix model explains 36 percent of the observed facility level variation in readmission. Over half of the VAMCs had numbers of readmissions that were significantly different than expected. There were also noticeable differences between the rankings based on actual and case mix-adjusted readmissions.
  
  
  CONCLUSIONS
  Secondary data can be used to build a reasonably stable case mix model for substance abuse treatment that will identify meaningful variation across facilities. Further, case mix has a large effect on facility level readmission rates for substance abuse treatment. Uncontrolled facility comparisons can be misleading. Case mix models are potentially useful for quality assurance efforts.
authors.affiliation: Program Evaluation and Resource Center, Stanford University, CA, USA.
authors.forename: C S
authors.initials: CS
authors.lastname: Phibbs
authors.forename: R W
authors.initials: RW
authors.lastname: Swindle
authors.forename: B
authors.initials: B
authors.lastname: Recine
mesh_terms: D000328:Adult; D003953:Diagnosis-Related Groups / Q000706:statistics & numerical data*; D006787:Hospitals, Veterans / Q000706:statistics & numerical data*; D006801:Humans; D016015:Logistic Models; D010359:Patient Readmission / Q000706:statistics & numerical data*; D011785:Quality Assurance, Health Care; D015203:Reproducibility of Results; D012307:Risk Factors; D016320:Substance Abuse Treatment Centers / Q000458:organization & administration*; D019966:Substance-Related Disorders / Q000628:therapy*; D014481:United States; D014728:Veterans / Q000706:statistics & numerical data*