title: Predictors of treatment resistance in patients with schizophrenia: a population-based cohort study.
abstract: BACKGROUND
  Identification of patients at high risk of treatment-resistant schizophrenia at the time of schizophrenia diagnosis would be of great clinical benefit in minimising the delay to clozapine treatment in patients unlikely to respond to non-clozapine antipsychotics. However, little is known about predictors of treatment resistance in this patient population. We used a treatment-based proxy for treatment-resistant schizophrenia to identify candidate predictors of treatment resistance at first hospital contact with a schizophrenia diagnosis.
  
  
  METHODS
  In this population-based cohort study, we obtained Danish national registry data for all adult patients (≥18 years) with incident schizophrenia diagnosed between Jan 1, 1996, and Dec 31, 2006, and followed up until Dec 31, 2010. Our main proxy definition of treatment-resistant schizophrenia was the earliest instance of either clozapine initiation or hospital admission for schizophrenia after having had two periods of different antipsychotic monotherapy. We did multivariable Cox proportional hazards regression analysis to estimate the association between baseline candidate predictors and treatment resistance.
  
  
  RESULTS
  8624 patients fulfilled the criteria for inclusion. In multivariable complete-case analyses, 1703 (21%) of 8044 patients fulfilled the main proxy definition of treatment-resistant schizophrenia during a median follow-up of 9·1 years (IQR 6·3-11·9). Younger age (hazard ratio 0·96 [95% CI 0·95-0·97]), living in a less urban area (provincial 1·38 [1·23-1·56], rural 1·44 [1·25-1·65]), primary education level (0·88 [0·79-0·98]), more than 30 bed-days in psychiatric hospital in the year before first schizophrenia diagnosis (1·54 [1·35-1·75]), inpatient at first schizophrenia diagnosis (2·07 [1·87-2·29]), paranoid subtype (1·24 [1·13-1·37]), comorbid personality disorder (1·24 [1·11-1·39]), psychotropic drug use (antipsychotics 1·51 [1·35-1·69], antidepressants 1·15 [1·03-1·29], and benzodiazepines 1·22 [1·10-1·37]), and previous suicide attempt (1·21 [1·07-1·39]) were all significantly associated with treatment-resistant schizophrenia.
  
  
  CONCLUSIONS
  Our study identifies several candidate predictors that could potentially be included in future prediction models for treatment-resistant schizophrenia. Notably, established risk factors for schizophrenia did not predict treatment resistance, suggesting that treatment-resistant disease might be a distinct subtype of schizophrenia and not merely a more severe form.
  
  
  BACKGROUND
  European Community's Seventh Framework Programme.
authors.affiliation: National Centre for Register-based Research, School of Business and Social Sciences, Aarhus University, Aarhus, Denmark. Electronic address: tw@econ.au.dk.
authors.forename: Theresa
authors.initials: T
authors.lastname: Wimberley
authors.affiliation: Section of Biostatistics, Department of Public Health, Aarhus University, Aarhus, Denmark.
authors.forename: Henrik
authors.initials: H
authors.lastname: Støvring
authors.affiliation: The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus University, Aarhus, Denmark; Research Unit, Mental Health Centre Copenhagen, Faculty of Health Sciences, University of Copenhagen, Gentofte, Denmark.
authors.forename: Holger J
authors.initials: HJ
authors.lastname: Sørensen
authors.affiliation: National Centre for Register-based Research, School of Business and Social Sciences, Aarhus University, Aarhus, Denmark.
authors.forename: Henriette T
authors.initials: HT
authors.lastname: Horsdal
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors.forename: James H
authors.initials: JH
authors.lastname: MacCabe
authors.affiliation: National Centre for Register-based Research, School of Business and Social Sciences, Aarhus University, Aarhus, Denmark; Centre for Integrated Register-based Research, CIRRAU, Aarhus University, Aarhus, Denmark.
authors.forename: Christiane
authors.initials: C
authors.lastname: Gasse
mesh_terms: D000293:Adolescent; D000328:Adult; D014150:Antipsychotic Agents / Q000627:therapeutic use*; D003024:Clozapine / Q000627:therapeutic use*; D015331:Cohort Studies; D003718:Denmark; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012042:Registries; D012307:Risk Factors; D012559:Schizophrenia / Q000175:diagnosis / Q000188:drug therapy*; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D014150:Antipsychotic Agents; D003024:Clozapine