Predictors of treatment resistance in patients with schizophrenia: a population-based cohort study

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. Findings 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 0.95-097]), living in a less urban area (provincial 138 [1z23-1.56], rural 1.44 [1.25-165]), 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 122 [1.10-137]), and previous suicide attempt (1.21 [1.07-1.39]) were all significantly associated with treatment-resistant schizophrenia. Interpretation 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.