Abstract
PurposeTo estimate the effect of selective sampling on first contact (FC) studies of the relation between migration and schizophrenia.MethodsWe compared the FC method directly with a more inclusive longitudinal psychiatric register (LPR) method, by letting both methods estimate age and sex adjusted incidence rate ratios (IRR) in the population of The Hague aged 20–54 years, for the three largest migrant groups (first and second generation Caribbean, Turkish, and Moroccan) relative to the native Dutch population.ResultsBoth methods found that the adjusted IRR was higher for migrants than for native Dutch [all migrants IRR = 1.70 (95% Cl 1.30–2.21) for the LPR method and 1.91 (95% Cl 1.15–3.25) for the FC]. The IRR for Moroccans was significantly lower in the LPR [IRR 2.69 (95% 2.10–3.41)] than in the FC study [4.81 (3.41–6.68)]. The FC method was relatively more inclusive for migrants presenting at earlier ages or with shorter durations of prior treatment (DPT) than the native Dutch. This resulted in differential sampling and artificially higher IRRs for Moroccan and, to a lesser extent, Turkish migrants.ConclusionWe confirm that the incidence of schizophrenia is raised twofold for migrants compared to nonmigrants. Using the LPR method, however, IRR estimates were less pronounced for most migrant groups than in a high quality FC study conducted in the same population. The FC method may overestimate the risk of schizophrenia for migrant groups who seek first mental health at a relatively younger age, or who present directly with schizophrenia.
Highlights
IntroductionA worldwide meta-analysis of studies using the first contact (FC) method and published between 1977 and 2008 estimated the overall incidence rate ratio (IRR) of schizophrenia at 2.1 (95% 1.8–2.4) for first generation migrants and at 2.4 (95% 2.0–2.9) for second generation migrants, compared to nonmigrants [10]
The incidence rate ratios (IRR) for Moroccans was significantly lower in the longitudinal psychiatric register (LPR) [IRR 2.69 (95% 2.10–3.41)] than in the first contact (FC) study [4.81 (3.41–6.68)]
Using the LPR method, IRR estimates were less pronounced for most migrant groups than in a high quality FC study conducted in the same population
Summary
A worldwide meta-analysis of studies using the FC method and published between 1977 and 2008 estimated the overall incidence rate ratio (IRR) of schizophrenia at 2.1 (95% 1.8–2.4) for first generation migrants and at 2.4 (95% 2.0–2.9) for second generation migrants, compared to nonmigrants [10]. We have reported that the FC method can seriously underestimate the incidence of schizophrenia. Using a longitudinal psychiatric register (LPR) to estimate the incidence of schizophrenia, we found that up to two thirds of incident cases had not been included in a FC study conducted in the same population and time frame [1]. Subjects had been missed in the FC study because they were no longer prototypical ‘first contact’ by the time they met criteria for schizophrenia, and at that point were not actively monitored within the FC design anymore (e.g. two thirds had been treated for more than five years before the onset of psychosis, or were aged 40 or older at the time of diagnosis)
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