Assessment of the consequences of caregiving in psychosis: a psychometric comparison of the Zarit Burden Interview (ZBI) and the Involvement Evaluation Questionnaire (IEQ)

BACKGROUND: The Zarit Burden Interview (ZBI) was originally developed to assess the level of subjective burden in caregivers of people with dementia. The Involvement Evaluation Questionnaire (IEQ) is amongst the leading scales to assess caregiving consequences in severe mental illness. We aimed to compare the psychometric properties of the ZBI, a generic tool, and of the IEQ, a more specific tool to assess the consequences of caregiving in schizophrenia and related disorders. METHODS: Secondary analyses of a 16-week, randomized controlled trial of a psychoeducational intervention in 223 primary caregivers of patients with schizophrenia or schizoaffective disorder. Psychometric properties (internal consistency, convergent and discriminative validity, and sensitivity to change) were evaluated for both ZBI and IEQ. RESULTS: Internal consistency was good and similar for both scales (ZBI: 0.91, 95% CI: 0.89, 0.94; IEQ: 0.86, 95% CI: 0.83, 0.89). Convergent validity was relevant for similar domains (e.g. ZBI total score vs IEQ-tension r = 0.69, 95% CI: 0.61, 0.75) and at least moderate for the rest of domains (ZBI total score, personal strain and role strain vs IEQ-urging and supervision). Discriminative validity against psychological distress and depressive symptoms was good (Area Under the Curve [AUC]: 0.77, 95% CI: 0.71, 0.83; and 0.69, 95% CI: 0.63, 0.78 – for ZBI against GHQ-28 and CES-D respectively; and AUC: 0.72, 95% CI: 0.65, 0.78; and 0.69, 95% CI: 0.62, 0.77 – for IEQ against GHQ-28 and CES-D respectively). AUCs against the reference criteria did not differ significantly between the two scales. After the intervention, both scales showed a significant decrease at endpoint (p-values < 0.001) with similar standardised effect sizes for change (-0.36, 95% CI: -0.58, -0.15 – for ZBI; -0.39, 95% CI: -0.60, -0.18 – for IEQ). CONCLUSIONS: Both ZBI and IEQ have shown satisfactory psychometric properties to assess caregiver burden in this sample. We provided further evidence on the performance of the ZBI as a general measure of subjective burden. TRIAL REGISTRATION: (ISRCTN32545295).