Epidemiology of multimorbidity in New Zealand: a cross-sectional study using national-level hospital and pharmaceutical data

OBJECTIVES: To describe the prevalence of multimorbidity (presence of two or more long-term health conditions) in the New Zealand (NZ) population, and compare risk of health outcomes by multimorbidity status. DESIGN: Cross-sectional analysis for prevalence of multimorbidity, with 1-year prospective follow-up for health outcomes. SETTING: NZ general population using national-level routine health data on hospital discharges and pharmaceutical dispensing. PARTICIPANTS: All NZ adults (aged 18+, n=3 489 747) with an active National Health Index number at the index date (1 January 2014). OUTCOME MEASURES: Prevalence of multimorbidity was calculated using two data sources: prior routine hospital discharge data (61 ICD-10 coded diagnoses from the M3 multimorbidity index); and recent pharmaceutical dispensing records (30 conditions from the P3 multimorbidity index). METHODS: Prevalence of multimorbidity was calculated separately for the two data sources, stratified by age group, sex, ethnicity and socioeconomic deprivation, and age and sex standardised to the total population. One-year risk of poor health outcomes (mortality, ambulatory sensitive hospitalisation (ASH) and overnight hospital admission) was compared by multimorbidity status using logistic regression adjusted for confounders. RESULTS: Prevalence of multimorbidity was 7.9% using past hospital discharge data, and 27.9% using past pharmaceutical dispensing data. Prevalence increased with age, with a clear socioeconomic gradient and differences in prevalence by ethnicity. Age and sex standardised risk of 1-year mortality was 2.7% for those with multimorbidity (defined on hospital discharge data), and 0.5% for those without multimorbidity (age and sex-adjusted OR 4.8, 95% CI 4.7 to 5.0). Risk of ASH was also increased for those with multimorbidity (eg, pharmaceutical discharge definition: age and sex-standardised risk 6.2%, compared with 1.8% for those without multimorbidity; age and sex-adjusted OR 3.6, 95% CI 3.5 to 3.6). CONCLUSIONS: Multimorbidity is common in the NZ adult population, with disparities in who is affected. Providing for the needs of individuals with multimorbidity requires collaborative and coordinated work across the health sector.