Incidence, prevalence and mortality of rheumatoid arthritis in Canterbury, Aotearoa New Zealand.

Background: There is relatively little research regarding the frequency of rheumatoid arthritis (RA) in Canterbury and no previous studies comparing the mortality of RA in Aotearoa New Zealand to a general population.

Aims: To determine the incidence, prevalence and mortality of RA in Canterbury, Aotearoa New Zealand.

Methods: Clinical records of persons attending the Christchurch Hospital rheumatology service between 1 January 2006 and 31 December 2008 (Period 1) and between 1 January 2011 and 31 December 2013 (Period 2) were reviewed to identify individuals who met the 2010 American College of Rheumatology/European League Against Rheumatism RA classification criteria. Medical records for persons treated by private sector rheumatologists were also reviewed. The period prevalence and average annual incidence for each time period were calculated. Prevalence and incidence rates for the total population were age- and sex-adjusted based on the usually resident population from the 2013 national Census. Standardised mortality ratios (SMRs) compared to the general population were calculated.

Results: The annual incidence was 24.40 per 100 000 (95% confidence interval (CI): 21.36-27.43) for Period 1 and 22.06 per 100 000 (95% CI: 19.32-24.81) for Period 2. The period prevalence was 442.65 per 100 000 (95% CI: 420.08-465.04) for Period 1 and 448.66 per 100 000 (95% CI: 427.31-470.01) for Period 2. The all-cause SMR for individuals with incident RA was 1.25 (95% CI: 0.95-1.60). The SMR for all prevalent RA during Period 1 was 2.01 (95% CI: 1.85-2.19) and for Period 2 was 1.87 (95% CI: 1.69-2.06).

Conclusions: The frequency of RA in the Canterbury population is comparable to other regions. Mortality for all individuals with prevalent RA was approximately two times higher than the general population.