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Abstract
Objective Age at rheumatoid arthritis (RA) onset varies by geographical latitude. We have investigated to what extent differences in patient-specific factors and country-level socioeconomic indicators explain this variability.
Methods Patients with RA from the worldwide METEOR registry were included. Bayesian multilevel structural equation models were used to study the relationship between the absolute value of (hospital) geographical latitude and age at diagnosis (as a proxy for age at RA onset). We examined to what extent this effect is mediated by individual patient characteristics and by country-specific socioeconomic indicators and disentangled whether the observed effects occurred at the patient, hospital, or country levels.
Results We included 37 981 patients from 93 hospitals in 17 geographically widespread countries. Mean age at diagnosis per country ranged from 39 (Iran) to 55 (Netherlands) years. Per degree increase in country latitude (between 9.9° and 55.8°), mean age at diagnosis increased by 0.23 years (95% credibility interval: 0.095 to 0.38) (reflecting >10 years difference in age at RA onset). For hospitals within a country, this latitude effect was negligible. Inclusion of patient-specific factors (eg, gender, anticitrullinated protein antibodies status) in the model augmented the main effect from 0.23 to 0.36 years. Inclusion of country-level socioeconomic indicators (eg, gross domestic product per capita) in the model almost effaced the main effect (from 0.23 to 0.051 (−0.37 to 0.38)).
Conclusions Patients living closer to the equator get RA at a younger age. This latitude gradient was not explained by individual patient characteristics, but rather by countries’ socioeconomic status, providing a direct link between countries’ level of welfare and the clinical onset of RA.
- Rheumatoid Arthritis
- Epidemiology
- Economics
Data availability statement
Data are available upon reasonable request. The data that have been used for the current study, which are by definition deidentified, are available upon reasonable request to the corresponding author. Data shall only be made available after approval by the individual study contributors of a submitted research proposal, with investigator support and after a signed data access agreement. A data dictionary and the syntaxes that have been used for the analyses are also available upon reasonable request to the corresponding author.
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Data availability statement
Data are available upon reasonable request. The data that have been used for the current study, which are by definition deidentified, are available upon reasonable request to the corresponding author. Data shall only be made available after approval by the individual study contributors of a submitted research proposal, with investigator support and after a signed data access agreement. A data dictionary and the syntaxes that have been used for the analyses are also available upon reasonable request to the corresponding author.
Footnotes
Handling editor Josef S Smolen
Twitter @AlexSepriano
Contributors SAB and RBML contributed to the conception and design of the work. SAB, XMEM and AS analysed the data. SAB, XMEM, AS and RBML interpreted the data and drafted the manuscript. AC, L-LW, DV-M and KS-E contributed to data acquisition. All authors critically revised and approved the final version of the paper. All authors accepted responsibility to submit the manuscript for publication. SAB acts as guarantor and accepts full responsibility for the conduct of the study, had access to the data and controlled the decision to publish.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests AC: none, SAB: none, RBML: none, XMEM: none, KS-E: none, AS received consultancing fees/speaker honoraria/meeting attendance support from UCB, Abbvie and Eli-Lilly. DV-M received speaker honoraria from Roche, Abbvie, Janssen.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Not commissioned; externally peer reviewed.
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