Comment on: Prevalence of systemic lupus erythematosus in Spain: higher than previously reported in other countries? Reply

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Deareditor, We would like to thank Chieh-Chun Yang et al. for their positive appraisal and comments on our work [1]. Undoubtedly, the explanations requested by them have improved the clarity of our work [2]. We collected information on the place of birth and nationality, but not on ethnic groups, as such data are not available in official statistics. The percentage of people born abroad was lower in the sample than in the general adult population in Spain (7% and 14.5%, respectively), which is likely attributable to the difficulty of accessing population from certain nationalities, due to language differences. In line with this, foreigners born in Africa and in Asia were under-represented in the sample (0.4% vs 2.1% and 0.02% vs 0.8%, respectively, in the general adult population), while foreigners born in America were over-represented (2.8% vs 2.5% in the general adult population). Although this could have an impact on the estimation of SLE prevalence, the magnitude of the figures was not high enough to invalidate our global estimation. We agree with Chieh-Chun Yang et al. regarding the comparison between urban and rural areas. As mentioned in the Discussion section of our manuscript, EPISER2016 was designed to determine the global prevalence of several rheumatic diseases. Our analysis of any associations with sociodemographic, anthropometric and lifestyle variables was a secondary objective. For pathologies with a low prevalence, such as SLE, the statistical power for this analysis was very limited and the estimate was quite imprecise (95% CI: 1.216, 13.424). We appreciate the comment on inter-observer agreement. Though it may help avoid a lower possibility of misdiagnosis, it could not be accomplished for logistical reasons (e.g. in some of the participating centres only one rheumatologist researcher was available). Nevertheless, only 1 of the 12 SLE cases detected in EPISER2016 had not been previously diagnosed; thus, even absent this case the prevalence would still be higher than the results reported by previous studies in different countries. We agree that increasing the sample size would allow us to obtain more precise estimates of SLE prevalence; however, feasibility issues would have been difficult to overcome. Finally, although higher than that described in most international epidemiological studies, SLE prevalence in EPISER2016 proved closer to that observed in another Southern European country, namely Greece [3]. This, together with the similarity between the observed and the estimated number of hospital admissions with one diagnosis code corresponding to SLE (as mentioned in the Discussion section of our manuscript), led us to believe that the prevalence of SLE as detected by EPISER2016 reflects the reality of this disease in Spain. Funding: EPISER2016 was supported by Celgene, Gebro Pharma, Merck Sharp & Dohme in Spain, Pfizer and Sanofi-Aventis, which had no role in the design, data collection, data analysis, interpretation, or writing of this manuscript. Disclosure statement: The authors have declared no conflicts of interest. Data are available upon reasonable request by any qualified researchers who engage in rigorous, independent scientific research, and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a Data Sharing Agreement (DSA). All data relevant to the study are included in the article. Supplementary data are available at Rheumatology online.

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