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

DearEditor, We read with great interest the article by Verdú et al. estimating the prevalence of SLE in the general adult population in Spain [1]. We highly appreciate the great efforts by the authors, using a population-based cross-sectional study and simultaneously exploring the association of SLE prevalence with sociodemographic, anthropometric and lifestyle variables. However, we hope there is a further clarification about the methodological issues and conclusions in this research.

Firstly, we suggest the reasons for high SLE prevalence in Spain are more detailed than in this study. Was there any bias in the study design leading to the higher prevalence, or does the study indeed reflect the actual status in Spain? In this study, the prevalence in Spain was estimated as 210 cases per 100 000 adults, obviously higher than the results of previous studies by different countries. (Korea: 35.45/100 000 [2]; Taiwan: 37.00/100 000 [3]; UK: 97.04/100 000 [4]; Italy: 71.00/100 000 [5]) Further investigations are recommended to confirm whether it represents the actual situation, and the reasons behind the high prevalence in Spain should be clarified.

Secondly, the ethnicity of the participants should be considered as a variable when it comes to cluster sampling. In this study, the sample was randomly selected based on rural/urban municipalities, sex and age. However, we did not see detailed description of the ethnic groups of participants. A previous research by Stojan et al. has reported that the prevalence of SLE in different ethnic groups could vary [6]. (Hispanics and Asians had similar prevalence of SLE, which was higher than Caucasians, but lower than African Americans.) In other words, if the ethnic composition of the sample is inconsistent with Spanish society, selection bias may exist in this study and cause the inaccurate result. Therefore, we recommend considering the ethnicity of the participants and confirming that their composition is the same as that of Spanish society.

In addition, we have queries about the different SLE prevalence in rural regions vs urban regions. According to previous research by Gergianaki et al., the prevalence of SLE was 165/100 000 in urban regions vs 123/100 000 in rural regions (P <0.001) [7]. Nevertheless, in Verdú’s study, SLE is more prevalent in rural areas compared with urban areas, with an odds ratio of 4.041. We suspect that only a few SLE cases (12 cases) in this study may not be suitable for drawing conclusions that rural areas are more prevalent than urban areas. Having a larger sample size and detecting more SLE cases may help determine the comparison between urban regions and rural regions.

Finally, to the best of our knowledge, inter-observer agreement is recommended to test the reliability of SLE diagnosis. For instance, individuals who have not been previously diagnosed but have positive results in the symptom-based screening can be assigned to two rheumatologists for clinical assessments. The inter-observer agreement can then be confirmed by calculating the Kappa statistic. In brief, the confirmation of inter-observer agreement may allow a lower possibility of misdiagnosis and enhance the credibility of this research.

As mentioned above, we look forward to further explanations for the reasons behind the high SLE prevalence in Spain. Besides, we are convinced that considering ethnicity as an important variable in cluster sampling and increasing your sample size to detect more SLE cases might be beneficial to your great work. Additionally, we sincerely recommend that the study design should include inter-observer agreement to test the reliability of the diagnosis of SLE.

Acknowledgements

All authors (C.-C.Y., S.-C.K. and J.C.-C.W.) provided their input by contributing to the conceptualization. C.-C.Y. and S.-C.K. contributed to the editing of the manuscript equally.

Funding: No specific funding was received from any funding bodies in the public, commercial or not-for-profit sectors to carry out the work described in this manuscript.

Disclosure statement: The authors have declared no conflicts of interest.

Data availability statement

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.

References

Author notes