Comment on: Sudanese and Swedish patients with systemic lupus erythematosus, immunological and clinical comparisons: reply.

Dear Editor, We have with interest read the response from Zaixing Yang and Yan Liang [1] to our recently published comparative paper on SLE in Sudan and Sweden [2]. The authors stress the fact that differences in how laboratory analyses are performed in different countries may result in a difference in which patients are actually included from the regions to be compared. We agree that this is a tentative problem with any study comparing SLE cohorts defined by classification criteria including laboratory measures, which is the case both for the 1982 criteria used in our study [3] and the recently published EULAR/ACR criteria [4]. This problem is especially prominent when using retrospective data. We also want to clarify that both the compared cohorts were incipient cohorts, and that when the Sudanese at an earlier time point were classified as SLE, the laboratory analyses were performed at the central laboratories serving the hospitals where the rheumatology clinics resided. Laboratory measures evaluated at the outpatient visit when the patients were included in the present study were, however, obtained from different hospital-based and private laboratories, making disease activity comparison based on such diverse data more difficult.

Drs Yang and Liang argue that we should have used the laboratory data obtained from each country instead of autoantibody analyses performed in Uppsala when comparing SLE patients from Sudan and Sweden, and that such an approach would yield information about the comparability of autoantibody laboratory data between the countries. We do not agree with this idea. Our main objective was to compare the patients living in Sudan and Sweden, and not to compare laboratory procedures. We think that a proper immunological comparison between patients living in different geographical areas relies on the use of identical laboratory analyses for both patient groups.

The Swedish population controls were chosen to individually match the Swedish SLE patients for age, sex and residential area, the only exclusion criterium being SLE. The authors are correct in that there is a difference between the Sudanese healthy controls and the Swedish population controls used to make national alignment of reference values. For that reason, we did not emphasize the difference between the Sudanese and Swedish control groups in our paper. Both this and our previous papers on rheumatoid arthritis in Sudan [5, 6] and Malaysia [7] have shown that such national alignment of reference values and ‘cutoffs’ increases the value of autoantibody analyses.

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.