Comment on: Development and validation of an alternative ankylosing spondylitis disease activity score when patient global assessment is unavailable: reply

Dear Editor, We would like to thank Aranda-Valera et al. for their interest in our manuscript ‘Development and validation of an alternative ankylosing spondylitis disease activity score when patient global assessment is unavailable’ [1].

Ankylosing Spondylitis Disease Activity Score (ASDAS) is a composite instrument used to assess disease activity in axial spondyloarthritis, with optimal psychometric properties [2]. However, in research settings, ASDAS calculation is sometimes hampered by unavailability of some of its components, especially in older databases. In our paper, we developed and validated an alternative formula (alternative-ASDAS), to be used when Patient Global Assessment (PGA) is not available. The final formula replaces PGA in the ASDAS equation with Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). Alternative-ASDAS was developed in such a way that the same cut-off values as original-ASDAS could be applied for clinically important improvement (≥1.1) and major improvement (≥2) [1].

Aranda-Valera et al. tested the alternative-ASDAS formula in their cohort [3, 4] and obtained a very similar agreement with original-ASDAS as we did. This observation is important, as it confirms the robustness of our findings, representing a first attempt to an external validation [4].

The authors were especially interested in comparing the performance of alternative-ASDAS to that of BASDAS, an additional surrogate formula for ASDAS they previously had developed for cases where only BASDAI total is available (but not its individual questions) [3]. When compared with alternative-ASDAS, BASDAS had a slightly lower agreement with original-ASDAS, a higher minimal detectable change, a slightly increased s.e.m., and wider limits of agreement in the Bland and Altman plots [4]. These results are probably due to the different methodology followed in the development of these two indices. In fact, alternative-ASDAS was selected as the best performing index among a variety of possible substitutes (using the individual questions of BASDAI), while the BASDAS formula was elaborated, deciding a priori to use BASDAI total score and C reactive protein for cases when individual BASDAI questions were not available [1, 3].

Appraising these differences, and based on their analysis, Aranda-Valera et al. propose, in cases where original-ASDAS calculation is not possible, to use alternative-ASDAS as a first, and BASDAS as a second alternative [4]. We agree with this solution, especially considering that BASDAS seems to have a better agreement with original-ASDAS than past tentative simplified scores [5]. Hopefully, in the near future, more data will be available about further validation of the BASDAS, particularly in terms of sensitivity to change as this aspect could not be studied in the original study due to the characteristics of the selected cohort [3]. Moreover, further studies will clarify whether, for BASDAS, the same cut-offs for improvement as original-ASDAS can be used, as can be done for the alternative-ASDAS.

We certainly support the view that neither alternative-ASDAS nor BASDAS should be used to replace original-ASDAS whenever its calculation is possible and prospective studies should include all elements to be able to calculate the original-ASDAS [2].

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

Disclosure statement: Authors declare 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.

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