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We thank the authors for the interest in our paper and are grateful for the opportunity to respond to the points raised.1 We agree that there is a clear distinction between composite measures of psoriatic arthritis such as disease activity in PsA (DAPSA) and composite measures of psoriatic disease such as minimal or very low disease activity (MDA/VLDA) and the PsA disease activity score (PASDAS). As the Vienna group rightly point out, these measures differ in terms of the components included, but not due to disagreement within the outcome measure community as suggested in the letter. The choice of components for each composite measure was decided using different methodology in the development of each one, thus resulting in different measures. We believe that this variation in scores is one reason for the need to compare such scores in different populations to establish the optimal measure or measures for psoriatic arthritis (PsA). Indeed when the DAPSA was originally suggested, it was because the same components used in the disease activity in reactive arthritis (DAREA) were identified in a principal component analysis (PCA) in PsA. Interestingly in this analysis, the variables tested were taken from the Outcome mesaures in Rheumatology (OMERACT) PsA domains and therefore DAPSA was not, a priori, designed specifically to be a unidimensional composite measure. The fourth component identified in the PCA was the psoriasis area and severity index (PASI) highlighting the importance of skin in PsA despite the fact that patients had a low baseline mean PASI of only 3.3. While PASI was not included in the DAPSA as the eigenvalue was 0.949 and therefore just under the threshold of 1,2 it is interesting to imagine how the results may have differed if …
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