| Summary: | We thank the authors for the interest in our paper and are grateful for the opportunity to respond to the points raised1. We agree that there is a clear distinction between composite measures of psoriatic arthritis such as DAPSA and composite measures of psoriatic disease such as MDA/VLDA and 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 PsA. Indeed when the DAPSA was originally suggested, it was because the same components used in the DAREA were identified in a principal component analysis (PCA) in PsA. Interestingly in this analysis, the variables tested were taken from the 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 PASI highlighting the importance of skin in PsA despite the fact that patients had a low baseline mean PASI of only 3.3. Whilst PASI was not included in the DAPSA as the eigenvalue was 0.949 and therefore just under the threshold of 12, it is interesting to imagine how the results may have differed if it were developed in a group with slightly more active skin disease.
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