• systemic lupus erythematosus
• disease activity
• arthritis

It was with great interest that we read the letter ‘Assessment of responsiveness of the musculoskeletal component of SLE-DAS in an independent cohort’ by Hassan et al.1

We recently published the derivation and validation of the Systemic Lupus Erythematosus Disease Activity Score (SLE-DAS), a new continuous measure of SLE disease activity with high sensitivity and specificity to change.2

Hassan et al report in their letter that in an analysis of 20 SLE patients, the responsiveness of the arthritis component of SLE-DAS (large effect size of −0.548) is superior to that of the arthritis component of SLE Disease Activity Index (DAI)-2K (medium effect size of −0.418).1 3 These results corroborate our data.2 4 Moreover, Hassan et al report that the arthritis component of SLE-DAS presents a similarly large effect size comparing with the arthritis component of the British Isles Lupus Assessment Group (BILAG) (−0.576). As highlighted by Hassan et al, the SLE-DAS is expected to present a more robust inter-reader reliability than physician visual analogue scale and BILAG, and to be less dependent on the training and expertise of the assessors than BILAG.

We further emphasise that SLE-DAS is a global disease activity index (non-organ specific), which presents a major advantage over BILAG for use in daily clinical practice, as it is much less time-consuming and requires a collection time similar to SLEDAI.

Hassan et al report that joint ultrasonography (US) is able to detect subclinical synovitis in SLE patients. We agree that in selected patients joint US can be useful to complement clinical assessment. However, the requirement for US joint assessment is not advisable to integrate a disease activity instrument, such as SLE-DAS, as it would constitute a major barrier for its practicality. Importantly, previous studies did not find any added value of systematic joint US assessment for treatment decision in chronic arthritis.5–7

In summary, this letter by Hassan et al corroborates the improved responsiveness of the arthritis component of SLE-DAS in an independent longitudinal cohort. The relative sensitivity and specificity of SLE-DAS and BILAG will be further tested in our upcoming study comparing BILAG and SLE-DAS.