• disease activity
• systemic lupus erythematosus
• ultrasonography
• arthritis
• synovitis

We read with interest the report by Jesus et al on the Systemic Lupus Erythematosus Disease Activity Score (SLE-DAS).1 We agree that the systemic lupus erythematosus disease activity index 2000 (SLEDAI-2K) in its original form has limitations as a disease activity tool. Since each organ manifestation can only be graded as present or absent, it is unable to differentiate between mild, moderate or severe disease activity, nor partial response in any feature.

Our group is researching outcome measures in musculoskeletal SLE. We published a previous study of responsiveness of various musculoskeletal outcome measures in patients treated with glucocorticoids.2 As expected, the musculoskeletal component of the SLEDAI-2K was less responsive than the physician visual analog scale (physician VAS), musculoskeletal component of the British Isles Lupus Assessment Group (BILAG) index or ultrasound, all of which allow for various grades of severity.

The musculoskeletal component of the new SLE-DAS should, in theory, improve its responsiveness as it includes a term for swollen joint count as a continuous variable. We therefore calculated musculoskeletal component of new SLE-DAS tool in our previously published data set of 20 patients. These patients had objective synovitis, were treated with 120 mg of intramuscular depomedrone and assessed at 0 and 4 weeks. We calculated the musculoskeletal component of the SLE-DAS using the terms:

As in our publication, change was assessed using Wilcoxon signed-rank test. Effect size was calculated using standardised test statistic, Z, using the formula r=Z/sqrt(n1+n2). Effect sizes were judged using Cohen’s criteria as large (≥0.5), medium (≥0.3) or small (≥0.1).3

As expected, and like the other variables, the musculoskeletal SLE-DAS significantly reduced from median 13.4 (IQR 12.6–14.7) to median 6.2 (IQR 0.0–12.5, p=0.001). The effect size was large at −0.548. As predicted by its developers, this was superior to the musculoskeletal component of SLEDAI-2K which had an effect size of −0.418. However, the musculoskeletal SLE-DAS was not as responsive as the musculoskeletal component of the BILAG index (−0.576), which could be graded as improving, or physician VAS (−0.599), which was a continuous variable. However, the disadvantage of these two variables is that they are heavily dependent on the training and expertise of the assessors. Measures based on joint counts like SLE-DAS may show more robust inter-reader reliability.

There is another crucial feature of musculoskeletal lupus in which the BILAG index and physician VAS are advantageous: subclinical synovitis. We recently showed that in patients with SLE presenting with inflammatory joint pain, 38% had swollen joints. However, a further 27% had ultrasound-proven synovitis without a swollen joint. Ultrasound-only synovitis was clinically significant, being associated with worse serology, and patient-reported and physician-reported outcomes.4 Since the musculoskeletal component of SLE-DAS is based entirely on swollen joint count, it would not be able to detect or monitor disease activity in this subgroup.

In summary, we validate the superior responsiveness of the musculoskeletal component of SLE-DAS compared with the SLEDAI-2K in an independent longitudinal study. However, our data suggest that a better outcome measure focusing on this specific organ system could be designed.