THU0246 Can disease activity in patients with psoriatic arthritis be adequately assessed by a modified disease activity index for psoriatic arthritis (DAPSA) based on 28 joints?
Authors: B. Michelsen, J. Sexton, J. Smolen, D. Aletaha, N.S. Krogh, D. van der Heijde, T.K. Kvien, M.L. Hetland
Journal: Annals of the Rheumatic Diseases (2018)

<h3>Background</h3> 66/68 vs. 28 joint count has higher face validity in PsA. However, many databases/registries routinely collect only 28-joint count. It is not known if these data can be used to provide sufficient information on disease activity and response to therapy. <h3>Objectives</h3> To compute and test the potential validity of a simplified Disease Activity index for PSoriatic Arthritis (DAPSA) using 28 instead of 66/68 joint count. <h3>Methods</h3> We included PsA patients from the Danish national quality registry DANBIO, divided into examination (n=3157 patients,24160 visits) and validation cohorts (n=3154 patients,24160 visits) according to odd/even IDs. We defined: DAPSA28=(28TJCxconversion factor<sub>1</sub>)+(28SJCxconversion factor<sub>2</sub>)+patient global[0–10VAS]+pain[0–10VAS]+CRP[mg/dL]. Identification of conversion factors was performed by Generalised Estimating Equations (GEE, multiple visits per patient) in the examination cohort, and <i>criterion, correlational and construct validity</i> explored in the validation cohort. <h3>Results</h3> Mean(SD) age: 52.0 (13.8) years, 54.4% females. GEE: C<i>onversion factor<sub>1</sub></i>=1.6, 95% CI(1.6–1.7), c<i>onversion factor<sub>2</sub></i>=1.6, 95% CI(1.5–1.6), leading to: DAPSA28=(28TJC+28 SJC)x1.6+patient global[0–10VAS]+pain[0–10VAS]+CRP[mg/dL]. <i>Criterion validity:</i> Physician global and DAPSA/DAPSA28 were similarly correlated (r=0.63/r=0.61, p&lt;0.001). DAPSA/DAPSA28 had comparable discriminative power, expressed as standardised mean difference (DAPSA,0.90; DAPSA28,0.93) to distinguish between patients in high (starting bDMARD) and low (not starting/changing s/bDMARD for ≥60 days) disease activity. Agreement between DAPSA/DAPSA28 disease activity states was best for remission/low disease activity (table 1). Kappa with quadratic weighting of DAPSA/DAPSA28 disease activity states indicated very good agreement; 0.92 95% CI (0.92–0.92). Standardised response means for DAPSA/DAPSA28 were −0.96/–0.92 (n=572) for visits after start of bDMARD. <i>Correlational validity:</i> Baseline DAPSA/DAPSA28 had strong correlation with DAS28CRP (r=0.87/r=0.93), SDAI (r=0.92/r=0.99), p&lt;0.001. Bland-Altman plot showed better agreement between DAPSA/DAPSA28 for low than high disease activity (figure 1). <i>Construct validity:</i> DAPSA/DAPSA28 were similarly correlated to HAQ; r=0.60/0.62,p&lt;0.001. DAPSA/DAPSA28 discriminated patients reporting their symptom state as acceptable (n=1140) vs. not acceptable (n=1045) equally well: mean(SD) 9.1 (8.7)/8.4 (8.0) and 24.2 (14.9)/22.5 (13.8), respectively. <h3>Conclusions</h3> DAPSA28 showed good criterion, correlational and construct validity, and sensitivity to change. However, agreement between DAPSA and DAPSA28 was better for low than high disease activity levels. We recommend that 66/68 joint count should be performed and the original DAPSA should be preferred in PsA. However, our study suggests that data sets with only 28-joint counts available can use DAPSA28, especially in patients with low disease activity. <h3>Disclosure of Interest</h3> B. Michelsen: None declared, J. Sexton: None declared, J. Smolen: None declared, D. Aletaha: None declared, N. Krogh: None declared, D. van der Heijde: None declared, T. Kvien: None declared, M. Hetland Consultant for: Abbvie, Biogen, BMS, CellTrion, MSD, Novartis, Orion, Pfizer, Samsung, UCB.