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?

Objective To test the psychometric performance of a modified Disease Activity index for PSoriatic Arthritis (DAPSA) using 28 instead of 66 swollen/68 tender joint counts (SJC/TJC). Methods We included patients with psoriatic arthritis (PsA) from the Danish national quality registry DANBIO, divided into examination (n=3157 patients, 23987 visits) and validation cohorts (n=3154 patients, 24160 visits). We defined DAPSA28 = (28TJC × conversion factor 1) + (28SJC × conversion factor 2) + patient global (0-10VAS) + pain (0-10VAS) + C reactive protein (CRP) (mg/dL). Identification of the conversion factors was performed by generalised estimating equations in the examination cohort and evaluation of criterion, correlational and construct validity in the validation cohort. Results We estimated DAPSA28 = (28TJC × 1.6) + (28SJC × 1.6) + patient global (0-10VAS) + pain (0-10VAS) + CRP (mg/dL). Criterion validity: DAPSA/DAPSA28 had comparable discriminative power expressed as standardised mean difference (DAPSA, 0.90; DAPSA28, 0.93) to distinguish between patients in high and low disease activity. Kappa with quadratic weighting of DAPSA/DAPSA28 disease activity states was high: 0.92 (95% CI 0.92 to 0.92). Standardised response means for DAPSA/DAPSA28 were -0.96/-0.92 for visits after biological DMARD-initiation. Correlational validity: Baseline DAPSA/DAPSA28 had high correlation with 28-joint disease activity score with CRP (r=0.87/r=0.93), simplified disease activity index (r=0.92/r=0.99), p<0.001. Bland-Altman plot showed better agreement between DAPSA/DAPSA28 for low than high disease activity. Construct validity: DAPSA/DAPSA28 were similarly correlated to Health Assessment Questionnaire; r=0.60/0.62, p<0.001. DAPSA/DAPSA28 discriminated patients reporting their symptom state as acceptable versus not acceptable equally well: Mean (SD) 9.1 (8.7)/8.4 (8.0) and 24.2 (14.9)/22.5 (13.8), respectively. Conclusion Our study suggests that data sets with only 28-joint counts