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 factor1) + (28SJC × conversion factor2) + 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 available can be used to calculate DAPSA28, especially in patients with low disease activity. DAPSA28 showed good criterion, correlational and construct validity and sensitivity to change. Still, our results support that 66/68 joint count should be performed and the original DAPSA should be preferred in PsA.
- psoriatic arthritis
- disease activity
- outcomes research
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Handling editor Francis Berenbaum
Contributors NSK and MLH were responsible for data acquisition. BM analysed the data and wrote the manuscript. All authors were responsible for study design, critical revision of the manuscript and approval of the final version.
Funding DANBIO is supported by unrestricted grants from AbbVie A/S, Biogen (Denmark) A/S, Bristol-Myers Squibb Danmark, Eli Lilly Denmark, MSD Danmark ApS, Novartis Danmark A/S, Pfizer Danmark ApS, Roche A/S, UCB Nordic A/S. These entities have no influence on DANBIO’s work, including user interface, data collection, statistical analyses or research projects. BM has received clinical research fellowships from Diakonhjemmet Hospital and the Hospital of Southern Norway Trust.
Competing interests TKK has received fees for speaking and/or consulting from AbbVie, Biogen, BMS, Boehringer Ingelheim, Celgene, Celltrion, Eli Lilly, Epirus, Hospira, Merck-Serono, MSD, Mundipharma, Novartis, Oktal, Orion Pharma, Hospira/Pfizer, Roche, Sandoz and UCB and received research funding to Diakonhjemmet Hospital from AbbVie, BMS, MSD, Pfizer, Roche and UCB. MLH has received research funding/consultancy/speakers fee from Abbvie, Biogen, BMS, CellTrion, MSD, Novartis, Orion, Pfizer, Samsung, UCB. All other authors have declared that no competing interests exist.
Patient consent Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement All relevant data are within the text.