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).
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
We estimated DAPSA28 = (28TJC × 1.6) + (28SJC × 1.6) + patient global (0-10VAS) + pain (0-10VAS) + CRP (mg/dL). 
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.