Background In Psoriatic arthritis (PsA) the pattern of joint involvement seems to be more heterogenic than Rheumatoid Arthritis (RA). Therefor the 66/68 joint count is frequently used in PsA scores. In this study we would like to investigate whether using less extensive joint counts influences PsA disease activity measures

Objectives To evaluate the effect of using a 28, 44 or 66/68 joint count on Minimal Disease Activity (MDA).

Methods Newly diagnosed PsA patients were included in the Dutch Early south-west Psoriatic Arthritis Registry (DEPAR) study between August 2013 and January 2017. Joint scores at baseline and 6 months were calculated using a 28, 44 and 66/68 joint count. Consecutively MDA was calculated using each of these joint counts. MDA is defined as minimal disease activity in 5 out of 7 domains (swollen joints, tender joints, PASI, patient vas pain, patient vas global, HAQ, enthesitis)

Results In total, 413 patients were included into the study, of which 320 had reached 6 months follow-up at the time of this abstract. Half of the patients were male (49%), and mean age was 50 years (SD 13.8).

The percentage of patients with at least one involved joint (swollen or tender) at baseline decreased from 91% using the 66/68 score to 88% with 44 joints and then to 80% with a 28 joint count (Table 1). At 6 month these scores were 66% 63% and finally 56% respectively for the 66/68, 44 and 28 joint count. After 6 months, 96 patients (30%) had achieved MDA using the original 66/68 joint count. Using a 28 joint count, 10 more patients (10% of the MDA population) were classified as having achieved MDA.

Conclusions Using reduced joint count joints in PsA misclassifies around 10% of patients as having no joint involvement. It also misclassifies about 10% of the MDA population of having achieved MDA while they have not. Full 66/68 joint counts remain recommended for measuring disease activity in PsA.

Disclosure of Interest None declared