Psoriatic arthritis (PsA) is a heterogeneous disease which may precede, occur concurrently, or after the development of psoriasis. In order to better understand the natural history of the disease researchers aimed to identify demographic and clinical characteristics associated with delay in transition from psoriasis to PsA.

A retrospective, population-based cohort of incident patients with PsA ≥18 years of age from a geographically-defined area meeting Classification of Psoriatic Arthritis (CASPAR) criteria for PsA (2000-17) was identified.  Onset of psoriasis was defined as confirmatory diagnosis of psoriasis by a dermatologist or rheumatologist. Date of fulfillment of CASPAR criteria was considered as the time of onset of PsA. Patients with PsA were divided into two groups: patients with concurrent psoriasis and PsA (within 1 year of each other) and patients with psoriasis onset before PsA (>1 year). Those patients with psoriasis development after PsA onset (>1 year) or only family history of psoriasis were excluded. Logistic regression models adjusted for age and sex were performed to identify factors associated with delay in transition from psoriasis to PsA.

There were 157 incident cases of PsA, mean age at diagnosis was 46.3 (standard deviation=12.0) years and 46% were females. Median lag time from psoriasis onset to PsA was 263 days (interquartile range=0, 2889). 51% (n=80) of patients had concurrent psoriasis and PsA while 49% (n=77) had onset of psoriasis before PsA. Family history of psoriasis (odds ratio [OR] = 2.71, 95% confidence interval [CI] 1.24 – 5.90) and psoriasis severity (OR=1.96, 95% CI 1.26 -3.06) were more likely in those who had a delay in transition from psoriasis to PsA than those who had a concomitant diagnosis. There were no statistically significant differences in demographic features, or any of the other psoriasis or PsA related factors between the two groups. Similarly, no difference in radiographic outcome was noted for the groups.

In this population-based study, approximately half of the patients had psoriasis onset before PsA, and the other half had concurrent psoriasis and PsA. Patients with a family history of psoriasis and severe psoriasis were more likely to have a delay in transition from psoriasis to PsA. Whether this difference is secondary to earlier detection of psoriasis in patients with a family history of psoriasis and severe psoriasis or a true phenotypic difference will require further studies.