Background:Dactylitis is a hallmark feature of Psoriatic arthritis (PsA) and Spondyloarthritis (SpA) defined as a uniform swelling of a digit (“sausage digit”). Dactylitis is associated with radiographic damage in chronic PsA. However, there are a paucity of data on the significance of dactylitis and its potential impact in disease burden in early PsA.Objectives:To characterize a very early DMARD naïve PsA cohort based on clinical presence or absence of dactylitis at disease onset.Methods:PsA subjects fulfilling the CASPAR classification criteria, were recruited into a prospective observational cohort, the Leeds Spondyloarthropathy Register for Research and Observation (SpARRO) after providing informed written consent. Clinical data including tender (TJC) and swollen joint counts (SJC) were independently assessed. Dactylitis was recorded per digit (finger or toe) as tender (hot) or non-tender (cold). Differences in baseline characteristics were evaluated using percentages to describe categorical variables and means and standard deviations for continuous variables, p value of the mean/proportion difference was calculated. Ultrasound (US) examination was conducted by trained ultra-sonographers blinded to clinical details. Bone erosions were defined on US if intra-articular discontinuity was present in two perpendicular planes at any of 46 joints: wrists, MCP1-5, PIP2-5, DIP2-5, MTP1-5, knees, ankles, subtalar, talonavicular.Results:A total of 177 PsA patients were recruited. Dactylitis was seen in nearly half the cohort [n=83 (47%)]. Patients with dactylitis had significantly more early morning stiffness, higher TJC and SJC, compared with non-dactylitis (Table 1). A total of 211 digits with dactylitis were recorded in 83 patients. Dactylitis of multiple digits was seen in 47/83 (57%) patients whilst a single dactylitic digit occurred in 36/83 (43%). Foot involvement was more prevalent (141/211, 67%) than hands (70/211, 33%). “Hot” or tender dactylitis was more frequently detected (153/211, 72.5%) than “cold” or non-tender dactylitis (58/211, 27.5%). The most prevalent sites for hot dactylitis were toes 2-4^thand fingers 2-3^rd.Table 1.VariableNo Dactylitis (n=94)Dactylitis (n=83)P valueAge, mean (SD) years44.4 (12.8)43.7 (13.2)>0.05Male38 (40.4%)42 (50.6%)>0.05Disease duration, median (IQR) weeks4.7 (0.0-11.4)5.2 (1-22.4)>0.05Early Morning stiffness, mean (SD) mins82.8 (145.4)170.5 (230.2)0.0025*TJC (78), mean (SD)8.3 (10.9)13.7 (14.0)0.004*SJC (76), mean (SD)2.2 (3.2)8.4 (8.0)<0.001*Psoriasis94/94 (100.0%)76/83 (91.6%)0.004*PASI, mean (SD)3.6 (4.0)3.0 (4.1)>0.05Nail Dystrophy51/94 (54.3%)41/83 (49.4%)>0.05mNAPSI, mean (SD)4.9 (7.1)7.5 (14.0)>0.05MASES, mean (SD)1.6 (2.9)1.5 (2.4)>0.05BMI, mean (SD)29.1 (6.4)28.6 (5.6)>0.05Smoker19.0 (20.2%)9.0 (10.8%)>0.05Elevated CRP (>10 mg/L)24 (25.5%)36 (43.4%)>0.05PsAQoL, mean (SD)6.2 (6.6)6.2 (6.1)>0.05HAQ, mean (SD)0.79 (0.71)0.84 (0.65)>0.05US Erosions (n=154)12/83 (14.4%)21/71 (29.5%)0.023*US defined erosions were significantly more prevalent in the dactylitis group: 34 erosions in 21/71 patients (29.5%) versus 16 erosions in 12/83 (14.4%) patients in non-dactylitis. Sites prone to erosive damage in both groups were the wrists, MCP1,2 and MTP4,5. The right MCP2 (n=6) and MTP5 (n=6) were most commonly eroded in the dactylitis group, but erosions corresponding at the dactylitic digit level were overall low.Conclusion:This study identifies a more severe phenotype in very early DMARD naïve PsA presenting with dactylitis with higher prevalence of ultrasound erosions. Longitudinal follow up will determine whether dactylitis represents a poor prognostic factor in very early PsA, which may be a useful discriminator for risk stratification in future PsA management recommendations.Disclosure of Interests:Sayam Dubash: None declared, Oras Alabas: None declared, Xabier Michelena: None declared, Gabriele De Marco: None declared, Leticia Garcia-Montoya: None declared, Richard Wakefield Speakers bureau: Novartis, Janssen, GE, Ai Lyn Tan: None declared, Philip Helliwell: None declared, Paul Emery Grant/research support from: AbbVie, Bristol-Myers Squibb, Merck Sharp & Dohme, Pfizer, Roche (all paid to employer), Consultant of: AbbVie (consultant, clinical trials, advisor), Bristol-Myers Squibb (consultant, clinical trials, advisor), Lilly (clinical trials, advisor), Merck Sharp & Dohme (consultant, clinical trials, advisor), Novartis (consultant, clinical trials, advisor), Pfizer (consultant, clinical trials, advisor), Roche (consultant, clinical trials, advisor), Samsung (clinical trials, advisor), Sandoz (clinical trials, advisor), UCB (consultant, clinical trials, advisor), Dennis McGonagle Grant/research support from: Janssen Research & Development, LLC, Helena Marzo-Ortega Grant/research support from: Janssen, Novartis, Consultant of: Abbvie, Celgene, Eli Lilly, Janssen, Novartis, Pfizer, UCB, Speakers bureau: Abbvie, Celgene, Eli Lilly, Janssen, Novartis, Pfizer, Takeda, UCB