Psoriatic arthritis (PsA) is recognised to have a significant impact on functional impairment, joint damage and quality of life. The aim of this thesis was\ud to investigate tools for early identification, to develop a clinical target for treatment and to utilise these tools within a clinical trial.\ud \ud The ClASssification of Psoriatic ARthritis (CASPAR) criteria, previously developed in established PsA, were tested in patients with recent onset inflammatory\ud arthritis (PsA and controls) to test their discriminative ability in early arthritis. The phenotype of early PsA was investigated further with clinical and ultrasound (US)\ud assessment. Clinical criteria for minimal disease activity (MDA) were developed using a questionnaire. These were subsequently tested in an observational cohort\ud and interventional trial dataset. Finally, they are being utilised prospectively in an RCT addressing the benefits of tight control in PsA.\ud \ud The CASPAR criteria were found to have good sensitivity and specificity for the diagnosis of recent onset PsA. No individual clinical parameters accurately distinguished PsA from other types of inflammatory arthritis, but there was evidence of more oligoarticular disease and enthesitis in PsA compared with rheumatoid arthritis. US imaging showed a small burden of subclinical arthritis and enthesitis\ud but found good correlation between clinical and imaging assessment of disease activity. Criteria for MDA were developed from expert consensus, covering all aspects of psoriatic disease. They were evaluated against the OMERACT filter and have supporting evidence for their validation in terms of truth, discrimination and feasibility. Early unblinded results from the clinical trial indicated that 53%\ud achieved MDA at 12 months.\ud \ud In summary, the CASPAR criteria can be used for early classification of PsA. In addition, a new composite outcome measure has been developed and validated and is now being utilised in a clinical trial
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