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Measuring Outcomes in Psoriatic Arthritis

Psoriatic arthritis (PsA) is a highly heterogeneous inflammatory musculoskeletal (MSK) disease. PsA affects a wide range of tissues, including the peripheral joints, axial skeleton, and enthesis (region where a tendon, ligament, or joint capsule inserts onto the bone), and is associated with dactylitis (swelling of an entire digit) (1). In addition, the majority of patients have skin psoriasis and some level of nail involvement (2). The impact of the disease on patients’ lives is similarly heterogeneous and affects physical function, work productivity, social participation, emotional well-being, and fatigue (3). Measuring this heterogenous disease requires the employment of a variety of outcome measures. Deciding on what to measure is challenging in a complex disease such as PsA. The Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) Outcome Measures in Rheumatology (OMERACT) working group developed a core domain set (Figure 1) to specify which key domains should be measured in randomized controlled trials (RCTs) and longitudinal observational studies (LOS) for PsA (3). Among the domains included in the core outcome set, MSK disease activity is considered one of the most important for both patients and clinicians (3). The MSK disease activity domain includes peripheral joints, enthesitis, dactylitis, and spine symptoms (among those with spinal involvement). Other domains included in the center circle (mandatory domains for PsA RCTs and LOS) include skin disease (encompassing both psoriasis and psoriatic nail disease), patient pain assessment, patient global assessment (PtGA) of disease, physical function, health-related quality of life (HRQoL), fatigue, and systemic inflammation. At some point in the life cycle of trials to test the efficacy of a new drug, the recommendation is to also measure economic cost, emotional well-being, participation, and structural damage (ie, via imaging). Finally, several measures were placed on the research agenda, including stiffness, independence, treatment burden, and sleep (3). In this article, we will review the key measures for assessment of PsA. We will focus primarily on measures with a sufficient body of evidence for the instrument and will use the GRAPPA-OMERACT Core Outcome Set to walk through instruments to measure these domains. Additionally, we will discuss composite measures that combine two or more of these domains. When addressing physical function, several of the patient-reported outcome (PRO) measures are available, including HRQoL, patient pain assessment, fatigue, emotional well-being, and participation. Many of these are covered in greater detail in other sections of this special edition, but we will discuss the instruments in the context of their use in PsA. Additionally, measurement of axial disease is covered in greater detail in the article “Measuring Outcomes in Axial Spondyloarthritis” in this special edition. Because of space considerations, we have decided not to review the psychometric properties of imaging modalities in PsA (4,5). Finally, it is important to recognize that there are no gold standards for disease activity in PsA. Thus, in discussing construct validity, we generally use the concept of convergent validity (a component of construct validity), in which a measure should be correlated with other measures of the same or similar constructs.