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Classification criteria for psoriatic arthritis and ankylosing spondylitis/axial spondyloarthritis

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The concept of spondyloarthritides (or spondyloarthropathies, SpAs) that comprises a group of interrelated disorders has been recognised since the early 1970s. While the European Spondyloarthropathy Study Group (ESSG) criteria and the Amor criteria have been developed to embrace the entire group of SpAs, new criteria for psoriatic arthritis have been developed recently. The Classification of Psoriatic Arthritis (CASPAR) study, a large one of more than 1000 patients, led to a new set of validated classification criteria for psoriatic arthritis. Since their publication in 2006 the CASPAR criteria are widely used in clinical studies. In ankylosing spondylitis, the 1984 modified New York criteria have been used widely in clinical studies and daily practice but are not applicable in early disease when the characteristic radiographical signs of sacroiliitis are not visible but active sacroiliitis is readily detectable by magnetic resonance imaging (MRI). This led to the concept of axial SpA that includes patients with and without radiographical damage; candidate criteria for axial SpA were developed based on proposals for a structured diagnostic approach. These criteria were validated in the Assessment of Spondyloarthritis International Society (ASAS) study on new classification criteria for axial SpA, a large international prospective study. In this new criteria, sacroiliitis showing up on MRI has been given as much weight as sacroiliitis on radiographs, thereby also identifying patients with early axial SpA. Both the CASPAR and the ASAS criteria for axial SpA are likely to be of use as diagnostic criteria.

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Classification criteria

Classification criteria are for case identification in clinical research and not for the diagnosis of individual patients in the clinical encounter. For the clinician, the value of classification criteria lies with the parameterised, abbreviated and refined nature of the diagnostic work-up contained within the criteria. The items within the criteria reflect the critical features of the disease in question and thus form a useful aide memoire to the key points used in making the diagnosis.

Pitfalls and limitations to current classification criteria for PsA and SpA

Psoriatic arthritis

Until the pioneering work of Wright [1] and Baker [2], an inflammatory arthritis occurring in the presence of psoriasis was felt to represent rheumatoid arthritis (RA) occurring coincidentally with psoriasis. The discovery of rheumatoid factor (RF) in the serum provided an important tool that helped categorisation of polyarthritis, but the distinction between RA and psoriatic arthritis (PsA) was achieved primarily on clinical and radiological grounds. Wright described the

Moll and Wright criteria

The original diagnostic criteria of Moll and Wright [5] are the simplest and the most frequently used in studies prior to 2006. The criteria of Moll and Wright are:

  • An inflammatory arthritis (peripheral arthritis and/or sacroiliitis or spondylitis).

  • The presence of psoriasis.

  • The (usual) absence of serological tests for RF.

The original criteria were designed to be sensitive without being too specific but it is possible that Moll and Wright were using other features of the disease to make their

Development and validation of CASPAR criteria

In recognition of the difficulties posed by multiple classification criteria and the unsatisfactory level of empirical validation for any of the existing criteria, a large collaborative study under the leadership of Philip Helliwell was initiated in 2001. The CASPAR study was one of the largest rheumatic disease classification criteria studies yet conducted [30] and paved the way towards the formation of the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) [31].

Ankylosing spondylitis

AS is considered the prototype disease of the SpA spectrum with a prevalence of 0.1–0.5% in Central Europe and many other countries worldwide [39], [40], [41], [42], [43]. In AS, axial involvement is the rule and patients may or may not have concomitant peripheral involvement such as peripheral arthritis or enthesitis. Therefore, the majority of AS patients suffer from back pain as the major clinical symptom while other manifestations such as peripheral arthritis or enthesitis occur in about

A structured diagnosis of early axial SpA

Attempts have been made to provide guidance on how to establish a diagnosis in non-radiographical axial SpA *[58], *[59], [62]. In AS, disease onset is usually in the third decade, and by the age of 45, more than 95% of patients are symptomatic [54]. Therefore, all efforts to improve diagnostic ascertainment have focused on patients with chronic (duration of at least 3 months) back pain and age at onset less than 45 years of age. These have included a recent diagnostic algorithm [59] as well as

Diagnostic performance of the ASAS criteria for axial SpA

Among the 649 patients included in this ASAS study [69], a diagnosis of axial SpA was made in 60.2% of the study population, reflecting the fact that the back pain patients who were referred to the ASAS rheumatology centre were suspected by the referring physician to possibly have SpA. In this regard, the patient population was rather representative for back pain patients referred to the rheumatologist in many rheumatology centres. Since ASAS aimed to assess the validity of using the new

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