Limitations of the ASAS Classification Criteria
Despite the aforementioned advantages of the new ASAS classification criteria, they also confront us with new challenges. First, these criteria were developed for classification, thereby to facilitate clinical research, but not for diagnosis. For example, a recent study compared the prevalence of axial SpA in at-risk patients with chronic back pain according to the ASAS axial SpA criteria with the diagnosis of axial SpA by the rheumatologist's expert clinical diagnosis and concluded that the concordance yielded a kappa of only 0.41 (low to moderate agreement), suggesting that the ASAS axial SpA criteria and rheumatology experts captured somewhat different patient populations. According to the ASAS axial SpA criteria, 24% of the patients were undiagnosed and 21% were misdiagnosed in clinical practice, which could be interpreted as a false-positive diagnosis (in comparison with the gold standard, the rheumatologist's expert opinion) in 24% of patients when applied as a diagnostic tool. This is probably related to the fact that MRI and HLA-B27 are major entry criteria in the ASAS classification but are diagnostically only useful in patients with a moderately high a priori chance of SpA.
A second limitation of the ASAS criteria is that the subdivision into either axial SpA or peripheral SpA is not a true reflection of the clinical reality since approximately 30% of the SpA patients have combined axial and peripheral disease manifestations. The ASAS proposed that the axial SpA criteria should be applied in patients with combined axial and peripheral disease. With this definition, however, there is a risk that a substantial number of SpA patients with peripheral disease and some degree of back pain will fail to be classified as axial SpA, and thus as SpA all together, despite fulfilling the peripheral SpA criteria. For example, an HLA-B27-negative patient with Crohn's disease with a monoarthritis of the knee, active inflammatory back pain for 6 months, but normal imaging of the sacroiliac joints would not classify as SpA, since neither the imaging nor the HLA-B27 criterion are met, which are required for axial SpA. However, the patient would be classified as SpA if the peripheral SpA criteria had been used. This questions whether patients with active axial (back pain) as well as peripheral symptoms (arthritis, enthesitis or dactylitis) should not enter the ASAS criteria by either one of the two criteria arms (Fig. 2).
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Figure 2.
Classification of patients with axial, peripheral or combined symptoms
Patients with pure active axial symptoms should be classified according to the axial SpA criteria and patients with pure peripheral symptoms should be classified according to the peripheral SpA criteria. However, according to the ASAS, the patients with combined axial and peripheral symptoms could only be classified as having SpA if they fulfil the axial SpA criteria. We propose a small modification to these criteria so patients with combined symptoms can enter either of the two criteria arms to reduce the chance that these patients are unfairly misclassified as not having SpA.
A third challenge of the new criteria is that we need to reassess carefully specific subgroups of axial and/or peripheral SpA (e.g. HLA-B27-positive vs -negative patients, and patients fulfilling the imaging vs clinical arm of the axial SpA criteria) to see whether they are different in terms of disease activity, disease burden, long-term outcome and response to treatment. Many studies addressing the latter issue are being conducted and results are awaited.