Approaches to the assessment of response in the treatment of SpA and PsA are similar between clinical practice, observational and clinical trials research. First and foremost, the patient and physician are focused on the assessment of signs and symptoms, such as pain, stiffness, fatigue and global well-being. Although standardized approaches such as VAS scales and the BASDAI have substantial discriminative power these assessments are non-specific and often do not answer the critical question relevant to management-are the symptoms related to active disease or do they reflect secondary mechanical complications. These measures as well as the ASAS20 response do not correlate at all with objective measures of treatment response such as MRI. Acute phase reactants are highly responsive and correlate well with MRI indices of axial inflammation but less well with clinical measures of response. An AS disease activity score has been developed and is now sufficiently validated for construct and criterion validity to be used in both clinical practice and research but it is unclear whether it should replace the BASDAI. The ASDAS has been shown to discriminate better than the BASDAI between states of high and low disease activity and to correlate better with MRI scores for inflammation. Cut-offs for the ASDAS have been validated that categorize patients into very high, high, moderate, and inactive disease activity states and also define the change in ASDAS that constitutes clinically important and major improvement. MRI assessment of bone edema reflecting inflammation may be very helpful but is often not conducted or interpreted in a standardized manner. In particular, lateral and posterior segments of the spine are often affected by inflammation but assessment and interpretation of abnormalities remains problematic. Assessment of disease activity by MRI in PsA requires further validation. Increasing attention has focused on the use of MRI to assess structural features that might reflect damage progression. The minimal disease activity and patient acceptable symptom state are concepts that have been proposed to define satisfactory health status and thresholds proposed for patient self-reported outcomes e.g. BASDAI that define PASS. But these thresholds appear to vary according to baseline factors such as sex and disease duration. The impact of disease on function requires monitoring but current recommendations focus on disease-specific self-report instruments such as the BASFI but systematic application of the International Classification of Function and Disability framework in patients with AS emphasizes the significant limitations of current approaches, particularly the lack of tools that assess participation. Mobility assessment should be standardized and focus on those measures that are most responsive, namely, lateral lumbar flexion and cervical rotation. Monitoring of peripheral inflammation should include evaluation of enthesitis which may reflect a more severe subset of disease but the optimal approach remains unclear.
Disclosure of Interest W. Maksymowych Grant/Research support from: Abbott, Amgen, Eli-Lilly, Merck, Pfizer, UCB, Janssen, Consultant for: Abbott, Amgen, Eli-Lilly, Merck, Janssen, Pfizer, UCB
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