Patient-centered psoriatic arthritis (PsA) activity assessment by Stockerau Activity Score for Psoriatic Arthritis (SASPA)

The main objective of this study was to assess whether a modified RADAI-5 questionnaire, namely SASPA, could be used as a fully patient-administered tool for daily PsA monitoring. SASPA should enable physicians to obtain reliable information about the disease course to support treatment decisions. It was not the intention with the development of SASPA, as with RADAI-5, to compensate for careful clinical examination [22]. Internal consistency, as a measure of the extent to which a variable or set of variables is stable, constitutes the primary prerequisite. Feasibility and acceptability to physicians and patients are also important requirements of any assessment tool. To cover the proposed core set measures for PsA as fully as possible, while keeping the balance between feasibility and reliability, it was decided to retain a five-question structure, as with RADAI-5 [24]. During all the studies to develop RADAI-5, support could be found for the arguments that RADAI-5 comprises questions targeting a patient’s pain perception and global health estimate, which can be seen as surrogates for functionality, and are likely also to include discomfort caused by enthesiopathy [22-24].

Self-report questionnaires, such as RADAI-5, can substitute for physician-derived disease activity scores, which were developed primarily for research purposes [19]. This may be expected highly likely also for SASPA in relation to PsA scores, such as DAPSA and PASDAS [11,12].

As with RADAI-5, SASPA allows better participation in the care of PsA patients, especially by non-specialists such as primary care physicians, without having to be trained in measuring formal joint counts.

PsA in contrast to RA often shows a highly variable course, including highly active as well as long-lasting inactive phases [1,2]. Nevertheless, PsA may cause severe joint destruction and functional loss in some patients [2]. From the patient’s position, PsA may cause joint problems, as well as skin manifestations, particularly of the nails. Moreover, and, of course individually aggravating, the disease burden’s focus may shift from the musculoskeletal system to the skin or vice versa [31]. Therefore, we are convinced that rheumatologists and dermatologists should pay attention to the skin and joint manifestations before deciding on treatment [32]. This was why we insisted on including the question targeting the severity of the skin manifestations. These considerations were the basis for the ultimate decision to leave out the question targeting the course of the disease during the past 6 months.

With regard to physician- or patient-reported outcome measures, SASPA, as for RADAI-5, can be regarded as a hybrid tool, namely, a tool assessed by patients, comprising domains selected by physicians. Patients agreed that the SASPA questionnaire was easy and rapid to complete. SASPA has the advantage that the individual patient – the primary target of all therapeutic interventions – is given the key role with respect to disease activity assessment. Beyond that, inter-physician, but also intra-physician variations in assessing joints or global disease activity are eliminated, and other pitfalls of joint counts are avoided [33]. The lower the number of involved joints, the more intra- and inter-observer variance exerts an influence upon relative assessment tools, which is of particular importance in PsA, because it constitutes an oligoarticular disorder in some patients [1,2,33].

Measurement of patient symptoms alone may have an important shortcoming, namely that symptom levels are differently evaluated and expressed among patients. However, physical function on a health assessment questionnaire, the most frequent patient-reported outcome, is the measure that is most significant in identifying and predicting work disability, explaining costs, and predicting mortality in RA – much more effectively than the other core data set measures [34].

The newly adapted SASPA was investigated for its psychometric properties. Cronbach’s alpha was >0.8, indicating high internal consistency. Moreover, the questionnaire proved to be a one-dimensional instrument with some differences in item weighing, with the question targeting the severity of skin disease showing the lowest one. One reason for that could be that PsA patients in our unit have a highly variable but generally mild psoriatic skin disease, and primarily seek to reduce the intensity of their joint complaints.

SASPA was shown to be in moderate agreement, however significantly correlated, with most of the ACR core set measures such as TJC, SJC, and MDglob [22,35]. This on the one hand can be regarded as proof of convergent validity, and on the other hand, also as a possibility to eliminate difficulties with joint counts in daily routine rheumatology [33]. The relationships obtained here are in line with the results for RADAI-5. One of the reasons for this moderate agreement may be differences in patients’ weightings of joint involvement and physicians’ joint counts. SASPA includes two questions that target all joints and entheses as a whole, enabling the patients to weigh their individual burden, which is not possible if single joints are simply counted, and identically weighted, for example, the fifth metacarpal joint being treated in the same manner as the knee.

PsA activity assessment without joint counts has the advantage of not losing activity by applying certain counting models. In contrast to RA, no consensus has yet been reached by which counting models can be applied in daily routine assessment of individual PsA patients [36]. The 28-joint count e.g. excludes the foot joints, which cannot be regarded appropriate in a disease frequently presenting with an oligoarticular pattern, and involving the lower extremities. Such joint counting models may lead to misclassification of some patients [1,2,33]. To refrain from routine joint counts does not mean disregarding joint examination, but rather giving a place for focused patient evaluation. Nevertheless, as individualized treatment becomes increasingly important, patient-related outcome tools could provide substantial advantages in identifying patients requiring particular attention.

The sensitivity to change of the questionnaire was tested in a subgroup of patients starting TNF-blocker therapy. SASPA was highly sensitive to change, resulting in a standardized mean difference for the particular intervention indicating high efficacy. This study was performed in a routine clinical setting, therefore, we took initiation of TNF-blocker as the standardized therapeutic intervention and ≥2 months thereafter as the decisive time point to assess efficacy.

The present study had several limitations. First, the study was performed in two closely related offices within a relatively small region. Second, increased self-efficacy, as a member of a study population, constitutes an important factor possibly influencing patient’s self- assessment [37]. Third, patients included in this observation did not have severe skin manifestations. SASPA should be compared to composite indexes for PsA. We are confident that the respective results will be in agreement with those obtained for RADAI-5 and RAPID 3 in RA patients.

However, as with the other instruments, only stable low SASPA values can be regarded as indicators of an uncomplicated disease course [38]. Significant changes, however, must be appraised with respect to the changes in the single items and possible coexisting or newly occurring diseases [39]. Our results provide robust evidence that patient-centered disease activity assessment is reliable and feasible for daily routine monitoring of PsA, as in RA or ankylosing spondylitis [22,40].