Firstly, the panel agreed on certain variables that should be included in the definition of remission such as: the absence of signs and symptoms, physical well-being, the absence of impact of the disease, the absence of inflammation in imaging tests and the absence of inflammation measured by biomarkers.

The panelists did not reach an agreement on the inclusion of the ‘absence of functional impairment’ in the concept of remission in PsA. Functional status is a crucial aspect of the disease, but we agree with other authors that inclusion of functional status in the definition of remission is not that straightforward, because functional status is not influenced by disease activity alone.1,4 Functional impairment may be due to previous sequelae (related or not to arthritis) despite the fact that the patient may be in remission, so it does not necessarily correlate with the current inflammatory activity of the disease but rather with the residual structural damage.7 In addition, functional status is influenced by other factors such as age or comorbid conditions. These arguments may explain why functional status, was not included into the definition of remission by the panel although it is critical for the patients.

Similarly, the inclusion of ‘psychic well-being’ and ‘social well-being’ in the definition of ‘remission’ in PsA presents serious difficulties, because there are multiple external factors that may influence these aspects and not only the disease activity itself. The panel considered particularly important that the presence of concomitant fibromyalgia should be taken into account, since it has a remarkable effect on measures of impact of the disease.8

The panelists agreed that the therapeutic goal of PsA should be to reach remission. However, this is not always possible, therefore, low or minimal disease activity is an acceptable goal. This statement is in line with recent guideline recommendations on the pharmacological management of PsA.6,9

Some studies, including one randomized clinical trial10 and real-world studies,11,12 have emphasized the need to apply a treatment strategy based on objectives (T2T) in PsA, but our panel did not agree on this. The use of T2T approach may be limited due to lack of universal consensus on how to measure this objective (remission or low activity). The application of this strategy comes from rheumatoid arthritis (RA) data, where the objective is usually defined based on DAS28. However, PsA has a much more heterogeneous presentation and it is more difficult to define a single objective. Therefore, different objectives may be defined taking into account all domains of the disease, as the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) suggests.13 Implementing this strategy may also require a more exhaustive follow-up, which is not always feasible in busy outpatient units.

The panelists did not agree on the item that assessed to what extent the rheumatologists know the different instruments to measure disease activity. Rheumatologists are likely to have little knowledge about the instruments used in the assessment of patients with PsA. The infrequent use of these indices in clinical practice may be associated with the wide range of instruments available, their complexity, their limitations to cover the entire spectrum of the disease, and the lack of consensus in the literature regarding which tool should be applied routinely in real world setting. In our opinion, in clinical practice many rheumatologists may be using DAS28 because they are more familiar with the use of this instrument even though it was developed for the assessment of RA and not for PsA. However, the use of DAS28 has important limitations in PsA: it only adequately evaluates polyarticular forms (≥5 joints), the 28 painful/swollen joints count does not include some that are frequently affected in PsA patients, and other domains such as skin involvement or enthesitis are not assessed. In any case, it would be interesting to carry out studies to determine to what extent clinicians know and use the different measurement instruments and strategies such as the T2T in clinical practice, since there is significant scarcity of data in this regard.

In this block of statements, the panel agreed on some practical recommendations on how to monitor disease activity. Firstly, it is recommended to use a composite index specifically designed for PsA to be used for the assessment of this condition. The panel considered that a minimum of variables should be included in the assessment of PsA in the outpatient setting, such as: joint pain, joint swelling, enthesitis, dactylitis, skin, nails, physical function, axial involvement, quality of life, structural damage by means of imaging techniques, C-reactive protein (CRP) levels, patient global assessment, assessment of the skin by the patient, physician global assessment of musculoskeletal manifestations, physician global assessment of the skin, and extra-articular manifestations. However, it has been pointed out that the application of these instruments can be time-consuming and it is not always realistic to evaluate all of these variables in routine clinical practice.

Fatigue was not included among the variables that might be routinely assessed. Although fatigue is prevalent in PsA patients, it may be the result of comorbidities or prior functional impairment not necessarily related to current disease activity.14 Emotional well-being assessment can also create confusion due to similar reasons. Therefore, fatigue and emotional well-being might be included in the anamnesis, however they do not necessarily measure disease activity systematically. On the contrary, it is important to bear in mind that these variables may interfere with the assessment of disease activity.15

The inclusion of imaging tests for the assessment of PsA provides an objective parameter, which could allow for the evaluation of both inflammatory activity and structural damage as well. However, there is currently no consensus on what tests should be done and how often. It has been established that radiography has a prognostic value in PsA, as joint damage detected by radiographs is an independent variable in the prognosis of further radiological progression.16 In the clinical trial ADEPT, radiographs of hands and feet were used successfully to assess the inhibition of structural damage by adalimumab compared to placebo in PsA patients at 48 weeks and 2 years.17 Furthermore, although ultrasonography (US) and magnetic resonance imaging (MRI) are more sensitive to detect erosions than radiography16 and both are the most sensitive methods for soft tissue assessment,16 their role in the follow-up of inflammatory activity has not been clearly established as yet.18 These tests may be necessary in some patients when there are doubts about the type of inflammatory involvement (synovitis, enthesitis and/or dactylitis), in locations where physical examination has limitations (spine, sacroiliac joints), or when there is disagreement between the subjective assessment of the patient and the objective data obtained by physical examination, laboratory tests or disease activity indices. The detection of subclinical inflammation on medical imaging complicates the definition of remission because the meaning and implications of this activity are not completely understood. Some studies suggest that it is related to the development of flares and structural damage.19,20 Persistent synovitis and enthesitis detected by US after 6 months of therapy might be predictors of subsequent structural progression. In addition, amongst patients with PsA in clinical remission, power Doppler ultrasound-detected synovitis is a strong predictor of short-term flare of the disease.21

Regarding acute phase response indices, such as CRP, it is important to emphasize that these laboratory markers of inflammation have limitations since both are elevated in only half of the patients with PsA. However, when these two markers keep increasing, their utility for the evaluation of disease activity is undeniable.22

No consensus was reached on what should be the optimal frequency of follow-up visits in patients who have achieved the objective of having the inflammatory activity of their disease controlled. A follow up at 4 months intervals has been suggested, but this may vary depending on several factors. We propose that in patients in clinical remission without treatment, the visits could be spaced even up to every 6–9 months. However, it is important to allow the patient the possibility to return in case of a flare. A plan that could be used is to progressively prolong the intervals between the visits once the patient has reached remission (6, 9, 12 months), especially if nursing consultations are used as suitable alternatives.

The panelists agreed that the most recommended tool to assess the disease activity in the clinic is the Disease Activity Index for Psoriatic Arthritis (DAPSA), with cut-off values of ≤4 for remission and >4–14 for low disease activity. Similarly, the most recommended index to assess whether the patient has reached minimum disease activity is the MDA,23 when at least 5 out of 7 criteria are met. The Ankylosing Spondylitis Disease Activity Score (ASDAS)24 may be used in cases with axial involvement, with the following cut-off values: <1.3 for remission and <2.1 for low disease activity. We propose that the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) (cut-off values: ≤2 for remission and ≤4 for LDA) may be used as an alternative to ASDAS. So the proposed definition of remission in PsA included: the absence of disease activity assessed by using DAPSA or MDA, and in patients with axial involvement, the absence of activity evaluated by ASDAS (or BASDAI as an alternative). There was also consensus to include the PsA Impact of Disease questionnaire (PsAID) and Patient Global Assessment questionnaire (PtGA) in the assessment of the impact of the disease during the follow-up visits.

DAPSA is an index that includes patient global and pain assessments, 68 tender joint count (68 TJC) and 66 swollen joint count (66 SJC) assessments, and CRP levels. One important advantage of DAPSA over other indices is that it provides a continuous measure and thresholds for high, moderate or low activity and remission based on a score.25 Higher DAPSA scores are significantly associated with higher probability of structural progression.23 Furthermore, DAPSA correlated with function23 and with the impact of disease as measured by PsAID.26 In addition, it has been validated in clinical trials.25 Limitations of DAPSA are that it does not contain domains for skin, enthesitis, dactylitis, or axial disease assessments.25

On the other hand, MDA is a dichotomous instrument (Yes/No answers) that includes seven variables (tender/swollen joint counts, tender entheseal points, Psoriasis Area and Severity Index or body surface area, patient pain and global activity visual analog scale, and functional evaluation by Health Assessment Questionnaire [HAQ]). It has been widely used in clinical practice, including clinical trials using T2T approach, and in real life observational studies.27 A sustained achievement of MDA is associated with improved prognosis in terms of joint damage progression confirmed both by observational studies and registries, and likely with improvements in function and quality of life.27 MDA has certain limitations as well. It has been argued that the lack of acute-phase reactants could limit the validity of MDA. Additionally, MDA includes a low level of HAQ, which may be difficult to be achieved in an established disease irrespective of disease activity levels.27 Finally, it has been recently showed that DAPSA-based remission/low disease activity performs better than MDA to detect patient-defined remission or remission/low disease activity.28

In axial SpA, evidence suggests that the ASDAS better reflects the inflammatory disease processes (both with biomarkers of inflammation and MRI inflammation scores) than BASDAI.29 In line with recent recommendations,2 we considered that ASDAS is preferred, and BASDAI may be used as an alternative.

The assessment of quality of life is an important outcome measure from the perspective of the patient and includes aspects not evaluated in the usual activity indices. In this consensus, the Psoriatic Arthritis Impact of Disease (PsAID)30 was the tool that obtained greater degree of agreement. The initial validation study by Gossec et al.30 demonstrated that PsAID has good correlation with patient global assessment, little variability in a retest when the patient was asked again 2–10 days later, and an acceptable sensitivity for treatment changes.

There are several limitations of our consensus that must be noted. The Delphi methodology prevents discussing the statements in detail and the questionnaire is designed by a limited number of experts so some issues may be overlooked. For instance, we do not include in the questionnaire statements related to other indices designed to evaluate disease activity. However, based on the evidence and our expertise we considered that the included indices were the most realistic in clinical practice. It might also be argued that the panelists agreed that the aim of treatment should be the clinical remission of musculoskeletal, skin and nail inflammatory manifestations, and that multiple domains should be evaluated, but they only selected tools to define remission that include a limited number of domains, such as DAPSA that only assesses the joints. Moreover, imaging is not directly included in these instruments. However, panelists’ statements in block I and II are related to an ideal definition of remission while DAPSA or MDA could have been selected taking into account a more practical definition of remission in real life, with limitations but realistic and practical, having also in mind that achieving these goals is associated with less radiographic progression.27