SUPPLEMENTARY ONLINE MATERIAL

PATIENTS AND METHODS

In our cross-sectional analyses we focussed on the following variables which all had received ≥50% of the votes at the OMERACT-7 PsA module [1]: 66 swollen and 68 tender joint counts (SJC, TJC); patient global assessment of disease activity (PtGA) and patient pain assessment, both using a 100mm visual analogue scale (VAS); physical function (PF), addressed by the health assessment questionnaire (HAQ) [2], validated in patients with PsA and used in a German version [3]; skin involvement according to the psoriasis area and severity index (PASI) [4], performed by the assessor who had been trained by expert dermatologists at the Dermatology Department of our institution; quality of life (QoL), employing the mental components summary score (MCS) of the generic health status questionnaire Medical Outcomes Study Short Form-36 Health Survey (SF-36), validated in patients with PsA [5]; acute phase reactants (APR, erythrocyte sedimentation rate, ESR, and C-reactive protein, CRP) determined using standard procedures; axial involvement measured by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) [6]; and enthesitis determined using the Maastricht Ankylosing Spondylitis Enthesitis Score (MASES).[7] With the HAQ and/or the SF-36 MCS, we accounted for the domain of participation, since according to a recent qualitative study [8], almost all (94%) concepts linked to “activities and participation” were covered by these two questionnaires. We did not include structural joint damage, since it does not constitute an actual disease activity measure.

While only the above variables were included in our primary analysis, in our sensitivity analysis we additionally performed assessments using all variables: reduced 28 joint counts were comprised in the 66/68 joint counts; evaluator global disease activity assessment (EGA; both by biometrician and physician), rated on a 100mm VAS; fatigue by 100mm VAS; and Beck Depression Index (BDI).[9] In addition, the Bath Ankylosing Spondylitis Metrology Index (BASMI) was obtained which includes 5 different measurements of spine mobility [10] and so was the Bath Ankylosing Spondylitis Functional Index (BASFI) [11] and the Dougados Functional Index (DFI).[12] Satisfaction of patient skin status was evaluated using a four point Likert scale (unsatisfied, not very satisfied, satisfied, very satisfied). The Dermatology Life Quality Index (DLQI) [13], a questionnaire assessing the impact of skin involvement in daily life, was used to assess skin related QoL.

Rheumatoid factor (IgM-RF) was measured by nephelometry; a level of >20 U/ml was considered positive (high titer ≥50 U/ml). Anti-CCP antibodies were measured by ELISA (Axis Shields Diagnostics, UK) and considered positive above a cut-off value of 5 units as suggested by the manufacturer. Anti-RA33 was assessed by immunoblotting using recombinant antigens.

Analyses

Statistical analysis was performed using SPSS 15. Principal component analysis (PCA)[14], was chosen to be an appropriate tool for producing a smaller number of combinations of the original variables in a way that accounts for most of the variability in the pattern of correlations. In PCA, two statistical measures are generated to assess the potential to summarize the variables into components: Bartlett`s test of shericity [15], and Kaiser-Meyer-Olkin (KMO) measure of sampling adequacy.[16] Bartlett's test is an indicator of the strength of the relationship among variables. When expecting relationships between variables, the Bartlett value should be significant in order to proceed with PCA. The KMO indicates whether or not the variables can be grouped into a smaller set of underlying components; it ranges from 0-1 with values above 0.6 regarded as good for PCA. Varimax rotation was performed for easier interpretation with the attempt to minimise the number of variables that have high loadings on each component.[17] For simplicity, only loadings above 0.3 were taken into consideration. The “Kaiser´s criterion“, or Eigenvalue, represents the amount of the total variance explained by that component and should be above 1.0 for each component.[17]

RESULTS

Baseline characteristics and therapy of the 105 PsA patients evaluated in our analysis are shown in Supplementary Table 1.

Principal component analysis

The Kaiser-Meyer-Oklin value was 0.806 and therefore well above the recommended value of 0.6, and Bartlett`s test of sphericity was highly significant (p<0.0001).

Supplementary Table 1. Patient characteristics and drug distribution at first visit

Age, mean (SD) / 50 (12)
Gender / Female, n (%) / 35 (33)
Male, n (%) / 70 (67)
Disease duration joints in months, median (IQR) / 98 (40/180)
Disease duration skin in months, median (IQR) / 169 (72/264)
Body weight in kg, mean (SD) / 81 (15)
Smoking / Non-smoker, n (%) / 71 (68)
IgM-RF / Negative, n (%) / 96 (91)
Low titer (20 and <50 U/ml) positive, n (%) / 4 (4)
High titer (≥50 U/ml) positive, n (%) / 5 (5)
Anti-CCP / Negative, n (%) / 103 (98)
Positive, n (%) / 2 (2)
Anti-RA33 / Negative, n (%) / 102 (97)
Positive, n (%) / 3 (3)
Morning stiffness in minutes, median (IQR) / 15 (0/30)
Prednisone, % / 20
NSAIDs, % / 40
DMARDs, % / None / 13.3
MTX / 57.2
TNF-inhibitors / 18
Leflunomide / 10.5
Sulphasalazine / 1
NSAID, non-steroidal anti-inflammatory drug; DMARD, disease modifying anti-rheumatic drug

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