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The measurement of health is difficult given its multidimensional nature (1, 2). The Outcome Measures in Rheumatology Clinical Trials (OMERACT) process and other research has made enormous progress with the valid measurement of disease activity in some rheumatic diseases (3). In psoriatic arthritis (PsA), there has been research that partially validates health-status measures, such as the Short Form 36 (SF-36) (4), Arthritis Impact Measurement Scales (5), and Health Assessment Questionnaire (HAQ) (6), but these are principally measuring at the level of disability and do not necessarily capture disease activity. There has been little attempt to validate disease activity measures in PsA. In rheumatoid arthritis (RA) (3) and ankylosing spondylitis (AS) (7), OMERACT recommends that core measures of disease activity include joint counts, markers of acute phase response, physician global assessment of disease activity, and patient self-report indices of disability and global perception of health.
Composite measures have some advantages over multiple measures, particularly when there is evidence for a single underlying construct, such as disease activity. The European League Against Rheumatism response criteria (8) for RA use a composite measure of disease activity (Disease Activity Score [DAS]) (9) established by factor analysis and correlation with physician treatment decisions. The DAS uses the tender and swollen joint count, erythrocyte sedimentation rate (ESR), and the patient's global assessment of disease activity. A randomized controlled trial of sulfasalazine for PsA (10) used a response criteria (referred to here as the Clegg response criteria) that depended upon a 30% improvement in tender and/or swollen joints and 20% improvement in global disease activity (patient or physician), but did not include an acute phase marker.
The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) (11) is a composite measure that may be potentially useful in spondyloarthropathies other than AS. We hypothesized that because PsA has both peripheral and axial clinical manifestations, indices that measure only peripheral joint activity, such as the DAS, the American College of Rheumatology (ACR) RA response criteria, or the Clegg PsA response criteria may not capture sufficient information to truly reflect disease activity in PsA. However, because BASDAI was developed for a mainly axial disease (AS), this measure may not sufficiently capture peripheral disease activity. The aims of this study were therefore 1) to establish that the relationship of BASDAI to patient's own perception of disease activity in PsA was sufficiently close to justify consideration of BASDAI as a possible index of disease activity; 2) to determine whether this relationship was similar for peripheral-pattern PsA and for axial-pattern PsA; and 3) to determine whether the BASDAI discriminates between patients with high and low disease activity (as indicated by rheumatologist treatment decisions).
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The Wellington Regional Rheumatology Unit provides secondary and tertiary rheumatology care to a predominantly urban population of ∼400,000 people. From a disease register maintained by the Unit since 1988, 257 patients with clinic-diagnosed psoriatic arthritis were identified and their case notes were reviewed to confirm a diagnosis of PsA. These patients were contacted by telephone to confirm their location and obtain consent. Patients were asked to complete a questionnaire by one of a number of methods convenient to the patient: by postal survey, at the time of their next clinic appointment, or at a research clinic where they would also be examined by a rheumatologist. From the clinic notes, patients who answered the postal survey were classified with axial- or peripheral-pattern disease based on whether inflammatory spinal signs or symptoms were present at their first presentation to clinic. Patients with peripheral and axial features were classed as having an axial disease pattern.
The questionnaire included questions related to aspects of demography, disease onset, medication use, and measures of disease activity and outcome (global visual analog scale [VAS], BASDAI, SF-36, and HAQ). The patients were asked to rate their current level of overall arthritis activity by means of a horizontal 10-cm VAS, ranging from “no problems at all” to “couldn't be worse.” A similar VAS asked patients to rate the current severity of their psoriasis.
The BASDAI (11) has been partially validated in patients with ankylosing spondylitis, being reliable and demonstrating good content, face, and discriminative validity. It asks patients to rate fatigue/tiredness, neck/back/hip pain, other joint pain, discomfort from tender areas, and morning stiffness on separate VASs. The 5 item scores are summed (range 0–50) and rescaled to a range of 0 to 10.
The SF-36 is a generic measure of health status (12) and has been validated for use in PsA (4). It comprises 36 questions, scored to give 8 subscales (physical function, role limitation due to physical function, mental health, role limitation due to emotional well being, social function, pain, vitality, and general health).
The HAQ disability index (13) is a measure of physical disability. It asks respondents to rate the degree of difficulty with activities under 8 subheadings on a 4-point scale (none, some, much, can't do), which are averaged to produce a single disability index between 0 and 3. Higher scores indicate more disability.
Because the instrument scores were not normally distributed, medians, interquartile ranges, Spearman correlation coefficients, and Mann-Whitney U tests are reported. All tests are two-tailed and because the aim of this study was more exploratory than confirmatory, statistical significance was set at P < 0.05 without adjustment for multiple tests. The underlying structure of the disease measures was analyzed using principal-components factor analysis with varimax rotation. Factors with eigenvalues of greater than unity were selected.
In a separate group of 50 consecutive patients with PsA attending any one of the rheumatology outpatient clinics, we measured BASDAI, patient and physician rating of disease activity (5-point scale), and ESR; we also recorded the treatment decisions at the time of this consultation. Patients in whom further treatment was initiated or in whom disease-modifying antirheumatic drug (DMARD) therapy was commenced or the dosage increased were assumed to have high disease activity. Patients in whom DMARD treatment was stopped because of remission or in whom there was no change in therapy were assumed to have low disease activity. The independent contribution of BASDAI, patient and physician ratings of disease activity, and ESR to disease activity was analyzed using multiple logistic regression (all variables entered).
SPSS (Chicago, IL) version 9 for Windows was used for all analyses.
The study received ethics approval from the Wellington Ethics Committee.
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Patients with PsA (n = 159) were identified and from these, 133 usable questionnaires were obtained (Figure 1). Eighty-six patients were also examined and the pattern of their arthritis defined. The demographic and disease characteristics of the 133 respondents are shown in Table 1.
Figure 1. Relationship between patient perception of arthritis disease activity (ARTHVAS) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) scores for peripheral and axial pattern disease.
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Table 1. Demographic and disease characteristics of respondents to the postal survey (n = 133)*
| ||Axial pattern (n = 37)||Peripheral pattern (n = 96)|
|Sex (% male)||59||47|
|Age, median (IQR) years||46.2 ± 18.9||51.7 ± 25.2|
|Duration of arthritis symptoms, median (IQR) years||10.9 ± 9.0||13.7 ± 14.7|
|Rheumatoid factor (% positive)||5.6||12|
BASDAI scores were somewhat negatively skewed. The median score was 2.95 (interquartile range 1.50–4.84, range 0.00–9.73). There were no important floor or ceiling effects (1.3% scored the minimum possible value and 0% scored the maximum possible value). BASDAI showed the greatest correlation with patient's perception of arthritis activity (r = 0.739) in comparison with other measures (Table 2). The BASDAI scores also showed similar or greater correlation with SF-36 subscales than did HAQ, except for the physical function scale.
Table 2. Bivariate correlation (Spearman) of BASDAI and HAQ with other indices of disease activity*
|Patient global arthritis VAS (n = 133)||0.74†||0.52†|
|Patient global skin VAS (n = 133)||0.38†||0.24‡|
|HAQ (n = 133)||0.59†||—|
|Swollen joint count (n = 86)||0.29||0.18|
|Tender joint count (n = 86)||0.35‡||0.34‡|
|Damaged joint count (n = 86)||0.15||−0.21§|
|Active joint count (n = 86)||0.29‡||0.27‡|
|Chest expansion (n = 86)||−0.20||−0.18|
|Dougados Articular Index (n = 86)||0.45†||0.48†|
|Schober's index (n = 86)||−0.19||−0.19|
|Tragus to wall measure (n = 86)||0.07||0.01|
|SF-36 (n = 133)|| || |
| Physical function||−0.58†||−0.67†|
| Role limitation—physical||−0.46†||−0.46†|
| Role limitation—emotional||−0.34‡||−0.26‡|
| Social function||−0.50†||−0.45†|
| Mental health||−0.34‡||−0.27‡|
| General health||−0.31‡||−0.32‡|
In the group of patients that was examined, those with a nonaxial pattern of disease (61 of 86) had a lower BASDAI score than patients with an axial pattern of disease (25 of 86); median 3.07 compared with 4.08 (Mann-Whitney U test P = 0.016). Individual BASDAI item scores by disease pattern are shown in Table 3. The patient perception of arthritis activity was nonsignificantly lower (median 27 versus 34; P = 0.134) in patients without axial pattern compared with axial pattern disease, suggesting there was a possibility that the higher BASDAI levels in axial pattern disease were due to worse disease rather than a special relationship of BASDAI with axial disease (Figure 2). The correlation between BASDAI and patient perception of arthritis activity in those with axial disease was very similar to the correlation in patients without axial disease (0.677 compared with 0.606). Using a multivariate analysis to regress patient perception of arthritis activity and type of disease (coded as 0 for nonaxial disease, 1 for axial disease) upon BASDAI, it appeared that the independent relationship between patient perception of disease activity and BASDAI remained dominant (standardized beta weight 0.621, P < 0.001) and that pattern of arthritis contributed little (standardized beta weight 0.165, P = 0.051).
Table 3. BASDAI item scores (median and interquartile range) by disease pattern at followup (n = 86)*
| ||Axial||Peripheral||P (Mann-Whitney U test)|
|Fatigue/tiredness||47 (24–64)||28 (14–59)||0.189|
|Neck, back, or hip pain||37 (13–62)||15 (1–37)||0.033|
|Pain and swelling in other joints||35 (11–48)||22 (10–50)||0.404|
|Discomfort from areas tender to touch||32 (10–64)||20 (8–52)||0.232|
|Level of morning stiffness||31 (13–54)||24 (6–40)||0.131|
|Duration of morning stiffness||38 (9–75)||23 (4–50)||0.056|
The following data items underwent factor analysis: arthritis VAS, skin VAS, BASDAI, Dougados Articular Index, HAQ, active joint count (joints both swollen and tender), damaged joint count (joints with deformity or loss of range of motion), chest expansion, Schober's index, tragus to wall measure. The analysis found evidence of 3 factors (Table 4) that appear to identify factors relating to self-report measures, structural damage, and actively inflamed joints. BASDAI loaded predominantly on self-report measures. The loading of the Dougados Articular Index onto self-report measures rather than actively inflamed joints is probably due to the design of this instrument, with scores being determined by reports of tenderness or pain with certain movements. In this sense, it is actually a self-report measure rather than an externally observed measure.
Table 4. Principle-components factor analysis with varimax rotation (showing factor loadings for each clinical measure)*
| ||Factor 1 (self-reported disability or well being)||Factor 2 (structural damage)||Factor 3 (active inflammatory joint disease)|
|Patient global VAS of arthritis activity||0.74||0.17||0.07|
|Dougados Articular Index||0.63||−0.06||−0.25|
|Patient global VAS of psoriasis activity||0.63||−0.19||0.12|
|Tragus to wall distance||0.06||0.72||−0.17|
|Damaged joint count||0.09||0.77||0.01|
|Active joint count||−0.10||−0.007||0.88|
In the second group of patients (n = 47 with complete data), there were 13 patients with high disease activity and 34 with low disease activity. Logistic regression analysis (Table 5) showed that only the doctors' perception of disease activity was independently associated with disease activity as distinguished by treatment decisions (odds ratio 18.4, 95% confidence interval 2.9–118.3). BASDAI, patient perception, and ESR failed to contribute significantly to this model.
Table 5. Logistic regression model for high versus low disease activity (all variables entered at once)*
|Variable||B (SE)||OR (95% CI)||P|
|Physician rating of disease activity||2.91 (0.95)||18.42 (2.87–118.25)||0.002|
|ESR||−0.0006 (0.0015)||1.00 (0.99–1.002)||0.71|
|Patient rating of disease activity||0.60 (0.68)||1.83 (0.49–6.88)||0.37|
|BASDAI||−0.07 (2.36)||0.93 (0.83–1.05)||0.24|
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We have shown that BASDAI is ambiguous as a measure of disease activity in patients with PsA. It correlates highly with patients' own perception of arthritis activity and this is independent of whether disease is axial or not. It demonstrates better correlation with patient perception than metrology measures or HAQ, and better correlation with SF-36 scores than HAQ for all dimensions other than physical function.
However, the factor analysis suggests that BASDAI is mainly related to other self-report indices rather than active inflammatory disease and that it may be the method of measurement rather than the underlying construct that causes these self-report indices to covary. Furthermore, when an attempt is made to measure disease activity independently by means of treatment decisions in rheumatology clinics in a different consecutive group of patients, BASDAI (and patients' perception of disease activity) fails to significantly discriminate between patients with high and low disease activity. Only the doctors' perception of disease activity was related to treatment decisions.
Compared with patients with ankylosing spondylitis, BASDAI scores in this cohort of patients with PsA were significantly lower overall, although the axial disease patients had values approaching those of AS patients (median 4.26). However, in the reported cohort of AS patients (11), disease duration was much greater (24.7 years) and included a significant number of patients undergoing inpatient treatment; therefore, it is likely that disease severity was significantly greater than in this cohort of PsA patients.
The cohort of patients was drawn from a register of all patients with PsA who had attended the Wellington Regional Rheumatology Unit from 1988. Given an approximate community prevalence of PsA of 0.01% (14), we would expect there to be about 400 patients with this disease in the region; the study identified 159 patients, or only ∼40% of prevalent cases in the region, which raises some concerns with generalizability to the population of patients with PsA as a whole. Nevertheless, the cohort was similar to other reported cohorts of PsA. The diagnosis of PsA was made by any of a number of consultant rheumatologists and, in the absence of validated diagnostic criteria for PsA, did not need to fulfill any criteria.
The absence of a gold standard for disease activity is problematic. The DAS was derived using rheumatologist treatment decisions as the gold standard. In the second part of this study we did not evaluate joint counts, which form integral components of the ACR criteria, DAS, and the Clegg responder criteria. It appears likely that self-report indices, such as BASDAI, are inadequate indicators on their own of disease activity; joint counts and physician ratings are likely to be the most important characteristics of disease activity, as they are in RA. We suggest that BASDAI not be further evaluated as a measure of overall disease activity in patients with PsA, and caution that self-report indices such as these may be measuring a somewhat different construct than active inflammatory pathology.