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Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Objective

The Disease Activity Score (DAS) is widely used in clinical trials. A DAS of 5.1 defines the level of severe rheumatoid arthritis (RA) and is the criterion for the initiation of anti–tumor necrosis factor therapy in the UK and The Netherlands. In North America, similar rules are sometimes imposed. However, it is not known how accurately the DAS characterizes RA activity. The present study was undertaken to determine the concordance between DAS scores and physicians' assessments of RA activity, to investigate factors relating to discrepancies, and to assess the suitability of using the DAS in individual patients.

Methods

Six hundred sixty-nine RA patients were assessed using the DAS and other clinical measures. A physician's global estimate of RA activity was performed using an 11-point predefined scale and a standard definition of disease activity.

Results

The DAS and physician global assessment had substantially different distributions of values. The level of agreement (Kendall's tau-a) between DAS scores and physician global assessments was 49% (95% confidence interval 45–53%), Lin's coefficient of concordance was 0.62, and the Bland-Altman 95% limits of agreement were −3.17 and 3.99. These results suggest poor-to-moderate concordance between the 2 measures of disease activity.

Conclusion

The DAS and the physician's assessment of RA activity do not approach, value, and weight RA variables to the same extent, suggesting that RA activity is not evaluated similarly by North American physicians and with the DAS. The scales do not have acceptable levels of concordance. There is too much inherent variability in the DAS and other RA scales (e.g., the Health Assessment Questionnaire) to recommend them as sole determinants of RA activity for clinical or regulatory purposes.

The 28-joint Disease Activity Score (DAS28) (1–4) is one of two currently used methods for describing the results of randomized clinical trials (RCTs), the other being the American College of Rheumatology (ACR) improvement criteria (5–7), and both have been shown to identify similar responder groups. The ACR criteria measure only change, while the DAS measures change as well as the level of rheumatoid arthritis (RA) activity.

There has been an increasing trend toward measuring disease activity in clinical practice (8–11). This is largely the result of attempts by governments and insurance companies to regulate prescription of expensive biologic agents, particularly anti–tumor necrosis factor (anti-TNF) therapy (12). They seek to have anti-TNF therapy prescribed to RA patients with severe or high levels of disease activity, and then to have the therapy continued only in those for whom there is a sufficient degree of response. The ability of the DAS to measure disease activity has led to its adoption in a number of countries. In the UK, for example, anti-TNF therapy is restricted to patients with DAS scores of >5.1 (12, 13). A DAS score of <3.2 is considered to indicate low disease activity, 3.2–5.1 moderate disease activity, and >5.1 high disease activity. The DAS has also been used to titrate the dosage of anti-TNF therapy (14).

The origins of the DAS come from the clinic (1–3). Patients in whom therapy was being changed because of worsening RA activity were studied, and a series of variables that best predicted change in therapy were identified. In an elegant series of analyses, a scale was constructed that reflected the distribution of 4 of these variables, the tender joint count, swollen joint count, erythrocyte sedimentation rate (ESR), and patient's estimate of global health. The Health Assessment Questionnaire (HAQ) (15) was not tested in this study, and neither pain nor physician global assessment is part of the index.

In the clinical setting the physician determines the level of RA activity but most often does not record it. To make this assessment, the physician may use a variety of data instead of or including the measures contained in the DAS. Such data might include degree of pain, pain sensitivity, functional ability, appearance, concomitant fibromyalgia, grip strength, response to therapy, other joints not included in the DAS28, extraarticular features, radiographic changes, and other laboratory results. Such assessments are weighted by the physician and produce the physician's measure of global RA activity. In clinical care this is the central determination that leads to prescription of therapy. Among the weaknesses of a physician's global evaluation is that each physician may rate these composite underlying activity variables differently or may not consider all of them. In addition, regulators may worry that such a single measure can easily be influenced so that treatment can be obtained, that it is not “objective,” and that it does not sufficiently document RA status. The DAS overcomes these problems and has become the standard evaluation tool in European clinics.

There are a number of reasons the DAS might not work well in the clinical setting. First, in the development of the measure, the valuation of the elements of RA activity was based on a single clinic in The Netherlands. It is possible that other physicians in other centers and countries might value these elements differently. Second, it is possible that physicians' estimates of RA global activity might not correlate strongly with the DAS. Third, classification into severity groups, while clearly acceptable and reliable for groups of patients, might not be accurate or reliable at the level of the individual patient. In this study we evaluated the use of the DAS as a clinic assessment tool by determining the concordance between the DAS score and the physician's assessment of RA activity. We then investigated factors relating to discrepancies and assessed the suitability of using the DAS in individual patients. However, our purpose was larger in that the DAS is representative of clinic assessment instruments in general. We hope the results of this study might be relevant when other assessment tools are considered for use in making regulatory determinations.

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

As part of a preliminary study to aid in the development of RA improvement criteria, we recruited Canadian and American academic and community physicians and asked them each to examine ∼10 consecutive RA patients in their clinics. Of 718 patients for whom data on the ESR were available, 27 had missing data on the physician assessment of RA activity and 48 had missing data on the patient global assessment. The results reported herein are from 669 patients for whom complete data were available on tender and swollen joint counts, ESR, patient global assessment, and physician rating of disease activity. Patient demographic and clinical characteristics are shown in Table 1. Data on these patients came from 61 physicians (mean ± SD 11.0 ± 6.9 patients each [median 9.0]).

Table 1. Demographic and clinical characteristics of the 669 RA patients*
  • *

    Except where indicated otherwise, values are the mean ± SD. RA = rheumatoid arthritis; DAS28 = Disease Activity Score in 28 joints; ESR = erythrocyte sedimentation rate; HAQ = Health Assessment Questionnaire; M-HAQ = modified HAQ; MD-HAQ = multidimensional HAQ; VAS = visual analog scale; SF-36 = Medical Outcomes Short Form 36; MTX = methotrexate; HCQ = hydroxychloroquine; DMARDs = disease-modifying antirheumatic drugs.

Age, years58.0 ± 13.5
Male, %27.3
Ethnicity, % 
 Non-Hispanic white80.3
 Black10.0
 Asian origin3.2
 Native North American1.9
 Hispanic2.8
 Other1.9
Years of education, % 
 0–84.5
 8–1113.2
 1229.3
 13–1524.4
 ≥1628.6
Disease duration, years12.5 ± 10.5
DAS28, 0–104.2 ± 1.7
Tender joint count, 0–286.1 ± 7.3
Swollen joint count, 0–285.1 ± 6.0
ESR, mm/hour27.0 ± 23.0
Patient global assessment, 0–104.0 ± 2.7
Physician-assessed RA activity, 0–103.7 ± 2.5
HAQ, 0–31.06 ± 0.75
HAQ-II (0–3)0.92 ± 0.69
M-HAQ, 0–30.49 ± 0.51
MD-HAQ, 0–30.73 ± 0.57
VAS fatigue, 0–104.5 ± 2.9
SF-36 vitality, 0–10045.7 ± 23.9
Regional Pain Scale, 0–196.3 ± 5.6
Fibromyalgia (survey criteria), %20.8
EuroQol utility, 0–10.61 ± 0.30
Patient self-reported joint count, 0–147.4 ± 4.3
Treatment, % 
 MTX ever78.9
 HCQ ever56.2
 Prednisone ever65.6
 Prednisone current30.9
 DMARDs or biologic agents current88.8
 Biologic agents current28.3

Physicians completed a 28-joint count of tender and swollen joints (4, 16) and a physician rating scale of RA disease activity. The scale consisted of 11 check boxes from 0 to 10. Under the 0, 1–3, 4–7, and 8–10 were brackets and the words “none, mild, moderate, severe” for the respective categories. Protocol instructions to the physician indicated “In making your evaluation you may take the patients' questionnaire responses into consideration or you may ignore them according to how you usually evaluate patients. You may ask any additional questions you wish or perform any examinations you would ordinarily do. That is, use your usual method of evaluation to determine the ‘Physician’s estimate of disease activity' … Disease activity does not mean structural damage. In addition, pulmonary fibrosis, pleuro-pericarditis and vasculitis does not necessarily mean disease activity, as these conditions may occur in patients with low levels of disease activity or without disease activity.” Physicians were asked if they considered extraarticular disease in making their assessments, and 19.6% indicated that they did.

Patients completed the HAQ (15), HAQ-II (17), multidimensional HAQ (18), modified HAQ (19), visual analog scales (VAS) for pain, global disease severity, and fatigue (20), a self-reported joint count (21), a count of nonarticular affected regions (the Regional Pain Scale) (22), and the Medical Outcomes Short Form 36 health survey (23, 24) from which the vitality scale was calculated.

The DAS was calculated from the tender and swollen joint count, the ESR, and the patient's global assessment, according to the instructions of the developers of the DAS.

Kendall's tau-a and associated confidence limits were calculated using the Somer's D package (25), adjusted for clustering within referring rheumatologists. Questionnaire agreement was also assessed using the Bland-Altman limits of agreement procedure (26) and Lin's coefficient of concordance (27). Other analyses were performed using Stata version 8.2 (Stata, College Station, TX). P values less than 0.05 were considered significant.

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

How concordant are the DAS and the physician-assessed RA activity scale?

Figure 1 presents the distribution curves of the DAS and physician-determined RA activity scale. The DAS was normally distributed. Using the cut points of 3.2 and 5.1, as provided by DAS developers and users, low disease activity was found in 29.0% of patients, moderate activity in 42.2%, and high activity in 28.9%. In contrast, the physician-determined RA activity scale was not normally distributed, with the predominance of values skewed to the left. No or mild disease activity was noted in 52.2% of patients, moderate activity in 37.6%, and severe activity in 10.3%. The differences in the scales according to disease activity groupings can be seen clearly in Figure 2, in which the DAS scale overlaps substantially with all categories of the physician RA activity scale.

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Figure 1. Left, Distribution of 28-joint Disease Activity Scores (DAS28). The line at 3.2 separates low disease activity from moderate disease activity, and the line at 5.1 separates moderate disease activity from high disease activity. Low disease activity encompassed 29.0% of patients, moderate activity 42.2%, and high activity 28.9%. Right, Distribution of physicians' estimates of rheumatoid arthritis (RA) activity. No or mild disease activity was noted in 52.2% of patients, moderate activity in 37.6%, and severe activity in 10.3%.

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thumbnail image

Figure 2. Distribution of the DAS28 by the 4 categories of physician-assessed RA activity. The line at 3.2 separates low disease activity from moderate disease activity, and the line at 5.1 separates moderate disease activity from high disease activity. The DAS overlapped substantially with physician-assessed RA activity in all categories. See Figure 1 for definitions.

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Because the determination of cut points for the groupings is or may be arbitrary, as are the words used to designate the groups, we further analyzed the 2 scales without references to grouping. The level of correlation between the scales was 0.69. Lin's concordance coefficient, which accounts for both precision and accuracy, was 0.62. These are relatively low values for scales that measure the same thing. The mean ± SD score on the DAS28 was 4.15 ± 1.67, and the comparable value for physician global estimate of disease activity was 3.73 ± 2.50. The between-scale differences can also be seen in the graph of the Bland-Altman limits of agreement shown in Figure 3. The average difference between scales was 0.41, with 95th percentile limits of agreement of −3.17 and 3.99. These differences are very wide for a 10-point scale. Figure 3 also shows that differences were greater for the DAS at the low end of the scale and greater for the physician assessment of RA activity at the high end of the scale. Overall, these data suggest poor-to-moderate concordance between the two measures of disease activity.

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Figure 3. Limits of agreement between the DAS28 and physician-assessed RA activity. The horizontal line at 0 indicates the region where all data points would fall if there were perfect agreement between the 2 scales. The line just above it is the mean difference between the 2 scales (0.41) (Table 2). The upper and lower lines are at the 95th percentile of differences in score (−3.17 and 3.99). The diagonal dashed line is the regression line of the differences in scores on the mean of the scores. See Figure 1 for definitions.

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Can the DAS be used in the clinic?

Figure 4 shows graphs of the scatter plot and regression of the physician RA activity scale on the DAS. In addition to showing that at the intercept the DAS was >2, the figure illustrates the difference between the confidence intervals for the mean and those for the forecast intervals, which refer to an individual patient prediction. The confidence intervals in the graph on the left side of Figure 4 are narrow, therefore allowing use of these variables in the analysis of clinical trials and groups of patients. Therefore it would be possible, if desired, to construct a conversion algorithm between the scales and predict a mean value of one variable from the other. However, it would not be possible to make any meaningful prediction from one value to the other for the individual patient (Figure 4, right). For example, with a physician-determined activity score of 3 (mild), all that could be concluded is that 95% of the time the expected value of the DAS would be between 1 and 5.9, or between very low activity and high activity. Such a large range makes this determination of no use. It is not just the DAS and physician-assessed RA activity scales that differ: the same would be true of the HAQ score and the VAS for pain (r = 0.633). The correlation between scales needs to be very high (>0.90) before prediction can be made from one scale to the other.

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Figure 4. Scatter plots and regression lines for regression of the DAS28 on physician-assessed RA activity. Shaded areas show the confidence interval for the mean of the regression line (left) and forecast intervals that refer to the individual patients (right). See Figure 1 for definitions.

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How and why do the DAS and physician-assessed RA activity scales differ?

Table 2 displays levels of agreement between important clinical variables and the DAS, the physician-assessed RA activity scale, and the patient global assessment, calculated using Kendall's tau-a. The tau-a coefficients reflect percent agreement. As seen in Table 2, the DAS28 and the tender joint count were 63% more likely to be concordant than to be discordant. In contrast, physician-determined RA activity and the ESR were only 17% more likely to be concordant than to be discordant.

Table 2. Levels of agreement (Kendall's tau-a) between RA activity functional assessment variables and other clinical variables*
VariableDAS28Physician-assessed disease activityPatient global assessment
Tau-a95% CITau-a95% CITau-a95% CI
  • *

    Tau-a coefficients reflect the percent agreement, e.g., the DAS28 and the tender joint count were 63% more likely to be concordant than to be discordant, physician-assessed RA activity and the tender joint count were 42% more likely to be concordant than to be discordant, and patient global assessment and the tender joint count were 27% more likely to be concordant than to be discordant. 95% CI = 95% confidence interval (see Table 1 for other definitions).

DAS28 1.00–1.000.490.45–0.530.410.36–0.45
Tender joint count0.630.60–0.660.420.38–0.460.270.22–0.32
Swollen joint count0.470.44–0.510.450.42–0.490.180.13–0.23
Patient global assessment0.410.36–0.450.320.27–0.36 0.94–0.95
ESR0.410.37–0.450.170.12–0.220.130.08–0.18
Physician-assessed RA activity0.490.45–0.53 0.88–0.890.320.27–0.36
Pain0.330.29–0.380.340.29–0.380.540.50–0.59
HAQ0.360.32–0.410.330.29–0.370.440.40–0.49
MD-HAQ0.360.32–0.410.350.30–0.390.470.43–0.52
HAQ-II0.350.31–0.400.320.28–0.370.470.42–0.51
M-HAQ0.340.30–0.380.330.29–0.380.440.40–0.49
Patient-reported joint count0.280.23–0.330.290.25–0.340.320.27–0.37
Fatigue0.260.21–0.300.260.22–0.310.510.47–0.55
Regional pain0.260.21–0.310.270.22–0.310.380.34–0.43
Vitality0.240.28–0.190.220.27–0.170.400.45–0.36

It is not surprising that the DAS was more strongly associated with the first 4 variables in Table 2, i.e., the tender joint count, swollen joint count, patient global assessment, and ESR, since they constitute the variables that make up the DAS. Of interest, however, is that the agreement was much stronger between the DAS and the tender joint count (tau-a 0.63) than between the DAS and the other DAS variables (tau-a 0.41–0.47). There was a much lower level of agreement between the DAS and the patient self-report variables (tau-a 0.24–0.36). Assessment of the association between individual variables and physician-assessed RA activity revealed that the strongest of the noncomposite clinical variables were swollen joint count (tau-a 0.45) and tender joint count (tau-a 0.42). Of particular interest was the low level of agreement between the physician-assessed activity variable and the ESR (tau-a 0.17). These data show that the North American physicians in this study did not value the ESR highly as a parameter for assessing disease activity. In addition to the low level of agreement with the ESR, there was reduced agreement between physician-determined RA activity and the patient self-report variables (tau-a 0.22–0.35).

A completely different pattern of agreement emerged when patient global assessment was considered. Self-report variables were important, and pain (tau-a 0.54) and fatigue (tau-a 0.51) were particularly strongly associated with patient global assessment. However, these variables were not strongly associated with the DAS or the physician assessment of RA activity, and patient global assessment showed weaker associations with the tender and swollen joint counts, physician global assessment, and ESR.

Thus, our analyses, as shown in the data presented in Table 2, revealed that 1) the DAS is associated with its “built-in” variables, although not equally as might have been expected from the DAS development data; 2) physicians do not value the ESR highly as a tool for evaluating overall disease activity; and 3) variables that are important to patients are not the same as those valued in the DAS and physician-assessed RA activity scale.

DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

The data obtained in this study show that RA variables are not approached, valued, and weighted to the same extent with the DAS and physician-assessed RA activity scales. The most obvious differences are in the tender joint count (tau-a 0.63 versus 0.42), ESR (0.41 versus 0.17), and patient global assessment (0.41 versus 0.32). We believe it is a fair conclusion that North American physicians' assessments and DAS variables do not lead to similar conclusions regarding RA disease activity. It should be understood that our data do not address which method of evaluating activity is more correct, only that they are different.

This difference results in distributions of scores that are substantially different for the 2 scales (Figures 1–3). We would go so far as to say that the scales are incompatible with regard to cut points and designations of mild, moderate, and severe activity (physician RA activity assessment) and low, moderate, and high activity (DAS). This is one reason we believe that application of scales to the assessment of the individual patient is tenuous. We also have concerns regarding the distribution of values for the DAS (Figure 1). This distribution is, in part, dependent upon the nonlinear transformation of the DAS variables since normally distributed disease activity is not what is seen in the clinic, where distributions of the HAQ score, pain on VAS, and physician-assessed RA activity (Figure 2) all have similar appearances. The DAS cut points of 3.2 and 5.1 result in low, moderate, and high disease activity being designated in 29.0%, 42.2%, and 28.9% of patients, respectively. This almost normal distribution does not parallel findings obtained with other scales.

Any scale that relies on fixed coefficients has other potential advantages as well as limitations. ESR, pain, and patient global assessment differ according to sex when measured in male and female patients separately. A physician's global assessment might (but often does not) take into consideration other factors such as pain, pain sensitivity, functional ability, appearance, concomitant fibromyalgia, grip strength, response to therapy, other joints not included in the DAS 28-joint count (for example, joints in the feet), extraarticular features, radiographic changes, and other laboratory results. The physician's global evaluation might account for a person who always scores a certain way on the tender joint examination or the ESR, for example. Because we seem to be criticizing the DAS, we want to reemphasize that these criticisms apply only to the use of the DAS as a sole clinical measurement tool and do not apply to RCTs. Whether the DAS is the best activity scale for use in RCTs was not evaluated in this study.

The second major conclusion from our findings is that relying on any one scale to make a regulatory decision is not appropriate. Figures 3 and 4 (right) show that there is too much variability in the DAS to enable reliance on it for important decision making. This is not just a function of the DAS; it is also true of scales such as the HAQ, pain assessments, or the various global assessments. As alluded to above, the degree of variability that makes the scales inappropriate as sole indicators in clinical care does not limit their effective use in RCTs and observational studies.

In summary, North American physicians' assessments and DAS variables do not lead to similar conclusions regarding RA disease activity. RA variables are approached, valued, and weighted differently with the 2 methods, and levels of concordance between the 2 scales are not acceptable. There is too much inherent variability in RA scales for them to be recommended as sole determinants of RA activity for clinical or regulatory purposes.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  • 1
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  • 2
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