Abstract
BackgroundThe 28-joint Disease Activity Score (DAS28) combines scores on a 28-tender and swollen joint count (TJC28 and SJC28), a patient-reported measure for general health (GH), and an inflammatory marker (either the erythrocyte sedimentation rate [ESR] or the C-reactive protein [CRP]) into a composite measure of disease activity in rheumatoid arthritis (RA). This study examined the reliability of the DAS28 in patients with early RA using principles from generalizability theory and evaluated whether it could be increased by adjusting individual DAS28 component weights.MethodsPatients were drawn from the DREAM registry and classified into a “fast response” group (N = 466) and “slow response” group (N = 80), depending on their pace of reaching remission. Composite reliabilities of the DAS28-ESR and DAS28-CRP were determined with the individual components' reliability, weights, variances, error variances, correlations and covariances. Weight optimization was performed by minimizing the error variance of the index.ResultsComposite reliabilities of 0.85 and 0.86 were found for the DAS28-ESR and DAS28-CRP, respectively, and were approximately equal across patients groups. Component reliabilities, however, varied widely both within and between sub-groups, ranging from 0.614 for GH (“slow response” group) to 0.912 for ESR (“fast response” group). Weight optimization increased composite reliability even further. In the total and “fast response” groups, this was achieved mostly by decreasing the weight of the TJC28 and GH. In the “slow response” group, though, the weights of the TJC28 and SJC28 were increased, while those of the inflammatory markers and GH were substantially decreased.ConclusionsThe DAS28-ESR and the DAS28-CRP are reliable instruments for assessing disease activity in early RA and reliability can be increased even further by adjusting component weights. Given the low reliability and weightings of the general health component across subgroups it is recommended to explore alternative patient-reported outcome measures for inclusion in the DAS28.
Highlights
If a concept or condition is too complex to measure with a single instrument, multiple measurements are often combined into a linear composite score
For rheumatoid arthritis (RA) the 28-joint Disease Activity Score (DAS28) is such an index measure, widely used for determining a patient’s degree of disease activity [1]. It consists of 4 different individual components: a 28-tender joint count, a 28-swollen joint count, a patient-reported rating of general health, and a non-specific acute phase reactant of systemic inflammation which can be either the erythrocyte sedimentation rate (ESR) or the C-reactive protein (CRP)
Since reliability is a concept defined relative to a specific population of patients [9], the second aim of this study was to examine whether the reliability of the index is acceptably high in relevant subpopulations and whether this reliability can be increased by adjusting the weightings of the individual component scores within the DAS28, which was shown to be the case
Summary
If a concept or condition is too complex to measure with a single instrument, multiple measurements are often combined into a linear composite score (i.e. an index measure). For rheumatoid arthritis (RA) the 28-joint Disease Activity Score (DAS28) is such an index measure, widely used for determining a patient’s degree of disease activity [1] It consists of 4 different individual components: a 28-tender joint count, a 28-swollen joint count, a patient-reported rating of general health, and a non-specific acute phase reactant of systemic inflammation which can be either the erythrocyte sedimentation rate (ESR) or the C-reactive protein (CRP). The components might measure completely different aspects of disease activity, which is the case with the DAS28 This poses significant methodological challenges for reliability testing. The 28-joint Disease Activity Score (DAS28) combines scores on a 28-tender and swollen joint count (TJC28 and SJC28), a patient-reported measure for general health (GH), and an inflammatory marker (either the erythrocyte sedimentation rate [ESR] or the C-reactive protein [CRP]) into a composite measure of disease activity in rheumatoid arthritis (RA). This study examined the reliability of the DAS28 in patients with early RA using principles from generalizability theory and evaluated whether it could be increased by adjusting individual DAS28 component weights
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