BACKGROUND: 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.
METHODS: Patients 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.
RESULTS: Composite 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.
CONCLUSIONS: The 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.