The Disease Activity Score (DAS) is integral in tailoring the clinical 
management of rheumatoid arthritis (RA) patients and is an important measure in 
clinical research. Different versions have been developed over the years to 
improve reliability and ease of use. Combining the original DAS and the newer 
DAS28 data in both contemporary and historical studies is important for both 
primary and secondary data analyses. As such, a methodologically robust means of 
converting the old DAS to the new DAS28 measure would be invaluable. Using data 
from The Early RA Study (ERAS), a sub-sample of patients with both DAS and DAS28 
data were used to develop new regression imputation formulas using the total DAS 
score (univariate), and using the separate components of the DAS score 
(multivariate). DAS were transformed to DAS28 using an existing formula quoted 
in the literature, and the newly developed formulas. Bland and Altman plots were 
used to compare the transformed DAS with the recorded DAS28 to ascertain levels 
of agreement. The current transformation formula tended to overestimate the true 
DAS28 score, particularly at the higher end of the scale. A formula which uses 
all separate components of the DAS was found to estimate the scores with a 
higher level of precision. A new formula is proposed that can be used by other 
early RA cohorts to convert the original DAS to DAS28.