The Disease Activity Score-28-C-reactive Protein 4 [DAS28-CRP(4)] composite measure for rheumatoid arthritis (RA) is based on 4 variables: tender and swollen joint counts, CRP, and patient global assessment. DAS28-CRP(3) includes only 3 variables, because patient global assessment has been omitted. Thresholds for low and high disease activity are the same for the 2 scores. The objective of our study was to compare the 2 DAS scores and their responses on the individual patient level.
Baseline and 12-week disease activity data from 239 patients with RA treated with a biological agent were extracted from the Danish registry for biological treatment (DANBIO). Cohen's effect sizes (ES) and disease activity levels according to the DAS thresholds were assessed. The Bland-Altman method was used to examine the bias between the DAS scores and the 95% limits of agreement (LoA).
Baseline values for DAS28-CRP(4) and DAS28-CRP(3) were 4.8 ± 1.2 and 4.6 ± 1.1, respectively. At 12 weeks, DAS28-CRP(4) had improved by -1.39 ± 1.34 (p < 0.0001). At that timepoint the bias of DAS28-CRP(3) was -0.07 (LoA -0.69, 0.55) (p < 0.0001). The bias of the DAS28-CRP(3) response was +0.21 (LoA -0.49, 0.91) (p < 0.0001). ES for DAS28-CRP(4) was 1.2 ± 1.1 versus 1.1 ± 1.1 for DAS28-CRP(3) (p < 0.0001). Compared to DAS28-CRP(4), DAS28-CRP(3) categorized 33% fewer patients as having a high level of disease activity, 8% fewer patients as good responders, and 12% more patients as nonresponders.
Mean values of DAS28-CRP(4) and DAS28-CRP(3) agreed well, but in the individual patient the difference between the scores and their responses may be substantial.