Table 1 compares the formulas used to calculate the different composite measures. Table 2 describes the baseline characteristics of the 223‐patient early‐RA cohort. The patients are similar to those in other early‐RA cohorts: mostly female (78%) with average disease duration of 6 months and of moderate to high disease activity with mean DAS44/ESR‐4 item 4.9 (and mean DAS28/ESR‐4 item 6.1). The average swollen joint count and tender joint count were 12 and 13 (using the 28‐joint count), the average Ritchie Articular Index was 17.8 (out of 78), and the swollen 44‐joint count was 18.7.
The centile distributions of the scores of 10 composite status measures (calculated for each of 705 distinct observations) are plotted in fig 1. Scores connected by each horizontal centile line are equivalent to each other in this population—for example, equivalent scores on the 20th centile line are DAS44/ESR‐4 (2.4), DAS44/ESR‐3 (2.4), DAS44/CRP‐4 (2.2), DAS44/CRP‐3 (2.2), DAS28/ESR‐4 (3.3), DAS28/ESR‐3 (3.4), DAS28/CRP‐4 (3.1), DAS28/CRP‐3 (3.1), SDAI (10) and CDAI (9). Thus, if the value of one of these status measures is known, the scores of the other measures can be approximated. For example, given a known CDAI value, equivalent values of DAS and SDAI can be easily estimated from this nomogram.
Correlations between the disease activity composite measures (table 1) ranged from 0.85 for DAS44/CRP‐3 with the DAS28/ESR‐3 item to 0.99 comparing DAS44/ESR‐4 with DAS44/ESR‐3 and SDAI with CDAI (data not shown).
For each of the 223 patients, the ACR20/50/70 response from baseline to the best follow‐up observation point was calculated. Maximum improvements attained at any time point during the 2 year follow‐up were ACR70 by 54 patients, ACR50 by 36 patients, ACR20 by 46 patients, and 87 patients did not satisfy ACR20 criteria at any evaluation during the 2‐year period (table 3). For each of 10 possible status measures, the change in score between baseline and the observation used to calculate the ACR20/50/70/none was determined. For all of the status measures, the mean change scores for the ACRnone group are significantly different from change scores for the ACR20, ACR50 and ACR70 groups. The baseline to ACR50 change scores are significantly different from the baseline to ACR70 change scores, but the change score differences between the ACR20 and ACR50 groups are not significantly different (table 3).
Comparing change scores of the DAS44/ESR‐4 item with the DAS44/ESR‐3 item, DAS44/CRP‐4 item, DAS44/CRP‐3 item, DAS28/ESR‐4 item, DAS28/ESR‐3 item, DAS28/CRP‐4 item, DAS28/CRP‐3 item, SDAI and CDAI, the correlation coefficients of these change scores were respectively 0.99, 0.97, 0.96, 0.87, 0.85, 0.82, 0.81, 0.83 and 0.81 (all p<0.001) (data not shown).
To evaluate how well the different composite disease activity measures categorised patients into the EULAR good, moderate and none criteria, we evaluated the percentage agreement between DAS44/ESR‐4 item categories (using published EULAR cut‐points) with categorisation of the same patients using each composite measure. The goal was to use the most recognised values for this analysis; thus, the published reached and change values were preferentially used if available, otherwise the calculated values from the Western Consortium cohort (table 4) were used to calculate the percentage agreement. When the DAS44/ESR‐3 item was compared with the DAS44/ESR‐4 item published cut‐point values, there was a percentage agreement of 92. Similarly, for the DAS44/CRP‐4 item, DAS44/CRP‐3 item, DAS28/ESR‐4 item, DAS28/ESR‐3 item, DAS28/CRP‐4 item, DAS28/CRP‐3 item, SDAI and CDAI, the percentage agreements with the DAS44/ESR‐4 item were calculated to be 87, 89, 80, 78, 80, 80, 80 and 74, respectively (table 5). The highest agreement is between the DAS44/ESR‐4 item and other DAS44 measures. Less agreement is seen with the DAS28 measures, and least with CDAI.
When we used the DAS28/ESR‐4 item as the gold standard in comparisons with the DAS44/ESR‐4 item, DAS44/ESR‐3 item DAS44/CRP‐4 item, DAS44/CRP‐3 item, DAS28/ESR‐3 item, DAS28/CRP‐4 item, DAS28/CRP‐3 item, SDAI and CDAI, the percentage agreements were 77, 76, 72, 73, 88, 83, 76, 73 and 74, respectively (data not shown).