DAS28 is a widely used composite score for the assessment of disease activity in patients with rheumatoid arthritis (RA) and is often used as a treatment decision tool in the daily clinic. Different versions of DAS28 are available. DAS28-CRP(3) is calculated based on three variables: swollen and tender joint counts and CRP. DAS28-CRP(4) also includes patient global assessment. Thresholds for low and high disease activity are the same for the two scores. Based on the Bland-Altman method, the interchangeability between DAS28-CRP with three and four variables was examined in 319 RA patients selected for initiating biological treatment. Data were extracted from the Danish registry for biological treatment in rheumatology (DANBIO). Multiple regression analysis was used to assess the predictability of the DAS28 scores by several measures of disease activity. The overall mean DAS28-CRP was 4.7 ± 1.2. The mean difference between the two scores (the bias) was -0.29 ± 0.33 (CI -0.33, -0.25) (p < 0.0001). Upper and lower limits of agreement were -0.95 (CI -1.01, -0.89) and 0.37 (CI 0.31, 0.43), respectively. Tender joint count was the most important predictor of both DAS28-CRP(4) (beta = 0.52, p < 0.0001) and DAS28-CRP(3) (beta = 0.62, p < 0.0001). The second most important predictor of DAS28-CRP(4) was patient global assessment (beta = 0.32, p < 0.0001) which did not add to the prediction of DAS28-CRP(3). In conclusion, overall DAS28-CRP(3) was only slightly underestimated compared to DAS28-CRP(4). In the individual patient, however, the two scores may differ considerably. The scores should not be used interchangeably in the daily clinic without caution.