The Disease Activity Score in 28 joints (DAS28) is a key measure in clinical practice and clinical trials. There are at least five different versions of the 'Patient Global' Visual Analogue Scale (PG-VAS) being used in the DAS28. The developers suggested that the PG-VAS can be an assessment of global health or disease activity, but did not specify the wording of the question. There is no consensus on what the PG-VAS is intended to capture, and the different words and phrases have not been evaluated. The aim of this study was to test if phrasing affects PG-VAS scores and hence yields different results for the DAS28.
Fifty patients with rheumatoid arthritis taking biologic agents in a rheumatology outpatient department completed a self-administered questionnaire containing five versions of the 100 mm PG-VAS.
All PG-VAS versions correlated strongly with each other (rho = 0.67-0.87, p < 0.0001). However, individual scores for each PG-VAS, when compared with the comparator on a Bland-Altman chart had wide limits of agreement--the largest being -42 mm to +45 mm. The five overall DAS28 scores were calculated for each patient using the five different PG-VAS. The largest difference in DAS28 scores was 0.63.
Different phrasing of the PG-VAS gives different DAS28 results. As the DAS28 is a key outcome measure, such differences have the potential to influence clinical decisions relating to eligibility for biologic agents and evaluation of new therapies. We urgently need to decide on the concept to be measured and the phrasing required to capture this. The PG-VAS phrasing should then be standardized and validated.