Development and validation of a new disease activity index as a numerical sum of four variables in patients with early arthritis.

Objective: To describe the development and validation of a disease activity index in early arthritis that can be easily applied in daily practice and clinical research.

Methods: The Hospital Universitario La Princesa Index (HUPI) was developed after analysis of data from an early arthritis cohort (202 patients with 756 visits). It is the sum of 4 variables (graded 0-3): tender joint count, swollen joint count, patient global assessment, and acute-phase reactants (erythrocyte sedimentation rate [ESR] and/or C-reactive protein [CRP] level, depending on availability at the moment of evaluation). The score for each variable was based on its quartile distribution in the cohort. The HUPI was validated using the following properties: feasibility, internal consistency (Cronbach's alpha), convergent validity (Pearson's r coefficients with other activity measures), criterion validity (area under the receiver operating characteristic curve [AUC ROC] to detect minimal disease activity [MDA]), and sensitivity to change (AUC ROC) to detect change with the physician's and patient's assessment of disease activity.

Results: Internal consistency is reasonable (α = 0.63). The HUPI correlates well with activity measures such as the Disease Activity Score in 28 joints (DAS28; r = 0.89) and the Simplified Disease Activity Index (SDAI; r = 0.70), and correlates slightly worse with the functional index of the Health Assessment Questionnaire (r = 0.69). It discriminates MDA correctly (AUC 0.95), and its sensitivity to change is slightly superior (AUC 0.902) to that of the DAS28-ESR (AUC 0.864), the DAS28-CRP (AUC 0.889), and the SDAI (AUC 0.791).

Conclusion: The HUPI has face validity, is easy to calculate, is sensitive, and is a valid composite index for the assessment of disease activity in patients with early arthritis, both in clinical research and in routine care.