Objective: To establish the validity of an index designed to measure activity in systemic necrotizing vasculitis (SNV).
Methods: The Vasculitis Activity Index (VAI) was designed to incorporate appropriately weighted clinical measurements that reflect disease activity in SNV. We performed a pilot study to guide the modification and subsequent testing of the initial design. The data necessary to calculate the VAI are direct ratings by a clinical observer of the degree of activity in 9 organ systems and 3 indirect measures of vasculitis activity. These data are recorded on 0-4 visual analog scales. Physician's global assessment (PGA) is used as the "gold standard" measurement of disease activity. The VAI was validated using 2 independent data sets: the questionnaire data set, derived from test case histories ("paper cases") sent to 100 practicing rheumatologists, and the clinic data set, obtained from use of the VAI in 204 regular care visits of 74 patients with SNV.
Results: The VAI correlated highly with the PGA: Pearson's correlation coefficient R = 0.84 (95% confidence interval [95% CI] 0.80-0.88) for the questionnaire data set, and R = 0.92 (95% CI 0.90-0.94) for the clinic data set. The mean of the interobserver coefficients of variation for the test case histories was lower for the VAI than for the PGA (mean difference 0.45; P = 0.002), indicating that the VAI has less interobserver variation than does the PGA. The change in VAI between clinic visits for individual patients correlated highly with the change in PGA (R = 0.88, 95% CI = 0.83-0.91). The VAI data collection form requires about 1 minute to complete, including computation of the score.
Conclusion: The VAI is a valid measure of vasculitis activity that correlates highly with the PGA. In addition, the VAI has less interobserver variation than the PGA and has a high level of sensitivity to change over time. Additional testing of the VAI appears warranted.