The Behcet's disease activity index

To identify a subset of clinical features of Behçet's disease (BD) that can be summated to form an overall index of disease activity appropriate for clinical and research use internationally.

Completed Behçet's Disease Current Activity Forms were collected from a total of 524 patients with BD from five countries. The data from 14 questions on the form were subjected to Rasch analysis to establish whether these items form a hierarchical and unidimensional scale of disease activity, both within and between countries.

The data showed a good fit to the Rasch model within three countries using a dichotomous scoring function. However, when the data from these three countries were pooled, the fit to the model was poor. Cross-cultural differential item functioning (DIF) was found in seven items in the pooled data. When the items with DIF by country were separated and two items were removed, the resulting 26-item scale showed a good fit to the Rasch model.

Within Turkey, Korea and the UK, the 14 items can be summated to give an index of disease activity. Analysis of the pooled data confirmed that the index is not suitable for comparison between countries or for pooling of data in the raw form, but after fitting the data to the Rasch model such comparisons can be made. This gives a scaling tool that is quick and easy to use in the clinical situation.