Clinical utility and validity of the Functional Disability Inventory among a multicenter sample of youth with chronic pain.

The Functional Disability Inventory (FDI) is a well-established and commonly used measure of physical functioning and disability in youth with chronic pain. Further validation of the measure has been called for, in particular, examination of the clinical utility and factor structure of the measure. To address this need, we utilized a large multicenter dataset of pediatric patients with chronic pain who had completed the FDI and other measures assessing pain and emotional functioning. Clinical reference points to allow for interpretation of raw scores were developed to enhance clinical utility of the measure, and exploratory factor analysis was performed to examine its factor structure. Participants included 1300 youth ages 8 to 18 years (mean=14.2 years; 76% female) with chronic pain. Examination of the distribution of FDI scores and validation with measures of depressive symptoms and pain intensity yielded 3 distinct categories of disability: No/Minimal Disability, Moderate Disability, and Severe Disability. Factor analysis of FDI scores revealed a 2-factor solution representing vigorous Physical Activities and non-physically strenuous Daily Activities. The 3-level classification system and factor structure were further explored via comparison across the 4 most commonly encountered pain conditions in clinical settings (head, back, abdominal, and widespread pain). Our findings provide important new information regarding the clinical utility and validity of the FDI. This will greatly enhance the interpretability of scores for research and clinical use in a wide range of pediatric pain conditions. In particular, these findings will facilitate use of the FDI as an outcome measure in future clinical trials.