Functional limitations in children and adolescents suffering from chronic pain: validation and psychometric properties of the German Functional Disability Inventory (FDI-G)

Our objective was to translate the Functional Disability Inventory (FDI) into German, to evaluate its validity and to assess functional limitation in a large cohort of children and adolescents with juvenile fibromyalgia syndrome (jFMS). We administered several questions (e.g., sociodemographics, school-related issues) and questionnaires to 329 patients and one parent. The questionnaires included, among others, a German version of the FDI, the CHAQ (parent report), KIDSCREEN, tender point score (TPS), Depression Inventory for Children and Adolescents (DIKJ) and others. Patients were asked about the severity of pain today (NRS = numerical rating scale) and other symptoms. Internal consistency was evaluated with Cronbach's alpha. Construct validity of the FDI was evaluated by correlating the FDI with the questionnaires as well as with the pain and other variables, e.g., days missed school. An exploratory factor analysis (EFA) was also performed. Mean age was 13.9 years (SD ±2.48). Means were for pain today 5.37 (±2.39) and for the TPS 39.71 (±21.56). Internal consistency was α = .90. Low-to-moderate correlations were obtained between the FDI and the CHAQ (ρ = .51**), KIDSCREEN (e.g., physical well-being ρ = -.62**; peers and social support ρ = -.28**) as well as the pain variables (NRS ρ = .24**; TPS ρ = .38**). Psychological variables were also correlated with the FDI (e.g., DIJK ρ = .28**). An EFA suggested a two-factor solution. The FDI is a valid instrument for measuring functional limitations in German children and adolescents with jFMS.