The Nepean Dyspepsia Index is a valid instrument for measuring quality-of-life in functional dyspepsia

Background The Nepean Dyspepsia Index (NDI) has been in widespread use since its publication in 1999 and the addition of a short form in 2001. The NDI was one of the first disease-specific quality-of-life instruments created for functional dyspepsia (FD). However, its psychometric properties have never been validated in an independent sample. Aim This study aimed to evaluate the validity and reliability of the NDI in an a-priori driven approach in an independent population. Patients and methods In 289 individuals who fulfilled the Rome criteria for FD enrolled in a randomized placebo-controlled trial (FD treatment trial), we examined construct validity, convergent validity, and internal consistency. Results Construct validity was supported in its 25-item unweighted and weighted forms as well as the 10-item short form. All items in the 25-item form yielded considerable (>0.5) standardized loadings on their respective latent variables and all reached statistical significance (P<0.0001), supporting their relationships with the hypothesized domains. Convergent validity was strongly supported, with every domain being correlated with multiple external instruments; the majority of correlations were in the range 0.3-0.5 (in absolute values). The items comprising each domain showed good internal consistency, with the lowest value of Chronbach α at 0.80. Scores based on the short form (10-item) version of the NDI correlated strongly with the full 25-item form (tension ρ=0.88, interference ρ=0.94, eat/drink ρ=0.95, knowledge ρ=0.84 and work/study ρ=0.97; all P<0.0001). Conclusion The NDI is a valid instrument that can be used to measure the disease-specific impact of FD on quality of life.