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.