Minimum clinically important difference for the Nepean Dyspepsia Index, a validated quality of life scale for...

The Nepean Dyspepsia Index (NDI) was specifically developed for trials in functional dyspepsia, but the smallest change on the total or subscale scores, which corresponds to a clinically meaningful change (minimal clinically important difference, MCID), has not been established. An MCID has been established for the SF-36 (> or =5 points on physical or mental health subscales); such information is critical for understanding clinical trial data. We aimed at calculating an MCID for the NDI to help guide the interpretation of future clinical trials.

Comprehensive clinical data were collected in outpatients (n=101) and community subjects (n=460), and the MCID for the NDI total score was examined in three ways. The first estimated the average change (over a 2-week period) in the NDI corresponding to a five-point change in the SF-36 mental and physical subscales, and the second repeated this using a 1-s.d. change in symptom level, whereas the third calculated Cohen's d effect size among individuals changing by at least five points on the SF-36 subscales. A separate cross-sectional study was used to obtain the receiver-operator characteristic curve for discriminating between dyspepsia and non-dyspepsia subjects.

Among individuals improving by at least 1 s.d. on the patient-reported symptom score, the corresponding improvement in NDI quality of life (QoL) was an average of 18 points (s.d.=12) compared with only 7 points (s.d.=15) in those with no/minimal change in symptoms, yielding a Cohen d of 1.0 and a proposed MCID of 10 points. Although the same pattern was found using the SF-36 physical scale, the effect size was smaller (Cohen's d=0.25). Smaller effect sizes were also obtained using the SF-36 mental subscale (Cohen's d=0.1) and the physician global assessment (Cohen's d=0.33). In a separate cross-sectional community study, the NDI-QoL score was shown to provide good discrimination between individuals meeting and not meeting the Rome criteria for functional dyspepsia, with an area under the receiver-operator characteristic curve of 0.80 (95 % confidence interval: 0.75, 0.85).

A change of at least 10 points on the NDI total scale corresponds to a clinically meaningful change in patient status.