• This study evaluated the validity of the dialysis diet and fluid non-adherence questionnaire (DDFQ) as a self-report instrument for clinical practice. The DDFQ was designed to measure non-adherence behaviour with diet and fluid guidelines in patients treated with hospital-based haemodialysis in Flanders (Belgium).
• In a multicentre cross-sectional study design, 564 patients from 10 dialysis centres in Flanders completed the DDFQ. Criterion and construct validity of the instrument were substantiated using correlation techniques between the DDFQ and biochemical and biological ratings of non-adherence.
• The results of this study suggest that the DDFQ is a valid self-report instrument to assess non-adherence behaviour in haemodialysis patients in Flanders and probably also in other cultures.