Development and Validation of a Food Frequency Questionnaire to Assess Potassium Intake of Patients on Hemodialysis

Abstract

To develop and to validate a food frequency questionnaire (FFQ) to assess potassium intake of patients on hemodialysis. This is a cross-sectional study that included 41 patients in the FFQ development step and 53 patients in the FFQ validation step. In the FFQ development step, the patients completed a 3-day food record (used as a reference method). Total potassium intake and potassium from each food item were calculated. Food items that contributed up to 90% of the total potassium intake were considered to be included in the FFQ food list. Then the FFQ was applied in person through the google-forms platform. The result of potassium intake obtained by the FFQ was compared with that obtained by the 3-day food record. A total of 94 patients were included: 53.2% women; age 55.7±15.0years; 47.9% had diabetes; body mass index 25.0±4.5kg/m2; and dialysis vintage 34.5 (17.0-68.2) months. Demographic, clinical, and laboratory parameters did not differ between the development and validation groups. From the 255 registered food items, 85 comprised the FFQ. The standardized Cronbach's alpha obtained was 0.71. No difference was found between the potassium intake obtained by the FFQ and by the 3-day food record (1,438.5±659.4mg/d vs. 1,464.8±529.4mg/d; P=.753, respectively) with an intraclass correlation coefficient of 0.66 (P=.001). No systematic bias or proportionality bias between the methods was observed in the Bland-Altman graphical analysis. The quantitative FFQ presented strong relative validity and may constitute a practical tool in the analysis of potassium intake of patients on hemodialysis.