Validation of a Semi-Quantitative Food Frequency Questionnaire Measuring Dietary Vitamin K Intake in Elderly People

Abstract

The study objective was to validate a semi-quantitative food frequency questionnaire (FFQ) specifically designed to measure dietary vitamin K intake. A 50-item FFQ was interviewer-administered and compared with data previously obtained from 5-day food records. Thirty-nine community-dwelling healthy men and women aged 65 to 85 years were recruited from the Montréal metropolitan area. Absolute and relative agreements between methods were assessed. Vitamin K intake measured by the vitamin K FFQ (mean±standard deviation; 222±186 μg/day) was significantly higher than that obtained by food records (135±153 μg/day; P<0.001). Bland-Altman analysis on log 10-transformed data indicated that vitamin K intake from vitamin K FFQ was 2.26 times (95% confidence interval: 1.90 to 2.67) higher than food records, limits of agreement ranging from 0.80 to 6.35. However, correlation between methods was strong and highly significant ( r=0.83; P<0.001). Cross-classification also showed that 72% of participants were correctly classified into thirds and only 8% were grossly miscategorized. Weighted κ value (κ=0.60) also indicated a good relative agreement. In light of these results, the vitamin K FFQ is a valid tool for ranking individuals according to their vitamin K intake. The poor absolute agreement likely results from the inability for food records to adequately measure the usual intake of episodically consumed foods, particularly those high in vitamin K. The vitamin K FFQ will be useful in large-scale, population-based research on vitamin K and disease as well as in clinical practice, especially that focusing on anticoagulant therapy.