Development of a self-assessment instrument to determine daily intake and variability of dietary vitamin K

OBJECTIVE: To develop and validate a brief, self-assessment instrument (K-Card) to determine daily variations in dietary vitamin K1 (phylloquinone) intake for use in patients receiving oral warfarin anticoagulant therapy. METHODS: The K-Card was designed to include a checklist of selected common foods and beverages providing > or = 5 microg vitamin K per serving in American diets and items with lower vitamin K content typically consumed in quantities which contribute significantly to total vitamin K intake. The K-Card was validated against records of weighed food intake from thirty-six healthy volunteers, 20 to 40 and 60 to 80 years of age, whose phylloquinone intakes and plasma concentrations had been previously measured by the Metabolic Research Unit, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, Boston, MA USA. Future use of the K-Card by patients was simulated by a single investigator using 108 one-day weighed food records to estimate phylloquinone intakes. Dietary phylloquinone calculated from the K-Card was compared to the values of phylloquinone intake from the diet records collected on the same days, and to fasting plasma phylloquinone concentrations obtained from the same individuals on the following day. RESULTS: The mean dietary phylloquinone intake (+/- SEM) was 138.8 +/- 15.7 microg for the K-Cards compared to 136.0 +/- 15.8 microg for the diet records (p = 0.067). Bland-Altman limits of agreement between quantities of dietary phylloquinone calculated from the K-Card and values obtained from the weighed food records were +/- 38 microg. CONCLUSION: In this simulation, the K-Card provided an accurate estimate of dietary phylloquinone intake and therefore deserves further testing for use by patients receiving coumarin-based anticoagulant therapy to determine whether variability in dietary patterns contributes to disruptions in anticoagulant drug efficacy and safety.