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

Objective: To develop and validate a brief, self-assessment instrument (K-C ard) to determine daily variations in dietary vitamin K-1 (phylloquinone) i ntake 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 greater than or equal to 5 mug 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 in take. 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 ph ylloquinone intakes and plasma concentrations had been previously measured by the Metabolic Research Unit, Jean Mayer USDA Human Nutrition Research Ce nter on Aging, Tufts University, Boston, MA USA. Future use of the K-Card b y patients was simulated by a single investigator using 108 one-day weighed food records to estimate phylloquinone intakes. Dietary phylloquinone calc ulated from the K-Card was compared to the values of phylloquinone intake f rom the diet records collected on the same days, and to fasting plasma phyl loquinone concentrations obtained from the same individual on the following day. Results: The mean dietary phylloquinone intake (+/- SEM) was 138.8 +/- 15.7 mug for the K-Cards compared to 136.0 +/- 15.8 mug for the diet records (p = 0.067). Bland-Altman limits of agreement between quantities of dietary p hylloquinone calculated from the K-Card and values obtained from the weighe d food records were +/- 38 mug. Conclusion: In this simulation, the K-Card provided an accurate estimate of dietary phylloquinone intake and therefore deserves further testing for us e by patients receiving coumarin-based anticoagulant therapy to determine w hether variability in dietary patterns contributes to disruptions in antico agulant drug efficacy and safety.