Vitamin K intake and sensitivity to warfarin in patients consuming regular diets

The effect of dietary vitamin K intake on warfarin sensitivity is known only from case reports and few small clinical studies. We followed 50 patients commencing warfarin and consuming their regular diets (for 8 weeks) to study this relationship. A one-week recall dietary questionnaire was completed at weeks 2 and 8. Daily intake of nutrients and vitamin K was calculated from standard tables. Warfarin sensitivity index (WIS) was defined as final INR/final warfarin dose (mg/day/m(2) of body surface area) (week 8). Vitamin K intake was 17-974 (median: 179) mu g/day. Median WSI was 0.82 (0.31-4.47). A WSI value of 1.1 significantly separated excess (greater than or equal to 250 mu g/day) from normal (<250 mu g/day) vitamin K consumers (16/18 vs. 15/32, respectively, p < 0.01), The former had lower day 5 INR (median: 1.9 vs. 3.0, p < 0.001), needed more warfarin to achieve INR greater than or equal to 2.0 (32.0 +/- 9.2 mg vs. 25.4 +/- 6.4 mg, p = 0.009) and required a higher maintenance steady state warfarin dose (5.7 +/- 1.7 mg/day vs. 3.5 +/- 1.0 mg/day, p < 0.001). We conclude that in 32% (16/50) of anticoagulated patients under usual dietary conditions sensitivity to warfarin is decreased by vitamin K intake greater than or equal to 250 mu g/day.