Anticoagulation effect of warfarin may be more stable in patients taking vitamin K

Kim KH. Thromb Haemost. 2010;doi:10.1160/TH10-04-0257.

Patients taking greater amounts of dietary vitamin K were more likely to experience long-term anticoagulation effects of warfarin than patients taking moderate amounts of vitamin K, according to study results.

Sixty-six patients who had been taking warfarin for 1 year or longer completed a 3-day food diary. The researchers used these entries to estimate the average daily vitamin K intake for the study population.

Patients were stratified into three groups of equal number based on vitamin K intake. The researchers compared the stability of anticoagulant effects between the three groups using the coefficients of variation for the prothrombin time expressed in INR and for warfarin doses.

The median intake of vitamin K overall was 161.3 mcg/day (31.3-616.6 mcg/day). The high vitamin K intake group was defined as patients taking at least 195.7 mcg/day and the low-intake group was defined as patients taking less than 126.5 mcg/day of vitamin K.

The coefficient of variations of the prothrombin time expressed in INR and for warfarin doses in relation to dietary vitamin K intake were negatively and independently correlated (For INR: r=–0.293, P=.017; for warfarin: r=–0.350, P=.004).

Analysis of variance results indicated that the coefficient of variation of the prothrombin time expressed in INR was different among the three intake groups (P<.05). Differences between the three groups also were observed for the coefficient of variation for warfarin doses (P<.05 in Jonckheere-Terpstra test).

Patients in the high-intake group had a lower coefficient of variation of the prothrombin time expressed in INR than patients in the low-intake group, 19.2 ± 8.96% vs. 25.5 ± 8.61% (P<.05). However, Mann-Whitney test results indicated only marginal differences between the high- and low-intake groups (P=.046).

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