Accounting for "loss to follow-up": We have not previously had patients lost to follow-up in our vitamin K studies, and we anticipate the rate in this study will be negligible (i.e. less than 5%). The low rate of loss to follow-up is due, in large part, to the fact that (a) patients will have only 2 clinic visits over the course of the study (day 1 and 14) and (b) all other follow-up will be performed over the telephone, Further, patients receiving warfarin are aware of the need for monitoring of their warfarin and thus accept the need for periodic medical and laboratory re-assessments. This increases the likelihood that they will be willing and able to participate in study follow-up.