Warfarin induction at 5 mg daily is safe with a low risk of anticoagulant overdose: results of an audit of patients with deep vein thrombosis commencing warfarin.

Background:  Conventionally warfarin therapy is initiated using a loading dose given over several days. Daily international normalised ratio (INR) monitoring is recommended to prevent overdose; however, with a large proportion of patients with deep vein thrombosis now receiving treatment out of hospital daily blood tests are inconvenient. We introduced a low-dose protocol for starting anticoagulant therapy that only required INR testing on days 4 and 6 and audited the results to assess safety and efficacy. Methods: Two-hundred and forty-eight patients with confirmed deep vein thrombosis were started on warfarin therapy at 5 mg daily for 3 days. INR measurements were taken at day 4 and day 6. Results:  Of these patients, 21% had an INR within the therapeutic range on day 4 and 52% had a therapeutic INR on day 6. The risk of overdose was small with only one case with an INR above 4.0 on day 4 and nine cases on day 6. There were no reported cases of bleeding. Conclusion: The low-dose protocol with infrequent testing is safe and convenient for outpatient management. However, our results suggest that patients on this protocol take between 6–10 days to achieve a stable INR. (Intern Med J 2005; 35: 717–720)