Post‐Operative Warfarin Re‐Initiation – Modelling Loading Dose Strategy

Abstract

ABSTRACTBackgroundGuidelines recommend re‐initiating post‐operative warfarin at the previous maintenance dose. This results in prolonged re‐establishment of a therapeutic international normalised ratio (INR). Modelling warfarin initiation and subsequent INR response using previously published data indicated that re‐initiation with twice the maintenance dose for 3 days may provide a rapid alternative.AimTo compare the time to a stable therapeutic INR for conventional maintenance dose re‐initiation of warfarin with a loading dose strategy.MethodWarfarin maintenance doses were adjusted by using the standardised maintenance dose – a theoretical dose resulting in an INR of 2.5. The test group was re‐initiated on warfarin doses twice their standardised maintenance dose on Days 1, 2 and 3 and on Day 4 reverted to their maintenance dose. The guideline group was re‐initiated on warfarin doses equivalent to their standardised maintenance dose on these days.ResultsThe test group (n = 17) achieved a therapeutic INR more rapidly than the guideline group (n = 23; p < 0.001) with median times to therapeutic INR of 3 and 6 days respectively. After 3 doses the test group INR on Day 4 was in the therapeutic range (2.1±0.4) and significantly higher than the guideline group (1.7±0.5; p = 0.002). No patient in the test group experienced an elevated INR (maximum INR 3.3).ConclusionINR response to the warfarin re‐initiation dose was dependent on the magnitude of the induction dose relative to the maintenance dose. The loading dose strategy achieved a stable therapeutic INR more rapidly than conventional practice. Prospective investigation in surgical patients is needed to confirm its clinical applicability.