Investigating a new approach to re-starting Warfarin after Surgery

INTERVENTION: Guidelines uniformly recommend pre‐operative cessation/reversal of the anticoagulant warfarin to ensure an INR <1.5 prior to surgery. Warfarin is recommenced orally on the night of surgery at the prior known maintenance dose. The patient has often been discharged prior to achieving a stable therapeutic INR, and is commonly followed by a hospital‐based extended care service until this occurs. This requires daily point‐of‐care INR monitoring and bridging anticoagulation, usually with a low‐molecular weight heparin. This study utilises the fact that the response to a warfarin loading dose is proportional to the maintenance dose to explore the utility of a structured loading dose strategy in these patients to shorten the time to re‐achieve a stable therapeutic INR. This would decrease the use of bridging anticoagulation and shorten the length‐of‐stay under extended care services. The oral loading dose strategy uses twice the Standardised Maintenance Dose daily for the first 3 days of re‐commencement (commencing on the night of surgery) and returning to the normal maintenance dose on the 4th day, where the Standardised Maintenance Dose is 2.5 x current dose / current INR. The new loading dose strategy will be compared to current guideline recommendations in a non‐blinded, randomised study. Medication adherence will not be monitored. CONDITION: elective surgery Re‐commencing post‐operative warfarin PRIMARY OUTCOME: Time to achieve a stable therapeutic INR (range 2‐3) where a stable INR is the first INR of two consecutive daily INRs in the therapeutic range or the first INR in the therapeutic range where the previous INR was within 0.5 SECONDARY OUTCOME: episodes of overanticoagulation (INR>=4) prior to achieving a stable therapeutic INR Length of stay under Hospital at Home care Sub‐group analysis of patients maintained on daily doses (measured as the Standardised Maintenance Dose) of <2mg Sub‐group analysis of patients maintained on daily doses (measured as the Standardised Maintenance Dose) of >7.5mg Subgroups analysis of patients commenced on drugs known to interact with warfarin that are commenced during the induction period. This will be separated into two groups ‐ interacting drugs known to increase the response to warfarin (elevate the INR) and drugs known to decrease the efficacy of warfarin. All new post‐operative drug use during warfarin re‐commencement will be recorded. INCLUSION CRITERIA: > 18 years old, receiving ongoing warfarin therapy, undergoing elective surgery, able to provide written, informed consent, and followed‐up by Hospital‐at‐Home services for INR monitoring