Introduction: Warfarin reduces stroke risk in atrial fibrillation (AF), but increases bleed risk. Frequent testing with dose adjustment is needed to maintain INR levels in the therapeutic range of 2.0-3.0. Novel anticoagulants (NOACs) now challenge warfarin as stroke-preventive therapy for AF. They are available at fixed doses but costlier. Warfarin anticoagulation at a time in therapeutic range (TTR) ≥70% is similarly effective and safe as NOACs. It is unclear whether AF patients with TTR ≥70% will remain stably anticoagulated and avoid the need to switch to a NOAC. We assessed stability of warfarin anticoagulation in AF patients with an initial TTR ≥70% primarily managed by anticoagulation clinics.
Hypothesis: AF patients who achieve TTR ≥70% in the first 6 months of warfarin therapy will maintain high TTR subsequently.
Methods: Within the community-based ATRIA cohort of AF patients, we identified 2521 new warfarin users who continued warfarin therapy over 15 months. We excluded months 1-3 to achieve stable dose. For patients with TTR1 (months 4-9) ≥70% (TTR by Rosendaal method), we describe the distribution of TTR2 (months 10-15) and assess multivariable (logistic regression) correlates of persistent TTR ≥70%.
Results: Of 1074 patients with TTR1 ≥70%, 57% (95%CI: 53-61%) persisted with TTR2 ≥70% (Figure). Of multiple patient features, only initial TTR1 ≥90%, aOR [95% CI]: 1.43 [1.05,1.93]); and heart failure aOR: 0.78 [0.57, 1.06]) independently predicted TTR2 ≥70%.
Conclusions: Nearly 60% of AF patients with high quality initial 6-month TTR on warfarin will maintain TTR ≥70% over the next 6 months. A minority deteriorate to very poor TTR. Patient features do not strongly predict deterioration. Our analyses support watchful waiting for AF patients with initial high quality warfarin anticoagulation before switching to a NOAC.