Background Reduced time in international normalized ratio therapeutic range (TTR) limits warfarin safety and effectiveness. In patients switched from warfarin to direct oral anticoagulants (DOACs), patient factors associated with low TTR could also increase risk of DOAC nonadherence. We investigated the relationship between warfarin TTR and DOAC adherence in warfarin-treated patients with atrial fibrillation switched to DOAC. Methods and Results Using data from the Veterans Health Administration, we identified patients with atrial fibrillation switched from warfarin to DOAC (switchers) or treated with warfarin alone (non-switchers). Logistic regression was used to evaluate association between warfarin TTR and DOAC adherence. We analyzed 128 605 patients (age, 71±9; 1.6% women; CHA₂DS₂-VASc 3.5±1.6); 32 377 switchers and 96 228 non-switchers. In 8016 switchers with international normalized ratio data to calculate 180-day TTR before switch, TTR was low (median 0.45; IQR, 0.26-0.64). Patients with TTR <0.5 were more likely to be switched to DOAC (odds ratio [OR],1.68 [95% CI,1.62-1.74], P<0.0001), as were those with TTR <0.6 or TTR <0.7. Proportion of days covered ≥0.8 was achieved by 76% of switchers at 365 days. In low-TTR individuals, proportion of days covered ≥0.8 was achieved by 70%, 72%, and 73% of switchers with TTR <0.5, 0.6, and 0.7, respectively. After multivariable adjustment, TTR <0.5 decreased odds of achieving 365-day proportion of days covered ≥0.8 (OR, 0.49; 0.43-0.57, P<0.0001), with similar relationships for TTR <0.6 and TTR <0.7. In non-switchers with TTR <0.5, long-term TTR remained low. Conclusions In patients with atrial fibrillation switched from warfarin to DOAC, most achieved adequate DOAC adherence despite low pre-switch TTRs. However, TTR trajectories remained low in non-switchers. Patients with low warfarin TTR more consistently achieved treatment targets after switching to DOACs, although adherence-oriented interventions may be beneficial.

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