oa Wits Journal of Clinical Medicine - A comparison between TTR and FIR as a measure of the quality of anticoagulation in patients with atrial fibrillation

Volume 1 Number 1
  • ISSN : 2618-0189
  • E-ISSN: 2618-0197



Background: Atrial fibrillation (AF) is a growing concern worldwide. In order to prevent AF-related adverse vascular events, adequate oral anticoagulation with warfarin is essential. The Rosendaal method has long been used to calculate time in therapeutic range (TTR) in clinical trials to assess the quality of anticoagulation but now suffers dwindling popularity due to its tedious method of calculation and inability to account for the duration spent in an out-of-range international normalized ratio (INR). Frequency in range (FIR) is being reassessed as to its value as it is easier to calculate.

We aimed to compare FIR and TTR (using the Rosendaal method) as a measure to assess the quality of anticoagulation with warfarin in a cohort of 102 consecutive patients with valvular and non-valvular AF at a tertiary South African hospital. Secondary objectives were to assess the predictive ability of FIR to categorize patients with a TTR ≥ 65% as well as to compare the CHA2DS2VASc and HASBLED scores with TTRs and FIRs.

Methods: We retrospectively analysed the INR values for all patients over a 2-year period and calculated both individual and overall mean TTR and FIR and assessed the agreement between these parameters.

Results: The mean overall TTR was 58.1% ± 16% and the mean FIR was 50.8% ± 16.7%. The mean TTR was significantly higher than the mean FIR (p < 0.0001). At the individual level, FIR was positively correlated with TTR in a linear fashion (r = 0.93, p < 0.001). However, the Bland–Altman method plot indicated lack of agreement between TTR and FIR, with a bias of 7.4% (95% CI: 6.1%–8.6%) and limits of agreement −4.6% to 19.3%, standard deviation (SD) = 6.1%. A cut-off value of FIR ≥ 53.3% was found to be a good predictor of TTR ≥65%.

Our study shows that although TTR and FIR are highly correlated with the individual INR levels, they are not equal. The two methods cannot be used interchangeably to assess warfarin control, and TTR should probably remain the gold standard.

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