The main objective was to assess whether the time-in-therapeutic range (TTR) in patients on warfarin is affected by the international normalized ratio (INR) range. We also evaluated whether the performance of the INR is negatively impacted as its absolute value increases. First, we extracted key performance indicators of the INR from 22 surveys of the UK National External Quality Assurance Scheme (NEQAS). We then analysed TTR in 6584 warfarin monitoring episodes that were categorized by range and indication. NEQAS surveys in which the sample INR was above 3.0 had more participants outwith consensus with higher coefficient of variation and standard error mean. The outwith consensus percentage was correlated with the mean reported INR value. In warfarinised patients, we found that the TTR was lower at INR ranges above the standard 2.0 to 3.0. We concluded that the performance of the INR assay decreases at higher values. Furthermore, increasing the INR range above the standard 2.0 to 3.0 in warfarinised patients has an adverse effect on TTR. This is important because the INR range may be increased to try and improve efficacy, whereas these data suggest that it may have the opposite effect.

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