Explaining and reducing the variation in inter-laboratory reported values for International Normalised Ratio.

INTRODUCTION: Monitoring of vitamin K antagonist (VKA) therapy is usually achieved using the International Normalised Ratio (INR). However, despite international standardisation, there remains considerable concern regarding ongoing high levels of inter-laboratory variation, as generated by different laboratories using the same homogeneous plasma sample. Notably, significant discrepancies continue to be evidenced in external quality assessment (EQA) environments, prompting additional investigations to determine causes and to identify potential inconsistencies of practice. MATERIALS AND METHODS: Several investigations involving all 580 participants in the Haemostasis program of the RCPAQAP Haematology were undertaken from 2009 to 2016, gathering details of methodology, and comparative assessments of INR values differentially obtained directly from participants versus values calculated using raw data for PT, ISI and MNPT provided by the same participants. RESULTS: Up to 6% of laboratories reported substantially different INR results compared to results calculated using differentially provided ISI, MNPT and PT data in 6 out of 8 surveys in 2009, highlighting discrepancies in ISI and MNPT values reported vs used by laboratories. Subsequent highlighting of issues to laboratories led to significant improvements in later surveys, with <1% of laboratories yielding different values in 2012, 2013 and 2016. CONCLUSIONS: Our study identified that pre- or post- analytical errors explained a large proportion of inter-laboratory variation in INR. These errors can lead to serious clinical consequences if such data discrepancies are applied to patients, with incorrectly reported INRs potentially leading to altered warfarin therapy. Further education in the importance of the INR process appears warranted.