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

Area of Science:

• Clinical Chemistry
• Laboratory Medicine
• Haemostasis and Thrombosis

Background:

• International Normalised Ratio (INR) monitoring is standard for vitamin K antagonist (VKA) therapy.
• Significant inter-laboratory variation in INR results persists despite international standardization.
• External Quality Assessment (EQA) programs reveal discrepancies, necessitating investigation into practice inconsistencies.

Purpose of the Study:

• To investigate the causes of inter-laboratory variation in INR measurements.
• To identify potential inconsistencies in laboratory practices affecting INR accuracy.
• To assess the impact of reporting discrepancies on VKA therapy monitoring.

Main Methods:

• Analysis of data from 580 participants in the RCPAQAP Haematology Haemostasis program (2009-2016).
• Comparison of directly reported INR values with INR values calculated from provided Prothrombin Time (PT), International Sensitivity Index (ISI), and Mean Normal Prothrombin Time (MNPT) data.
• Evaluation of laboratory methodology and reported data accuracy.

Main Results:

• In 2009, up to 6% of laboratories reported substantially different INR results due to discrepancies in reported vs. used ISI and MNPT values.
• Targeted feedback to laboratories led to significant improvements.
• <1% of laboratories showed differing values in subsequent surveys (2012, 2013, 2016).

Conclusions:

• Pre- and post-analytical errors are major contributors to inter-laboratory INR variation.
• Data discrepancies can lead to clinical consequences, including altered warfarin therapy.
• Enhanced education on the INR testing process is recommended to improve accuracy and patient safety.