Discrepant INR values: a comparison between Manchester and Thrombotest reagents using capillary and venous samples

Area of Science:

• Clinical Pathology
• Pharmacology
• Medical Laboratory Science

Background:

• The International Sensitivity Index (ISI) calibrates thromboplastin reagents against WHO standards.
• The International Normalized Ratio (INR) aims to standardize reporting of warfarin's anticoagulant effect.
• Variations in laboratory techniques may impact INR accuracy and patient management.

Purpose of the Study:

• To evaluate the consistency of INR values across different common laboratory techniques.
• To identify potential discrepancies in warfarin effect reporting based on reagent and sample type.
• To assess the clinical implications of differing INR values on warfarin dosing.

Main Methods:

• Analysis of 128 warfarin patient samples.
• Comparison of INR values generated by four distinct methods: venous Thrombotest, capillary Thrombotest, venous Manchester reagent, and capillary Manchester reagent.
• Statistical analysis to identify significant differences between INR values.

Main Results:

• Significant discrepancies in INR values were observed across the four tested methods.
• Mean INR values from the Manchester venous reagent were consistently lower than those from the other three methods (P < 0.0001).
• This suggests a potential for under-dosing or over-dosing depending on the method used for dose adjustment.

Conclusions:

• Current laboratory techniques for determining INR values exhibit significant variability.
• The Manchester venous reagent appears to yield lower INR values compared to Thrombotest methods.
• These findings highlight a critical need for standardization to ensure accurate warfarin dosing and patient safety.