Standardizing the prothrombin time. Calibrating coagulation instruments as well as thromboplastin

Area of Science:

• Clinical Pathology
• Pharmacology
• Medical Instrumentation

Background:

• Warfarin anticoagulation indications are rising, necessitating reliable monitoring.
• Point-of-care and home prothrombin time (PT) monitoring are increasingly common.
• International Sensitivity Index (ISI) aims to standardize warfarin dosing but may not fully account for instrument variability.

Purpose of the Study:

• To systematically compare prothrombin time (PT) and international normalized ratio (INR) results between a portable clinic monitor and a hospital reference laboratory.
• To assess the clinical significance of discrepancies in PT/INR measurements from different coagulation instruments.

Main Methods:

• Two groups of 50 patients on stable warfarin doses were studied over 3 months.
• Prothrombin times (PTs) and international normalized ratios (INRs) were measured independently from single venipuncture samples.
• Regression analysis was used to compare results from the portable monitor and the hospital reference laboratory.

Main Results:

• While PT/INR results showed high correlation between instruments (r values ranging from 0.88 to 0.94), they were not identical.
• Significant discrepancies were observed, with only 82% of all sample differences falling within 1.0 INR unit.
• Regression analysis revealed systematic differences between the monitor and reference laboratory measurements.

Conclusions:

• Coagulation instrument variability can lead to clinically meaningful differences in warfarin therapy.
• Calibration of both coagulation instruments and thromboplastins is essential for standardizing warfarin dosing and ensuring patient safety.