Coagulometer international sensitivity index (ISI) derivation, a rapid method using the prothrombin...

Area of Science:

• Clinical Chemistry
• Hematology
• Medical Device Technology

Background:

• The original World Health Organization (WHO) prothrombin time (PT) standardization is largely obsolete due to widespread use of PT coagulometers.
• Coagulometers often yield variable International Normalized Ratio (INR) results compared to manual methods and across different instrument types.

Purpose of the Study:

• To develop and validate a simple procedure for deriving local INR using PT coagulometers.
• To modify the PT/INR Line method for coagulometer-specific calibration, determining International Sensitivity Index (ISI) and mean normal PT (MNPT).

Main Methods:

• The PT/INR Line method, previously established using certified plasmas, was adapted for coagulometers.
• This adapted method generates a coagulometer PT/INR Line to derive local INR, ISI, and MNPT.
• Results were compared against conventional ISI calibrations performed in the same laboratories.

Main Results:

• Local calibration with human thromboplastins yielded a mean ISI of 0.93.
• The PT/INR Line method resulted in a higher mean coagulometer ISI (0.99) but produced comparable local INR values when using the derived MNPT.
• Inter-center INR variation for a certified plasma was reduced using both local ISI calibrations and the PT/INR Line method.

Conclusions:

• The PT/INR Line-ISI method, with its derived MNPT, reliably provides accurate local INR.
• This method demonstrated effectiveness across 13 reagent/coagulometer combinations in a 28-center international study.