Simplified method for international normalized ratio (INR) derivation based on the prothrombin time/INR line: an international study.
BACKGROUND
The need to perform local International Sensitivity Index (ISI) calibrations and in particular the requirement for a manual method for prothrombin time (PT) determination, have proved to be obstacles to application of the WHO scheme for PT standardization.


METHODS
We used international normalized ratio (INR) derived with a set of only 5 European Concerted Action on Anticoagulation (ECAA) lyophilized calibrant plasmas, certified manually by expert centers with reference thromboplastins, to determine a local PT/INR Line. We compared results of an independent set of validation plasmas with INRs from conventional ISI calibrations and with manually certified INRs.


RESULTS
The mean certified INR of 5 lyophilized validation plasmas was 2.41 with human thromboplastin, 2.04 with bovine/combined, and 2.80 with rabbit. With 42 human reagents, the mean observed INR of the validation plasmas was 2.68 (11.2% deviation from certified INR). Deviation was reduced to 0.4% with both local ISI calibration and the PT/INR Line. Eight results based on bovine/combined thromboplastin gave an INR deviation of 4.9%, becoming 0.5% after ISI calibration and 2.4% with the PT/INR Line. Six results with rabbit reagents deviated from certified INR by 2.5%. After ISI calibration, deviation became 1.1%, and with the PT/INR Line, 0.7%. The PT/INR Line gave similar results with both linear and orthogonal regression analysis. The total proportion of validation plasmas giving INR within 10% deviation from certified values was 42.5% with uncorrected INR, which increased to 92.1% with local ISI calibration and 93.2% with the PT/INR Line.


CONCLUSIONS
The PT/INR Line procedure with 5 ECAA calibrant plasmas successfully substitutes for local ISI calibrations in deriving reliable INRs.
Poller
Leon
L
European Action on Anticoagulation (EAA) Central Facility, Faculty of Life Sciences, University of Manchester, Oxford Road, Manchester, UK. leon.poller@manchester.ac.uk
Ibrahim
Saied
S
Keown
Michelle
M
Pattison
Albert
A
Jespersen
Jørgen
J
D000818:Animals; D002138:Calibration; D002417:Cattle; D006801:Humans; D007202:Indicators and Reagents; D019934:International Normalized Ratio / Q000379:methods*; D010949:Plasma; D011517:Prothrombin Time; D011817:Rabbits; D012044:Regression Analysis; D013925:Thromboplastin / Q000032:analysis
D007202:Indicators and Reagents; D013925:Thromboplastin