In-house calibration of the international sensitivity index or calibration curve for determination of the international normalized ratio.
BACKGROUND
The international normalized ratio (INR) has been used since 1983 to standardize prothrombin time results for patients on oral anticoagulants. However, significant interlaboratory variations have been noted. Attempts have been made to address these differences with the use of instrument-specific International Sensitivity Index (ISI) values and in-house calibration of ISI values.


OBJECTIVE
To assess the performance of laboratories using a calibration curve for INR testing.


METHODS
Attempts to improve performance of the INR include the use of instrument-specific ISI values, model-specific ISI values, in-house calibration of ISI values, and more recently, the preparation of a calibration curve. Several studies have shown an improvement in performance using these procedures. In this study of licensed laboratories performing routine coagulation testing in the Province of Ontario, Canada, the determination of the INR by a calibration curve was compared with the laboratories' usual method of assessment. These methods were subsequently analyzed by comparing the results to instrument-specific ISI, model-specific ISI, and in-house calibrators. International normalized ratios derived by both methods were analyzed for accuracy and precision. The stability of a calibration curve was also investigated.


RESULTS
Performance of INR testing has improved with use of a calibration curve or in-house calibrators.


CONCLUSIONS
The results confirm that either in-house calibrators or the calibration curve improve performance of INR testing. The calibration curve may be easier to use and appears stable up to 4 months.
University Health Network, Department of Laboratory Hematology, Toronto, Ontario.
William F
WF
Brien
Linda
L
Crawford
Anne
A
Raby
Harold
H
Richardson
D002138:Calibration; D006801:Humans; D019934:International Normalized Ratio / Q000592:standards*; D011517:Prothrombin Time / Q000592:standards