Results of calibration by the Dutch National Reference Laboratory of the thromboplastins included in the ICTH/ICSH collaborative study of prothrombin time standardization.

Results obtained with the original calibration procedure of Biggs and Denson obtained by one expert laboratory compare well with those obtained from the ICTH/ICSH collaborative study. The simplified calibration procedure described in 1975 should only be used for the assessment of inter-batch variability of a given brand of thromboplastin; for the calibration of unlike thromboplastins, the simplified procedure should be revised by using more than two abnormal plasmas, e.g. different plasmas representing seven levels of anticoagulation between international calibrated ratios (ICRs) from 1.5 to 4.5. The formula for the calculation for the proposed ICRs based on the calibration constant should be modified to allow for instances where the calibration line for dissimilar thromboplastins, fitted in the therapeutic range, does not pass through the origin of ratio 1,1.