Stability of international reference thromboplastins

Long-term stability of the WHO International Reference Preparation (IRP) of thromboplastin 67/40 and of two secondary preparations, WHO IRP 68/434 and the Dutch RP 73/528 was assessed by two criteria on the basis of the basis of data obtained in the period 1969-79. The clotting times of normal plasmas determined with these preparations over this 10 year period did not show changes which were of practical importance. Moreover, the slopes of the lines relating clotting-time ratios of 67/40 to ratios obtained with 68/434 or 73/528 did not give any firm reasons to assume a systematic change of these slopes with time. These lines were determined by linear regression analysis of individual patient ratios. Corrections were applied to the original data for modifications of the assay technique made during the past 10 years.