Long-term stability of international standards for thromboplastin stored at -20°C, -70°C, and -150°C.

Background: Long-term stability is an essential requirement for all international biological standards. The main stocks of the current international standards for thromboplastin, i.e. RBT/05 (rabbit brain thromboplastin) and rTF/09 (recombinant human tissue factor), are stored at -20°C. The aim of the present study is to assess the long-term stability of the international sensitivity index (ISI) for RBT/05 and rTF/09.

Methods: Part of the main stocks of RBT/05 and rTF/09 were stored at -70°C and -150°C, up to 38months. At various time points samples were taken from the materials stored at -20°C, -70°C, and -150°C. The samples were reconstituted and analysed in the prothrombin time (PT) test using plasma samples derived from healthy subjects and patients treated with vitamin K-antagonists (VKA). The PT's obtained with the standards stored at -20°C were compared to the PT's obtained with the standards stored at -70°C and at -150°C. The PT's were used to calculate relative ISI values by means of orthogonal regression.

Results: There were no important differences between the ISI values for the materials stored at -20°C, -70°C, and -150°C. There was no significant trend with storage time.

Conclusion: The ISI values for the international standards RBT/05 and rTF/09 appear to be stable at storage temperatures of -20°C, -70°C, and -150°C.