title: Long-term stability of international standards for thromboplastin stored at -20°C, -70°C, and -150°C.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Thrombosis and Hemostasis, Leiden University Medical Center, Leiden, the Netherlands. Electronic address: a.m.h.p.van_den_besselaar@lumc.nl.
authors.forename: A M H P
authors.initials: AM
authors.lastname: van den Besselaar
authors.affiliation: Department of Thrombosis and Hemostasis, Leiden University Medical Center, Leiden, the Netherlands.
authors.forename: C F
authors.initials: CF
authors.lastname: Abdoel
authors.affiliation: National Institute for Biological Standards and Control, Potters Bar, Hertfordshire, United Kingdom.
authors.forename: A R
authors.initials: AR
authors.lastname: Hubbard
mesh_terms: D000818:Animals; D015925:Cryopreservation; D005615:Freezing; D006801:Humans; D019934:International Normalized Ratio / Q000592:standards; D055550:Protein Stability; D011517:Prothrombin Time / Q000592:standards*; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry; D013925:Thromboplastin / Q000737:chemistry* / Q000592:standards
chemical_list: D011994:Recombinant Proteins; D013925:Thromboplastin
keywords: International Sensitivity Index; International Standard; Stability monitoring; Thromboplastin; Vitamin K Antagonist