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To cite this article: Hubbard AR, Hamill M, Beeharry M, Bevan SA, Heath AB, on behalf of the SSC SubCommittees on Factor VIII/Factor IX and von Willebrand factor of ISTH. Value assignment of the WHO 6th International Standard for blood coagulation factor VIII and von Willebrand factor in plasma (07/316). J Thromb Haemost 2011; 9: 2100-2. Harmonization in the diagnosis and monitoring of the bleeding disorders hemophilia A and von Willebrand disease (VWD) relies on the availability of the World Health Organization International Standard (WHO IS) for factor (F)VIII and von Willebrand factor (VWF) in plasma, which provides the primary definition of the International Unit (IU) for five analytes in human plasma [factor VIII:coagulant activity (FVIII:C), factor VIII:antigen (FVIII:Ag), VWF:antigen (VWF:Ag), VWF:ristocetin cofactor (VWF:RCo) and VWF:collagen binding, (VWF:CB)]. The WHO IS provides a common traceable source of calibration in IU for secondary plasma standards throughout the world and the annual demand exceeds 800 ampoules. Depletion of the stocks of the WHO 5th IS (02/150), which was established in 2003 [1], made it necessary to prepare a replacement standard and this report describes the value assignment of the WHO 6th IS. The candidate WHO 6th IS (07/316) was prepared from recovered plasma from 80 normal healthy donors, pooled and buffered with HEPES, to a final concentration of 40 mmol L)1 , before freeze drying under conditions suitable for the preparation of international biological standards [2]. Value assignment of the proposed WHO 6th IS was achieved through assays relative to the WHO 5th IS in an international multi-center study involving 44 laboratories from 14 countries which comprised 20 clinical laboratories, 21 manufacturers and three regulators. Estimates for FVIII:C, relative to the WHO 5th IS, produced similar mean values for all three methods (0.67 IU per ampoule by 1-stage clotting, n = 31; 0.72 IU per ampoule by 2-stage clotting, n = 1; 0.70 IU per ampoule by chromo-genic, n = 20) and there was good agreement between laboratories for the overall combined mean of 0.68 IU mL)1 [n = 52, geometric coefficient of variation (GCV) 4.06%] (Table 1). Estimates for FVIII:Ag, relative to the WHO 5th IS, showed good agreement between laboratories with a GCV of 4.24% and an overall mean of 1.04 IU mL)1 (n = 10). Estimates for VWF:Ag, relative to the WHO 5th IS, showed excellent agreement between the ELISA (mean 1.00 IU per ampoule, n = 21) and immuno-turbidimetric (mean 1.00 IU per ampoule, n = 10) methods and produced an overall mean of 1.00 IU mL)1 (n = 31) with low inter-laboratory variability (GCV 4.29%) (Table 1). Estimates for VWF:RCo relative to the WHO 5th IS by the automated aggregometric methods were less variable (mean 0.86 IU per ampoule, GCV 6.52%) but not significantly different to estimates by manual visual agglutination (mean 0.90 IU per ampoule, GCV 12.0%). A combination of all estimates relative to the 5th IS produced a mean of 0.87 IU mL)1 (n = 28) with acceptable inter-laborator...

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To determine the effectiveness of an antiviral to prevent herpes labialis during a brief, high-risk circumstance, 147 persons with a history of sun-induced recurrences were treated prophylactically with oral acyclovir or matching placebo and were observed during their ski holidays. Five (7%) of 75 acyclovir-treated subjects developed lesions compared with 19 (26%) of 72 persons in the placebo group.