Establishment of the World Health Organization 2nd International Standard for Factor XI, Plasma, Human

Abstract

The 1st International Standard (IS) for blood coagulation factor XI (FXI), plasma, has been successfully used for potency labeling of FXI therapeutics and for diagnosis of FXI deficiency in patients. With stocks of the 1st IS near depletion, a replacement is required. In addition to the functional activity value, assignment of an antigen value to the 2nd IS would allow harmonization of antigen assay methods and differentiation of patients who have low functional activity but normal antigen FXI levels from patients who have both low functional and antigen FXI levels. The aims of this study were, therefore, to assign FXI functional activity to the 2nd IS for FXI, plasma, and to additionally assign a new analyte, FXI antigen, to the same International Standard. The candidate material was prepared from double-spun, virology negative, normal plasma, which was pooled and filled into siliconized glass ampoules and subsequently freeze-dried. Assignment of the functional activity (FXI:C) value in International Units (IUs) was performed by one-stage clotting assay by 29 laboratories, relative to the 1st IS. The overall geometric mean (GM) was 0.71 IU/amp with extremely low inter-laboratory variability (expressed as geometric coefficient of variation) of 1.8%. The antigen value assignment was performed by 11 laboratories and was calculated relative to normal plasma pools, as is customary with new coagulation factor analytes. The amount of antigen present in 1 ml of normal plasma was taken to be 1 U. The overall GM for the antigen assays was 0.78 IU/amp with an inter-laboratory variation of 10%. The candidate (National Institute for Biological Standards and Control code, 15/180) was established by the World Health Organization (WHO) Expert Committee on Biological Standardization in 2016 as the WHO 2nd IS for blood coagulation FXI, plasma, with a functional activity value (FXI:C) of 0.71 IU/amp and an antigen value (FXI:Ag) of 0.78 IU/amp.

An international collaborative study to establish the WHO 3rd International Standard for Thrombin: Communication from the ISTH SSC subcommittee on factor XIII and fibrinogen

“…11 In this analysis log-trans- ples, that this range is appropriate for similar functional assays. 12,13 Non-linearity was assessed by visual inspection of the plotted data, to rule out anomalously significant results due to tight replicates (and underestimation of the residual error). In other cases, invalid assays due to significant non-linearity (P < .01) were further investigated by removal of non-linear points.…”

“…The strict range of 0.90 to 1.11 allows any samples with doubtful parallelism to be excluded and not bias the final relative potency estimates obtained. Several previous (larger) studies have demonstrated, through analysis of data from coded duplicate samples, that this range is appropriate for similar functional assays 12,13 . Non‐linearity was assessed by visual inspection of the plotted data, to rule out anomalously significant results due to tight replicates (and underestimation of the residual error).…”

Establishment of the World Health Organization First International Standard for Factor XII, Plasma, Human

“…The candidate material (15/180) is the same as used for the second IS for FXI, Plasma, details of which have been published elsewhere ( 9 ). Briefly, plasma that had been prepared by the centrifugation of whole blood collected into CPD (citrate-phosphate-dextrose) adenine anticoagulant was purchased from the UK Blood Service.…”

“…This study was carried out alongside the study on the second IS for FXI, Plasma ( 9 ), and the participants’ laboratory number was assigned arbitrarily across the two studies.…”

“…The processing conditions required to make a standard for FXII are the same as those required for a FXI standard, since both are contact activation factors and exposure to glass or cold results in the activation to FXIIa and FXIa, respectively. The first IS for FXI has just been replaced by the second IS due to stock depletion ( 9 ), so this presented an ideal opportunity to use the same processed material to generate the first IS for FXII. This study presents the results of the collaborative study conducted to assign the material with values for functional activity (FXII:C) and antigen (FXII:Ag).…”