A radial immunodiffusion method for the assay of factor VIII:C Antigen (VIII:C Ag) in plasma

The influence of D.D.A.V.P. on the survival of factor VIII in severe haemophiliacs

The effect of D.D.A.V.P. (desamino-cys'-D-arg8-Vaso-pressin) on the survival characteristics of transfused VIII concentrate was studied in six severe, adolescent, haemophiliacs (VIII:C 1 iu/dl). D.D.A.V.P. was administered at a concentration of 0.3 ug/kg immediately following or 24 hours post infusion. No significant alteration in the half disappearance (1/2 d) or biological half-life (t 1/2) was detected for either VIII procoagulant (VIII:C) or its antigenic counterpart VIII:C Ag. As anticipated there was a significant (if sub-optimal) increase in VIII:R Ag within 2-4 hours of D.D.A.V.P. administration.

Agarose gel method: its usefulness in assaying factor VIII inhibitors, evaluating treatment and suggesting a mechanism of action for factor IX concentrates

Plasmas from 14 patients with factor VIII inhibitors, 10 haemophiliacs and four non-haemophiliacs, were assayed by both the agarose gel and Bethesda methods. Good correlation was observed in 34 samples from 13 patients, but there was poor correlation in three samples from a single haemophilic patient. The sensitivity of the method was increased by diluting normal platelet rich plasma (PRP) with congenital factor VIII deficient plasma. With this modification, as little as 0.4 of a Bethesda unit (Bu) could be measured accurately. The agarose method is easier to perform and requires much less technician time than the Bethesda assay (Ba). Inhibitory activity can be measured even in the presence of large amounts of transfused factor VIII or factor IX concentrates. To study the effect of factor IX concentrates on the inhibitor, the method was modified by incorporating into the gels plasmas specifically deficient in either factor VII, IX or X. Our data suggest that factor VII is probably responsible for the 'bypassing' activity of factor IX concentrates.

Standardization of factor VIII: establishment and use of secondary standards

Two secondary standards for use in routine assays of Factor VIII in therapeutic concentrates and in patients, plasmas, respectively, have been established in a multicenter collaborative study. In order to assess the effect of the adoption of these preparations as common Secondary Standards a comparative assay has been performed: one sample of a Factor VIII concentrate of intermediate purity and one plasma sample have been tested in two laboratories for Factor VIII:C activity using as reference, among others, the common working standard. Analysis of the results shows that with the plasma sample the differences of the estimates obtained with any of the references in our two laboratories were not statistically significant (P greater than 0.3), while with the concentrate sample the differences were always statistically significant (P less than 0.005). The study shows that the adoption of common working standards (besides the uniformity in assay method, reagents and basic equipment) is not sufficient to eliminate interlaboratory variation in the measurement of Factor VIII:C.

The stability of VIII:C in frozen stored plasma samples--re-examined

VIII procoagulant activity (VIII:C) was estimated by a two-stage assay method, on 100 pre- and post-transfusion samples obtained from severe haemophiliacs receiving regular treatment with a variety of VIII concentrate preparations. Close and significant correlation (r = 0.97 P less than 0.01) was obtained between freshly tested samples and samples stored for one month at -35 degrees C. No significant difference between the two samples was demonstrated by Student's paired-t-test. Forty-nine of the stored samples were also measured for VIII procoagulant antigen (VIII:C Ag) level. A similar close and significant correlation was found for this parameter (r = 0.9 P less than 0.01), when compared with the VIII:C of the freshly tested samples. On average the VIII:C Ag value was 10% lower than the corresponding VIII:C value, although the difference was not statistically significant.

The 'in vivo' survival characteristics of factor VIII procoagulant antigen (VIII:C Ag) in haemophilia A subjects

Initial half disappearance times and biological half-lives were determined for factor VIII procoagulant antigen (VIII:C Ag) and compared with VIII procoagulant (VIII:C) survival for each of two commercial concentrates, transfused into seven severe (less than 1 i.u./dl) non-bleeding haemophiliacs. The results show that VIII:C Ag has similar half disappearance and biological half-life to that of factor VIII:C. The relevance of these results to recently published findings is discussed.