Blood-coagulation factor 8: genetics, physiological control, and bioassay

How to assess parallelism in factor assays: coefficient of variation of results with different dilutions or slope ratio?

INTRODUCTION

Non-parallelism in factor assays can lead to incorrect factor activities. Parallelism can be assessed by calculating the coefficient of variation (CV) of results obtained on 3 dilutions of the same sample. Some authors have proposed that if there is <15% then the average activity is reportable. Some analysers use a slope ratio (SR) to calculate parallelism, with an acceptance range of approximately 0.9-1.1.

METHODS

We evaluated CV and SR in one stage FII-FXII assays on Sysmex CS5100i using Innovin or Actin FS. Frozen normal and pathological plasmas, plasmas containing Direct Oral Anticoagulants, Direct Thrombin Inhibitors or Lupus Anticoagulant were analysed to assess possible non-parallelism.

RESULTS

In plasmas with factor levels >25 IU/dl (plus no interfering substances) all CVs were < 15%. One sample (low factor activities 10-15 IU/dl), had CVs > 15% in FII, FVII and FXII assays only. SR outside of 0.9-1.1 were seen in FII and FXII assays at different levels of clotting factor including some within the normal range. Non-parallelism was detected more frequently with SR than CV for those with interfering substances.

CONCLUSIONS

SR outside of 0.9-1.1 were seen in different levels of clotting factors, including samples which did not contain interfering substances. The target of 15% CV was a better discriminator than a SR for acceptance. When factor levels were reduced to around 10-15 IU/dl, a target 20 %CV was more appropriate than 15%. It might be appropriate for laboratories to assess locally whether their acceptance criteria need to be wider at low levels of clotting factors.

Accuracy of FVIII: C assay by one-stage method can be improved using hemophilic plasma as diluent

BACKGROUND

the basic prerequisite of Factor VIII clotting assay (FVIII:C) by One-Stage Method is that all other than FVIII clotting factors are present in constant concentration in each dilution of both standard reference and patient's plasma curves. On the contrary, the plasma content of each dilution is decreasing as the dilution factor increases.

OBJECTIVES AND METHODS

to keep exactly constant the plasma content in each mixture, we performed all dilutions of both standard reference and patient's plasma with FVIII deficient plasma and further with a fixed amount of buffer (method B). To show the discrepancies between this method and regular method A, using buffer to make dilutions, a comparative study was conducted on FVIII: C assay on samples at known FVIII concentration and in patients' plasma. Imidazole or Owren's buffers and five different aPTT reagents were employed, both in method A and B.

RESULTS

a discrepancy between FVIII: C assays obtained by method A and B was observed, mainly when Pathrontin SL and Imidazole buffer were used. The assays derived from method B always better fit with the expected, calculated, values of FVIII:C concentrations. Furthermore, FVIII: C was assayed in 60 patients: the outcome of method A was always higher than values of method B. The discrepancy between the two methods was higher at FVIII concentrations below 50 U/dL but null at 100 U/dL. The A slope was steeper than B slope and the difference was statistically significant starting from the 1/10 dilution. Accordingly, FVIII: C of patients' plasma obtained by method A was always higher that those obtained by method B, even 2 or 3 times for FVIII level < or = 10 U/dL or 1.4-1.6 times for FVIII levels between 10 and 25 U/dL.

CONCLUSIONS

only method B is able to give FVIII: C assays in agreement with the expected values. The dilution of reference standards and samples with FVIII deficient plasma is crucial to accurately evaluate the post-infusion FVIII concentrations in pharmacokinetic studies or the trough level during prophylactic therapy and to investigate the discrepancy among different FVIII: C assays. In addition, the assessment of severity and classification of hemophilia should be reviewed.

A multicenter pharmacokinetic study of the B-domain deleted recombinant factor VIII concentrate using different assays and standards

When the one-stage clotting assay is used in comparison with the chromogenic and immunological assays, plasma levels of factor (F)VIII are underestimated by 40-50% after infusion of B-domain deleted recombinant FVIII (BDD-rFVIII) in patients with hemophilia. A possible way to counteract the underestimation of FVIII levels by the one-stage assay is the adoption of a recombinant FVIII reference standard instead of a plasma standard. To evaluate the usefulness of such a standard [ReFacto Laboratory Standard (RLS)], the pharmacokinetic parameters of a single dose of BDD-rFVIII (25 U kg(-1)) were evaluated in a multicenter study carried out in 18 patients with severe hemophilia A. The very low in vivo recovery, obtained with the combination of the one-stage assay and plasma reference standard, was increased up to the values obtained by the chromogenic assay when the results were expressed in terms of RLS. When the plasma standard was used, the one-stage/chromogenic ratio was 0.82 +/- 0.12 for FVIII levels above 25 U dL(-1) and 1.42 +/- 0.99 for FVIII levels below 25 U dL(-1). Using the RLS, the one-stage/chromogenic ratio increased to 1.01 +/- 0.19 at FVIII levels above 25 U dL(-1), as a consequence of a complete overlap of the two decays; however, at FVIII levels below 25 U dL(-1), the one-stage/chromogenic ratio was still 1.6 +/- 0.85. After the twelfth hour, FVIII concentrations obtained by chromogenic assay were always lower than those resulting from the one-stage clotting assay, independently of the standard used. Results obtained by chromogenic assay were not affected by the type of standard used. Compared with those obtained by the one-stage assay, higher values of clearance, lower volume of distribution area and shorter plasma half-life or mean residence time were obtained by chromogenic assay because of a shape change of the decay curve due to a shift to higher values in the first part (time interval 0-12 h) and to lower values in the second part of the decay curve (time interval 12-48 h). As a consequence, the slope of the decay curve obtained by means of chromogenic assay was steeper. In conclusion, the more homogeneous results of in vivo recovery and pharmacokinetic analysis, due to the decrease of discrepancy between the two methods when RLS was used, make the cheaper and more widely used one-stage assay preferable to the more expensive chromogenic assay, on condition that the ReFacto specific standard has used.

Circulating immune complexes in haemophilia and von Willebrand's disease

Sera from 63 patients with haemophilia A, 21 with haemophilia B and 29 with von Willebrand's disease were screened for the presence of circulating immune complexes (CICs), serological markers of hepatitis A and B virus, autoantibodies and factor VIII or factor IX inhibitors. CICs were detected by the 125J Clq binding assay (ClqBA), the solid phase conglutinin assay (KgBSP) and the solid phase Clq assay (ClqSP). The incidence of CICs detected by the ClqBA and the ClqSP methods in haemophiliacs and in von Willebrand patients was higher than that observed in normal subjects, while the prevalence of CICs detected by the KgBSP method was not. The presence of CICs was not correlated with patient age, severity of disease, presence of hepatitis B virus serological markers, abnormal liver function tests or factor VIII inhibitors. A significant connection was demonstrated between CICs detected by the ClqBA method and replacement therapy when the dose administered over 1 year was over 20 000 U of factor VIII or IX concentrates. The high proportion of CICs in von Willebrand's disease, not connected with the replacement therapy or the presence of serological markers of hepatitis virus, is in agreement with the possibility that immune complexes may be related to the disease itself and independent, at least in part, of exogenous agents.

Immunological heterogeneity of haemophilia B: a multicentre study of 98 kindreds

An electroimmunoassay with a precipitating rabbit anti-human factor IX antiserum and an inhibitor neutralization assay with a non-precipitating homologous antibody were used to measure factor IX antigen (IX:Ag) in 117 patients from 98 kindreds with haemophilia B; and to investigate in a mixed population the incidence of different immunological types of the disease. Although the two assays showed an excellent correlation, the electroimmunoassay was selected for its simplicity as a criterion for classification. 52 kindreds, referred to as haemophilia B-, were characterized by severe deficiency of factor IX coagulant activity (less than 0.01--0.03 u/ml) and unmeasurable IX:Ag (less than 0.12 u/ml): this genetic variant of the disease appears to be related to a complete or marked suppression of factor IX synthesis. In 16 kindreds, a severe or moderately severe IX:C deficiency was associated with normal or increased levels of IX:Ag (haemophilia B+): among them, a subgroup of five kindreds could be identified by the additional abnormality of a prolonged Thrombotest clotting time (haemophilia BM). These patients are likely to be the expression of normal or increased synthesis of a factor IX molecule markedly defective in the site(s) responsible for coagulant activity. Reduced levels of IX:Ag (0.12--0.65 u/ml, characterized the remaining 30 kindreds, presenting with IX:C levels ranging from less than 0.01 to 0.21 mu/ml. In 28 there was a significant excess of IX:Ag over IX:C, suggesting a reduced capacity to synthesize the factor IX molecule accompanied by a variable defect in the coagulant site; the remaining two kindreds, which showed a concomitant reduction of IX:C and IX:Ag, are likely to be examples of a true reduction of factor IX synthesis.

Factor VIII coagulant activity in an African population in relation to a recognized standard

The normal range of factor VIII coagulant activity (derived from log potency ratio) in some sections of the Nigerian population has been established at 0.65--5.55 iu/ml with a geometric mean of 1.90 iu/ml. This was determined against an acceptable standard (MRC Human 68/413 with activity o.66 iu/ml). The distribution of the potency ratio was log normal. The level was not affected by age or an abnormal haemoglobin (Hb A + S or A + C). The mean activity in females was significantly higher than the mean value in male subjects. With the use of a stable standard, our results show that the conclusions of some previous studies in respect of some of the parameters such as population distribution but which did not use a recognized standard, were valid. Within the age limits of our subjects, age did not affect the population level of factor VIII coagulant activity.

Sensitivity and precision of activated partial thromboplastin time (APTT) methods. A multicenter study

The Activated Partial Thromboplastin Time (APTT) test, Cephotest, was compared to other APTT methods in current use in 4 specialized coagulation laboratories. In 3 of 4 laboratories, the sensitivity of Cephotest was superior (P less than 0.001) to that of the local APTT method. There was no statistically significant difference between the APTT methods with regard to precision of repetitive testing. In each laboratory, the normal range of Cephotest was estimated on freshly collected plasma samples from healthy subjects. A mean value between 28.8 and 35.8 s, with a standard deviation of 1.1-3.3 s, was obtained. It is concluded that the composition of the APTT method if of importance for the sensitivity of this test, but does not influence the precision of repetitive testing to a significant degree. The use of a standardized reagent facilitates comparison of the results obtained with the APTT method from one laboratory to another.

Immunoradiometric assay of factor VIII related antigen, with observations in 32 patients with von Willebrand's disease

A solid phase non-competitive immunoradiometric assay (IRMA) has been developed which allows measurement of factor VIII related antigen (VIIIR:AG) levels in normal plasma as low as 2.5 x 10(-4) U/ml. The assay is based on the extraction of VIIIR:AG from test plasma by means of polystrene tubes coated with a specific unlabelled anti-VIIIR:AG rabbit antiserum and subsequent labelling of the extracted antigen with 125I-labelled anti-VIIIR:AG rabbit IgG.

Mechanism of plasminogen activator and factor VIII increase after vasoactive drugs

Adrenaline, nicotinic acid (NA), vasopressin (LVP) and other drugs affecting vascular motility are known to increase plasminogen activator (PA) and factor-VIII plasma levels in man. To evaluate the hypothesis that NA, LVP and adrenaline release PA from the endothelial cells of the vessel wall through their common effect on vascular motility, PA has been characterized by means of a histochemical technique on vein biopsies obtained from human volunteers after infusion of the compounds. Furthermore, the effect of single and repeated administration has been compared in order to investigate whether the pattern of PA and factor-VIII variations in plasma is similar with the three drugs. There was no major difference in the PA content of the veins following the marked and sustained increase of the corresponding plasma activities. A simple explanation is that the intensity and duration of the stimulus may not be sufficient to deplete the large stores of the vessel walls. The magnitude, time course and duration of the plasmatic response after single and repeated infusions was on the whole different and peculiar for each drug. A derivative of LVP which is free of vasoactive actions was more effective than LVP in inducing the responses, which could also be elicited in two anephric subjects. These findings suggest that vasoactivity is unlikely to provide the clue to a common pathway for the fibrinolysis and coagulation response after the compounds, and support the existence of different specific receptors.