Factor VIII and Factor IX Activity Measurements for Hemophilia Diagnosis and Related Treatments

Accurate measurement of clotting factors VIII (FVIII) or IX (FIX) is vital for comprehensive diagnosis and management of patients with hemophilia A or B. The one-stage activated partial thromboplastin time (aPTT)-based clotting assay is the most commonly used method worldwide for testing FVIII or FIX activities. Alternatively, FVIII and FIX chromogenic substrate assays, which assess the activation of factor X, are available in some specialized laboratories. The choice of reagent or methodology can strongly influence the resulting activity. Variation between one-stage FVIII or FIX activities has been reported in the measurement of some standard and extended half-life factor replacement therapies and gene therapy for hemophilia B using different aPTT reagents. Discrepancy between one-stage and chromogenic reagents has been demonstrated in some patients with mild hemophilia A or B, the measurement of some standard and extended half-life factor replacement therapies, and the transgene expression of hemophilia A and B patients who have received gene therapy. Finally, the measurement of bispecific antibody therapy in patients with hemophilia A has highlighted differences between chromogenic assays. It is imperative that hemostasis laboratories evaluate how suitable their routine assays are for the accurate measurement of the various hemophilia treatment therapies.

Testing strategies used in the diagnosis of rare inherited bleeding disorders

INTRODUCTION

Rare Bleeding Disorders have a low population prevalence and may not be recognized by most clinicians. In addition, knowledge gaps of the indicated laboratory tests and their availability add to the potential for delayed diagnosis or misdiagnosis. The lack of widely available commercial, regulatory body approved esoteric tests limit them to reference laboratories, thus limiting easy access for patients.

AREAS COVERED

A literature search of Pubmed, Medline, Embase and review of international society guidelines was performed. Additional references from published articles were reviewed. A patient-centered approach to recognition and evaluation of RBD is discussed.

EXPERT OPINION

Recognition of RBD relies on obtaining a detailed patient personal and family hemostatic history. Inquiry into a history of involvement of other organ systems is important and if present should lead to suspicion of an inherited platelet disorder or a variant of Ehlers Danlos Syndrome. Multiple factors contribute to the complexity of development of efficient algorithms for diagnostic testing. Limitations in diagnostic sensitivity and specificity of screening tests, diagnostic tests, and esoteric tests further compound the complexity of establishing a diagnosis. Educational efforts focusing on clinician awareness of RBDs and available testing options are vital for optimal management of such patients.

Coagulation assay discrepancies in Japanese patients with non-severe hemophilia A

Patients with non-severe hemophilia A often show discrepancies in factor VIII (FVIII) activity. However, information on variant-specific coagulation assay characteristics in Japanese patients is limited. Pathogenic variants were classified into three groups, thrombin-cleavage site (TC), A1-A2-A3 interface (IF), and non-discrepant, with reference to previous studies. Cutoff values for the one-stage assay (OSA)/chromogenic substrate assay (CSA) ratio, which is suitable for distinguishing discrepancies, were determined for all five aPTT reagents. TGA and CWA parameters and bleeding scores were compared between groups. Two of the 39 patients with non-severe hemophilia A (5%) were classified as TC, 10 (26%) as IF, and 27 (69%) as non-discrepant. The OSA/CSA cutoff values between the groups varied widely by aPTT reagent and tended to be relatively low compared to previous studies. As an indicator of bleeding tendency, TGA had a low correlation coefficient for the IF variant, but this was not significant and was comparable to FVIII activity and CWA. Moreover, various parameters and bleeding tendency differed among patients with the same variants. Thus, our findings suggest that it is difficult to adequately assess the bleeding tendency of individual patients, even with the various assessments currently available.

Genetic analysis of non-severe hemophilia A phenotype with A discrepancy between one-stage and chromogenic factor VIII activity assays

INTRODUCTION

The diagnosis of hemophilia A (HA) is based on the measurement of factor VIII activity (VIII:C). About one-third of non-severe HA patients show a discrepancy of VIII:C measured by one-stage (VIII:C 1st) and chromogenic (VIII:C chr) assays. Different mutations in the F8 gene may cause the discrepancy in results of the FVIII activity assay. The aim of this study was to investigate F8 gene mutations in patients with assay discrepancies and to evaluate their impact on the results of VIII:C assays.

METHODS

Mutation analysis was performed on 41 individuals with a discrepancy in VIII:C 1st and FVIII: C chr assays by direct sequencing. In addition, the effect of the variants on FVIII macromolecule structure was investigated by in silico and bioinformatics tools.

RESULTS

Genetic analysis disclosed 22 different variants, of which 19 were identified for the first time to be involved in the phenotype of VIII:C discrepancy. Most of the variants related to the higher VIII:C 1st were found in A1, A2, A3 domains. The variant related to VIII:C chr > VIII:C 1st was located in the thrombin cleavage site. In silico analysis showed the effect of variants on FVIII macromolecule stability, which may be the possible mechanism causing the discrepancy.

CONCLUSION

Our data shed light on the impact of genetic defects on VIII:C assay and provided evidence that the consideration of these mutations may open a new window to the proper diagnosis and treatment monitoring of non-severe HA patients.

Impact of variation in reagent combinations for one-stage clotting assay on assay discrepancy in nonsevere haemophilia A

INTRODUCTION

Factor VIII activity (FVIII:C) is measured by one-stage clotting assay (OSA) or chromogenic substrate assay (CSA). Significant differences in FVIII:C between OSA (FVIII:C_1st ) and CSA (FVIII:C_Chr ) are described as assay discrepancy in nonsevere haemophilia A (HA). A large number of reagent combinations (APTT reagent and FVIII-deficient plasma) are used for OSA, but the impact of variations in reagent combinations on assay discrepancy has not been fully characterized.

AIM

To clarify the variations in FVIII:C_1st /FVIII:C_Chr ratios according to OSA reagent combination in HA subjects with/without assay discrepancy.

METHODS

Thirty-nine patients previously diagnosed with nonsevere HA were enrolled, and their FVIII genes were investigated and FVIII:C levels were assessed by a single CSA reagent and 11 OSA reagent combinations. Receiver operating characteristic (ROC) curve analysis was used to predict possible cut-off values of the FVIII:C_1st /FVIII:C_Chr ratio to define FVIII assay discrepancy for each reagent combination.

RESULTS

Patients were categorized into nondiscrepant (n = 25), discrepant (n = 5) and unclassified (n = 9) groups according to their genotypes and information in the database. The FVIII:C_1st /FVIII:C_Chr ratio in nondiscrepant HA varied widely, depending on the APTT reagents and FVIII-deficient plasma used. The ratio in discrepant HA patients differed with respect to their genotype and the reagent combination used. ROC curve analyses revealed that cut-off values to distinguish the assay discrepancy differed depending on the reagents used, but revealed two novel genotype variants, p.Cys573Gly and p.Gly582Arg, associated with FVIII assay discrepancy.

CONCLUSION

Our findings showed that the FVIII:C_1st /FVIII:C_Chr ratio is dependent on the reagent combination used for OSA.

Genetics and Hemostatic Potential in Persons with Mild to Moderate Hemophilia A with a Discrepancy between One-Stage and Chromogenic FVIII Assays

BACKGROUND

 Factor VIII (FVIII) activity (FVIII:C) can be measured by different methods including one-stage clotting assays (OSAs) and chromogenic assays (CSAs). Discrepancy between FVIII:C assays is known and associated with genetic variations causing mild and moderate hemophilia A (HA). We aimed to study the discrepancy phenomenon and to identify associated genetic alterations. Further, we investigated if hemostatic global assays could discriminate the group with discrepant FVIII:C from them.

METHODS

 The study contained plasma samples from 45 patients with HA (PwHA) from Hemophilia Centers in Stockholm, Sweden, and Belgrade, Serbia. We measured FVIII:C with OSA and CSA, sequenced the F8 gene, and performed two global hemostatic assays; endogenous thrombin potential and overall hemostatic potential.

RESULTS

 Nineteen of 45 PwHA had a more than twofold higher FVIII:C using OSA compared to CSA and were considered discrepant. Thirty-four causal mutations were detected, where of five had not previously been associated with assay discrepancy. These novel mutations were p.Tyr25Cys, p.Phe698Leu, p.Met699Leu, p.Ile1698Thr, and Ala2070Val. We found no difference between discrepant and nondiscrepant cases with either of the global assays.

CONCLUSION

 There was a discrepancy between FVIII:C assays in almost half of the PwHA, which for some could lead to missed HA diagnoses or misclassification of severity. Genotyping confirmed that mutations associated with FVIII:C discrepancy cluster in the A domains of F8, and five mutations not previously associated with FVIII:C discrepancy was identified. Global hemostatic assays did not contribute to distinguish assay discrepancy in PwHA.

Discrepant Hemophilia A: An Underdiagnosed Disease Entity

OBJECTIVES

The term discrepant hemophilia A (DHA) denotes the discrepancy between factor VIII activity (FVIII:C) measured by different assay methodologies in patients with nonsevere hemophilia A (HA). The objective was to review the characteristics and the current understanding of mechanisms contributing to assay discrepancy in DHA.

METHODS

Characteristics of the DHA patients treated were examined by retrospective chart review. In addition, a literature review was performed to determine the current understanding of DHA.

RESULTS

Three cases of DHA were diagnosed based on bleeding phenotype: 2 cases represented missed diagnoses of HA, and 1 represented misclassification of hemophilia severity. The revised diagnosis and classification of hemophilia directly affected clinical management. Review of the literature identified 18 articles with an estimated pooled prevalence of 36% (95% CI, 23%-56%; I2 = 85%; P < .01) among nonsevere HA. Furthermore, literature indicated that DHA is a feature of how different FVIII gene mutations affect FVIII:C activity within different assay methodologies.

CONCLUSIONS

Our experience and literature review suggested that DHA is not only a laboratory phenomenon-it can affect clinical management in a subset of patients. A high index of suspicion for DHA is necessary while evaluating bleeding patients and/or classifying nonsevere HA.

Hemophilia management: Huge impact of a tiny difference

Hemophilia A and B are inherited X-linked disorders of hemostasis, associated with an increased bleeding tendency. Patients with severe hemophilia have undetectable clotting factor levels and experience spontaneous bleeds. In patients with nonsevere hemophilia, the clotting factor levels are 2% to 40% of normal and bleeds predominantly occur after provocative events such as trauma and surgery. Despite this milder phenotype, patients with nonsevere hemophilia may suffer from considerable morbidity and have an increased mortality risk. However, many aspects of the course of disease and treatment remain unclear. Information on the factors influencing interindividual differences in bleeding phenotype is lacking, and misdiagnosis may occur due to assay discrepancies in the diagnostic workup. Desmopressin is the preferred treatment modality, but some patients and indications require treatment with clotting factor concentrates. This may elicit inhibitor formation, which is associated with an increased burden of disease and a higher mortality rate. It has been found that patients with nonsevere hemophilia A carry a lifelong risk for this serious complication. In this review, we provide an overview of the current knowledge of the diagnosis and management of nonsevere hemophilia. A report of science presented at the International Society on Thrombosis and Haemostasis 2019 Annual Congress is also provided.

Clinical utility and impact of the use of the chromogenic vs one-stage factor activity assays in haemophilia A and B

Treatment of haemophilia A/B patients comprises factor VIII (FVIII) or factor IX (FIX) concentrate replacement therapy, respectively. FVIII and FIX activity levels can be measured in clinical laboratories using one‐stage activated partial thromboplastin time (aPTT)‐based clotting or two‐stage chromogenic factor activity assays. We discuss strengths and limitations of these assays, providing examples of clinical scenarios to highlight some of the challenges associated with their current use for diagnostic and monitoring purposes. Substantial inter‐laboratory variability has been reported for one‐stage assays when measuring the activity of factor replacement products due to the wide range of currently available aPTT reagents, calibration standards, factor‐deficient plasmas, assay conditions and instruments. Chromogenic activity assays may avoid some limitations associated with one‐stage assays, but their regulatory status, perceived higher cost, and lack of laboratory expertise may influence their use. Haemophilia management guidelines recommend the differential application of one or both assays for initial diagnosis and disease severity characterisation, post‐infusion monitoring and replacement factor potency labelling. Efficient communication between clinical and laboratory staff is crucial to ensure application of the most appropriate assay to each clinical situation, correct interpretation of assay results and, ultimately, accurate diagnosis and optimal and safe treatment of haemophilia A or B patients.

Haemophilia A mutations in patients with non-severe phenotype associated with a discrepancy between one-stage and chromogenic factor VIII activity assays

About one-third of patients with non-severe haemophilia A (HA) show a discrepancy of factor (F)VIII activity (FVIII:C) measured by one-stage (FVIII:C1st), two-stage assays or the chromogenic method (FVIII:Cchr). The aim of the study was to characterise the mutation profile in patients with FVIII:C assay discrepancies. FVIII:C discrepancy was considered significant if the calculated ratio between FVIII:C1st and FVIII:Cchr was ≤ 0.6. In 16 patients FVIII:C1st was higher than FVIII:Cchr. The reverse phenomenon was observed in 83 patients. Genetic analysis revealed 23 different missense mutations of which 17 were novel. Most mutations, exhibiting a higher FVIII:C1st were localised in the A1-A2-A3 interface. The majority of mutations associated with FVIII:Cchr>FVIII:C1st discrepancy were located close to or within the thrombin cleavage sites, FIX or vWF binding sites. Our data show a correlation between FVIII:C and thrombin generation testing with a clear differentiation between patients with haemophilia and normal controls. However, in the subgroup of FVIII:C1st>FVIII:Cchr discrepancy, the endogenous thrombin potential and peak thrombin parameters were similar to non-discrepant haemophilia patients, while in the inverse discrepancy FVIII:Cchr>FVIII:C1st, these variables showed values close to that of the normal control group. Certain hereditary F8 mis-sense mutations cause discrepancy in FVIII:C as measured with different assays. This can lead to failure in diagnosing HA or incorrectly classifying the severity of the disease. Therefore, we recommend that initial diagnosis of non-severe HA phenotypes should be based on results of both FVIII:C1st and FVIII:Cchr assays.

Significance of F8 missense mutations with respect to inhibitor formation

We have identified 1,135 haemophilia A patients with missense mutations associated with mild (46%), moderate (22%), severe (16%), and mixed haemophilia phenotypes (11%). Altogether, we detected 374 different missense mutations of which 195 are not listed in the HAMSTeRS database. While missense mutations are strongly underrepresented within the factor VIII (FVIII) B-domain, they are evenly distributed throughout the entire F8 cDNA sequence. Only 36 (5%) of 720 patients with missense mutations and known inhibitor status showed an association with inhibitor formation. Inhibitor prevalence was four-fold higher for severe haemophilia compared to mild/moderate phenotypes. Mutations associated with inhibitor formation were especially clustered within the C1/C2 domain compared to the other domains (8.7% C1/C2 domain vs. 3.6% non-C1/C2-domain; p-value: 0.01). Three different missense mutations (T314A [T295A], S2010P [S1991P], R2169H [R2150H]) were associated twice with inhibitor formation. Importantly, we found that the risk of inhibitor formation in association with FVIII missense mutations is significant higher if the amino acid substitution belongs to another physicochemical class than the original residue (p-value 0.039). For this purpose distinct classes of substitutions were grouped in association with side chains properties: class I, small/hydrophobic; class II, neutral; class III, acidic; class IV, basic. Thus, although missense mutations were associated with an overall lower risk of inhibitor formation compared to other F8 gene mutation types, different missense mutations correlate with specific risks for inhibitor formation. These differences have to be identified in assigning risk profiles to aid in choice of preventative treatments designed to prevent inhibitor formation.

A critical appraisal of one-stage and chromogenic assays of factor VIII activity

Accurate and precise potency determination by manufacturers of different types of factor VIII product (plasma-derived and recombinant FVIII [rFVIII]) is vital to clinicians and patients using FVIII concentrates. A separate, but related, requirement is ascertaining the FVIII activity levels in clinical samples for diagnosing and treating hemophilia A. The one-stage clotting assay (OSA) and the chromogenic substrate assay (CSA) are the main assays used for these measurements, with both assays being used for potency assignments, and the OSA also being widely used for clinical monitoring. Although the assays can produce concordant results, discrepancies often occur, e.g. when measuring FVIII levels in patients with mild or moderate hemophilia A, or when assaying high-purity FVIII products. Modifications to rFVIII proteins, such as B-domain deletion (BDD), and technologies for improving the pharmacokinetic profile of rFVIII may exacerbate assay discrepancies. The CSA appears to be essentially unaffected by these modifications. However, the OSA underestimates the FVIII activity levels and therapeutic potential of some further modified BDD rFVIII products, especially those conjugated to poly(ethylene glycol); the extent of the effects is dependent on the specific OSA reagents used. Although the OSA remains the preferred choice for clinical monitoring in Europe and the USA, an awareness of the limitations of that assay has prompted more laboratories to adopt the CSA.

Factor VIII assay mimicking in vivo coagulation conditions

Under certain circumstances, the determination of coagulation factor VIII (FVIII) is hampered by assay discrepancies between clotting and chromogenic approaches. These are observed in certain patients' plasma as well as in certain concentrates. We intended to develop a novel assay for the quantification of coagulation FVIII which reflects the physiological situation better than the established assays. It is based on plasma without chelation of divalent cations and simultaneously minimizes the generation of activated factors which could function as uncontrolled triggers of coagulation. FVIII deficient plasma is prepared with the aid of biotinylated antibodies against FVIII from normal plasma in presence of inhibitors of contact activation. To start the assay only tiny amounts of activated FIX serve as trigger. The FVIII determination is performed in a kinetic experiment and is based on the cleavage of a fluorogenic substrate for activated FX. FVIII concentrations between 0.01 and 1 IU mL(-1) are easily determined. Plasma-derived and recombinant FVIII concentrates were compared. All plasma-derived concentrates were found to contain FVIII activities within the specification of the manufacturer. Recombinant concentrates yielded only 35-50% of the claimed potency. The novel in vivo-like assay avoids the undue advantage or disadvantage of certain product characteristics by eliminating unphysiological assay conditions. Its usefulness could turn out in future experiments with plasma from haemophilia A patients.

Prevalence, biological phenotype and genotype in moderate/mild hemophilia A with discrepancy between one-stage and chromogenic factor VIII activity

BACKGROUND

In most laboratories, the severity of hemophilia A is assessed by the factor VIII activity (FVIII:C) one-stage assay. However, comparisons of these results with those of two-stage assays can reveal discrepancies and suggest misdiagnosis.

PATIENTS/METHODS

In this monocentric study, we measured FVIII:C with two methods (one-stage chronometric and chromogenic assays) in 307 (173 families) patients with moderate/mild hemophilia A. To compare results, we used a chronometric/chromogenic ratio. Discrepancy was defined as a ratio < 0.5 or > 1.5. We studied their putative involvement at known FVIII functional sites, their interspecies conservation status, and their spatial position within the FVIII structure.

RESULTS

Thirty-six patients from 17 families exhibited a discrepancy between the two assays: 12 (6.9%) families had a low ratio (< 0.5), and five (2.9%) families had a high ratio (> 1.5). Qualitative deficiency was diagnosed in about 16% of the families. Molecular studies were performed in 15 of these 17 families, resulting in each case in the identification of missense mutations, including three novel mutations. We were further able to propose a pathophysiologic explanation.

CONCLUSIONS

In this monocentric study, we have demonstrated a discrepancy between FVIII:C assay results in 10% of families with moderate/mild hemophilia A. The prevalence of 'inverse' discrepancy (i.e. low chronometric/chromogenic ratio) is high as compared with previous reports. We suggest that both FVIII:C assays are recommended in patients with moderate/mild hemophilia A for a complete biological phenotype. This could also improve our knowledge of the FVIII structure-function relationships.

Mild haemophilia: a disease with many faces and many unexpected pitfalls

SUMMARY

Despite major advances in diagnosis and treatment, the management of patients with mild haemophilia (MH) remains a major challenge. Mild haemophilia is defined by factor levels between 0.05 and 0.40 IU mL(-1). The bleeding associated with mild haemophilia is most frequently episodic, occurring during surgery or following trauma. Spontaneous bleeding is rare. Diagnosis is sometimes delayed because of insensitivity of screening clotting assays or discrepancies in factor VIII activity as measured by different assays. The treatment of choice in mild haemophilia A is desmopressin, which typically induces a 2-6-fold increase of factor VIII over baseline. However, desmopressin has its limitations in this setting such as the occurrence of tachyphylaxis and failure to respond in an undetermined proportion of patients. Factors underlying poor biological response or magnitude of response to desmopressin are incompletely understood. Inhibitor development in mild haemophilia is particularly distressing. This complication arises at an older age in this patient group because of infrequent need for factor VIII replacement. Inhibitors in mild haemophilia patients often cross-react with endogenous factor VIII resulting in severe spontaneous bleeding frequently in a postoperative setting. Intensive perioperative use of factor VIII and some specific mutations induce a particularly high risk for inhibitor development, but risk factors are incompletely understood. For reasons of the older age of the patients, treatment of bleeding with bypassing agents may cause major thrombotic complications. Data on therapeutic options for inhibitor eradication in patients with mild haemophilia are particularly scarce. With increased life-expectancy for all haemophilia patients, the group of elderly patients with mild haemophilia requiring major surgery will further increase. Prevention of inhibitors, particularly in this patient group, should be a major topic of interest in both clinic and research.