Limit of detection and threshold for positivity of the Centers for Disease Control and Prevention assay for factor VIII inhibitors

International Council for Standardization in Haematology recommendations for laboratory measurement of factor VIII and FIX type I inhibitors

This guidance document has been prepared on behalf of the International Council for Standardisation in Hematology. The aim of the document is to provide guidance and recommendations on the measurement of factor VIII (FVIII) and factor IX (FIX) inhibitors. After an introduction on the clinical background and relevance of factor VIII and factor IX inhibitor testing, the following aspects of laboratory testing are included: screening for inhibitors, assay principle, sample requirements, testing requirements and interpretation, quality assurance, interferences and recent developments. This guidance document focusses on recommendations for a standardised procedure for the laboratory measurement of FVIII and FIX type I inhibitors. The recommendations are based on published data in peer-reviewed literature and expert opinion.

Hemophilia A subjects with an intron-22 gene inversion mutation show CD4+ T-effector responses to multiple epitopes in FVIII

Background

Almost half of severe hemophilia A (HA) is caused by an intron 22 inversion mutation (Int22Inv), which disrupts the 26-exon F8 gene. Inverted F8 mRNA exons 1-22 are transcribed, while F8B mRNA, containing F8 exons 23-26, is transcribed from a promoter within intron 22. Neither FVIII activity nor FVIII antigen (cross-reacting material, CRM) are detectable in plasma of patients with an intron-22 inversion.

Objectives

To test the hypothesis that (putative) intracellular synthesis of FVIII proteins encoded by inverted F8 and F8B mRNAs confers T-cell tolerance to almost the entire FVIII sequence, and to evaluate the immunogenicity of the region encoded by the F8 exon 22-23 junction sequence.

Patients/Methods

Peripheral blood mononuclear cells (PBMCs) from 30 severe or moderate HA subjects (17 with an Int22Inv mutation) were tested by ELISPOT assays to detect cytokine secretion in response to FVIII proteins and peptides and to map immunodominant T-cell epitopes. Potential immunogenicity of FVIII sequences encoded by the F8 exon 22-23 junction region was also tested using peptide-MHCII binding assays.

Results

Eight of the Int22Inv subjects showed robust cytokine secretion from PBMCs stimulated with FVIII proteins and/or peptides, consistent with earlier publications from the Conti-Fine group. Peptide ELISPOT assays identified immunogenic regions of FVIII. Specificity for sequences encoded within F8 mRNA exons 1-22 and F8B mRNA was confirmed by staining Int22Inv CD4⁺ T cells with peptide-loaded HLA-Class II tetramers. FVIII peptides spanning the F8 exon 22-23 junction (encoding M2124-V2125) showed limited binding to MHCII proteins and low immunogenicity, with cytokine secretion from only one Int22Inv subject.

Conclusions

PBMCs from multiple subjects with an Int22Inv mutation, with and without a current FVIII inhibitor, responded to FVIII epitopes. Furthermore, the FVIII region encoded by the exon 22-23 junction sequence was not remarkably immunoreactive and is therefore unlikely to contain an immunodominant, promiscuous CD4⁺ T-cell epitope. Our results indicate that putative intracellular expression of partial FVIII proteins does not confer T-cell tolerance to FVIII regions encoded by inverted F8 mRNA or F8B mRNA.

Optimization and evaluation of a two-stage chromogenic assay procedure for measurement of emicizumab plasma levels

Emicizumab mimics the hemostatic activity of activated factor VIII (FVIIIa) within the tenase complex. Despite functional similarities between FVIIIa and emicizumab, conventional laboratory methods designed for monitoring of FVIII activity are inappropriate for the measurement of emicizumab. At present, a modified one stage (FVIII) assay (mOSA) is mainly used for emicizumab monitoring. Two-stage chromogenic FVIII assays based on human factors can be used, although limited performance due to lack of corresponding optimization might be observed. Furthermore, the presence of FVIII or anticoagulants in the patient sample may falsify assay results. To address these issues, we optimized and evaluated a two-stage chromogenic assay (emi-tenase) for measurement of emicizumab in plasma samples. Heat inactivation of samples was established to abolish the influence of endogenous or substituted FVIII. The lower limit of quantification (LLoQ) was found to be 2 μg/ml in a manual assay format and 9.5 μg/ml on an automated coagulation analyzer. Intra- and inter-assay coefficients of variation (CV) did not exceed 20%. Analysis of 17 patient plasma samples with severe haemophilia A under emicizumab treatment showed good correlation of results between the emi-tenase assay and the mOSA (Cohens Kappa coefficient = 0.9). Taken together, the emi-tenase assay allows specific measurement of emicizumab plasma levels over a broad concentration range (10 μg/ml to 100 μg/ml). The assay can be applied on an automated coagulation analyzer, demonstrating its applicability within a routine laboratory setting.

An Update on Laboratory Diagnostics in Haemophilia A and B

Haemophilia A (HA) and B (HB) are X-linked hereditary bleeding disorders caused by lack of activity of coagulation factors VIII (FVIII) or IX (FIX), respectively. Besides conventional products, modern replacement therapies include FVIII or FIX concentrates with an extended half-life (EHL-FVIII/FIX). Two main strategies for measuring plasma FVIII or FIX activity are applied: the one-stage clotting assay (OSCA) and the chromogenic substrate assay (CSA), both calibrated against plasma (FVIII/FIX) standards. Due to the structural modifications of EHL-FVIII/FIX, reagent-dependent assay discrepancies have been described when measuring the activity of these molecules. Assay discrepancies have also been observed in FVIII/FIX gene therapy approaches. On the other hand, nonfactor replacement by the bispecific antibody emicizumab, a FVIIIa-mimicking molecule, artificially shortens activated partial thromboplastin time–based clotting times, making standard OSCAs inapplicable for analysis of samples from patients treated with this drug. In this review, we aim to give an overview on both, the currently applied and future therapies in HA and HB with or without inhibitors and corresponding test systems suitable for accompanying diagnostics.

Guidance for establishing a factor VIII testing protocol for the myriad of factor VIII products

INTRODUCTION

Management of hemophilia A has changed significantly in the past few years with the expansion of new and/or modified products as treatment options. Unfortunately, many of the standard factor VIII assays do not always accurately measure all available treatment products; therefore, the laboratory must investigate various assay algorithms to ensure the reporting of the correct results.

METHODS

Requirements for factor testing, diagnosis and severity levels, product testing, factor VIII inhibitor detection and titers, are evaluated, and potential algorithms are created for optimal assessment of patients with hemophilia A.

RESULTS

The potential for inaccurate result reporting for patients with hemophilia A or those being treated with the myriad of products has left many laboratories uncertain as to which assay algorithm to implement to ensure reporting the correct results for all products used in their hemophilia program. Algorithms for using either One-stage Clotting assays or Chromogenic assays or a combination of both types of assays are presented for each laboratory to implement based on their clinical situation.

CONCLUSIONS

Several algorithms are considered based on the needs of the clinical providers and their patients. Each laboratory must select a testing algorithm that is cost-effective and within available resources, yet that encompasses the needs of their providers and patients. Laboratory personnel must consider all assay uses (factor VIII levels, different products, interfering products, and inhibitor titers) in determining the best algorithm for their laboratory. This paper is a starting guide for developing the best factor VIII testing assays and protocols for your laboratory.

An In Silico Exploration of the Factors That Affect the Precision of the Bethesda Assay

OBJECTIVES

Despite more than 40 years of experience performing the Bethesda assay (BA), poor intra- and interlaboratory precision remains the biggest laboratory challenge to date.

METHODS

The BA procedure was modeled using stochastic simulation techniques to determine the precision of the BA up to dilutions of 1:4,096, to estimate the minimum significant relative change at various inhibitor titers, and to understand the laboratory procedural variables that could significantly affect the performance of the BA at high dilutions.

RESULTS

Selecting the lowest dilution tube with a residual activity closest to 25% for calculating the reported Bethesda titer (BT), using a factor activity assay with a coefficient of variation less than or equal to 7.5% in the range of 15% to 50% factor activity level, performing the factor activity measurement in replicates, and minimizing pipette volumetric error resulted in the lowest imprecision in the reported BT. The factor neutralization kinetics of the inhibitor appear to have little impact on the precision of the assay if the incubation time is greater than 90 minutes.

CONCLUSIONS

This in silico model will assist future laboratory efforts in standardizing the quantification of specific coagulation factor inhibitors and improving the precision of the reported results.

Evaluation of CDC's Hemophilia Surveillance Program - Universal Data Collection (1998-2011) and Community Counts (2011-2019), United States

Problem/Condition

Hemophilia is an X-linked genetic disorder that primarily affects males and results in deficiencies in blood-clotting proteins. Hemophilia A is a deficiency in factor VIII, and hemophilia B is a deficiency in factor IX. Approximately one in 5,000 males are born with hemophilia, and hemophilia A is about four times as common as hemophilia B. Both disorders are characterized by spontaneous internal bleeding and excessive bleeding after injuries or surgery. Hemophilia can lead to repeated bleeding into the joints and associated chronic joint disease, neurologic damage, damage to other organ systems, and death. Although no precise national U.S. prevalence estimates for hemophilia exist because of the difficulty identifying cases among persons who receive care from various types of health care providers, two previous state-based studies estimated hemophilia prevalence at 13.4 and 19.4 per 100,000 males. In addition, these studies showed that 67% and 82% of persons with hemophilia received care in a federally funded hemophilia treatment center (HTC), and 86% and 94% of those with the most severe cases of hemophilia (i.e., those with the lowest levels of clotting factor activity in the circulating blood) received care in a federally funded HTC. As of January 2020, the United States had 144 HTCs.

Period Covered

1998–2019.

Description of the System

Surveillance for hemophilia, which is a complex, chronic condition, is challenging because of its low prevalence, the difficulty in ascertaining cases uniformly, and the challenges in routinely characterizing and tracking associated health complications. Over time, two systems involving many stakeholders have been used to conduct ongoing hemophilia surveillance. During 1998–2011, CDC and the HTCs collaborated to establish the Universal Data Collection (UDC) surveillance system. The purposes of the UDC surveillance system were to monitor human immunodeficiency virus (HIV) and bloodborne viral hepatitis in persons with hemophilia, thereby tracking blood safety, and to track the prevalence of and trends in complications associated with hemophilia. HTC staff collected clinical data and blood specimens from UDC participants and submitted them to CDC. CDC tested specimens for viral hepatitis and HIV. In 2011, the UDC surveillance system was replaced by a new hemophilia surveillance system called Community Counts. CDC and the HTCs established Community Counts to expand laboratory testing and the collection of clinical data to better identify and track emerging health issues in persons with hemophilia.

Results

This report is the first comprehensive summary of CDC’s hemophilia surveillance program, which comprises both UDC and Community Counts. Data generated from these surveillance systems have been used in the development of public health and clinical guidelines and practices to improve the safety of U.S. blood products and either prevent hemophilia-related complications or identify complications early. Several factors have played a role in the effectiveness of the UDC and Community Counts systems, including 1) a stable data collection design that was developed and is continually reviewed in close partnership with HTC regional leaders and providers to ensure surveillance activities are focused on maximizing the scientific and clinical impact; 2) flexibility to respond to emerging health priorities through periodic updates to data collection elements and special studies; 3) high data quality for many clinical indicators and state-of-the-art laboratory testing methods for hemophilia treatment product inhibitors (developed and refined in part based on CDC research); 4) timely data and specimen collection and submission, laboratory specimen testing, analysis, and reporting; and 5) the largest and most representative sample of persons with hemophilia in the United States and one of the largest and most comprehensive data collection systems on hemophilia worldwide.

Interpretation

CDC has successfully developed, implemented, and maintained a surveillance system for hemophilia. The program can serve as an example of how to conduct surveillance for a complex chronic disease by involving stakeholders, improving and building new infrastructure, expanding data collection (e.g., new diagnostic assays), providing testing guidance, establishing a registry with specimen collection, and integrating laboratory findings in clinical practice for the individual patient.

Public Health Action

Hemophilia is associated with substantial lifelong morbidity, excess premature deaths, and extensive health care needs throughout life. Through monitoring data from Community Counts, CDC will continue to characterize the benefits and adverse events associated with existing or new hemophilia treatment products, thereby contributing to maximizing the health and longevity of persons with hemophilia.

Anti-C1 domain antibodies that accelerate factor VIII clearance contribute to antibody pathogenicity in a murine hemophilia A model

Essentials Inhibitor formation remains a challenging complication of hemophilia A care. The Bethesda assay is the primary method used for determining bleeding risk and management. Antibodies that block factor VIII binding to von Willebrand factor can increase FVIII clearance. Antibodies that increase clearance contribute to antibody pathogenicity.

SUMMARY

Background The development of neutralizing anti-factor VIII (FVIII) antibodies remains a challenging complication of modern hemophilia A care. In vitro assays are the primary method used for quantifying inhibitor titers, predicting bleeding risk, and determining bleeding management. However, other mechanisms of inhibition are not accounted for in these assays, which may result in discrepancies between the inhibitor titer and clinical bleeding symptoms. Objectives To evaluate FVIII clearance in vivo as a potential mechanism for antibody pathogenicity and to determine whether increased FVIII dosing regimens correct the associated bleeding phenotype. Methods FVIII-/- or FVIII-/- /von Willebrand factor (VWF)-/- mice were infused with anti-FVIII mAbs directed against the FVIII C1, C2 or A2 domains, followed by infusion of FVIII. Blood loss via the tail snip bleeding model, FVIII activity and FVIII antigen levels were subsequently measured. Results Pathogenic anti-C1 mAbs that compete with VWF for FVIII binding increased the clearance of FVIII-mAb complexes in FVIII-/- mice but not in FVIII-/- /VWF-/- mice. Additionally, pathogenic anti-C2 mAbs that inhibit FVIII binding to VWF increased FVIII clearance in FVIII-/- mice. Anti-C1, anti-C2 and anti-A2 mAbs that do not inhibit VWF binding did not accelerate FVIII clearance. Infusion of increased doses of FVIII in the presence of anti-C1 mAbs partially corrected blood loss in FVIII-/- mice. Conclusions A subset of antibodies that inhibit VWF binding to FVIII increase the clearance of FVIII-mAb complexes, which contributes to antibody pathogenicity. This may explain differences in the bleeding phenotype observed despite factor replacement in some patients with hemophilia A and low-titer inhibitors.

Effects of pre-analytical heat treatment in factor VIII (FVIII) inhibitor assays on FVIII antibody levels

INTRODUCTION

The use of pre-analytical heat treatment (PHT) with the Nijmegen-Bethesda assay (NBA) for inhibitors to factor VIII (FVIII) can remove/destroy infused or endogenous FVIII from patient plasma samples, allowing testing of recently infused patients with haemophilia. Two PHT methods have been described as follows: heating to 56°C for 30 minutes and heating to 58°C for 90 minutes. Data examining the effects of PHT on anti-FVIII IgG4 , the antibodies known to correlate most closely with the presence of FVIII inhibitors, are limited.

AIM

To assess the effect of PHT on the levels of detectable anti-FVIII IgG4 .

METHODS

Nijmegen-Bethesda assay-positive specimens were incubated at 56, 58 or 60°C for 90 minutes, and anti-FVIII IgG4 was measured by fluorescence immunoassay (FLI) at 30-minute intervals. The effects of PHT on the ability of recombinant FVIII (rFVIII) to inhibit detection of patient antibodies by FLI was also examined to assess the stability of rFVIII under the various PHT conditions tested.

RESULTS

Levels of anti-FVIII IgG4 showed little change following incubations at 56°C (mean 101% of original value at 30 minutes and 100% at 60 minutes) but decreased upon exposure to 58°C (mean 85% at 30 minutes and 66% at 60 minutes). In addition, heating to 56°C effectively decreased the ability of rFVIII to block antibody binding compared to unheated rFVIII.

CONCLUSION

The optimal temperature for PHT in the FVIII NBA is 56°C. Higher temperatures may lead to loss of inhibitory antibodies.

Laboratory testing for factor VIII and IX inhibitors in haemophilia: A review

Inhibitors are antibodies directed against haemophilia treatment products which interfere with their function. Factor VIII (FVIII) inhibitors in haemophilia A and factor IX (FIX) inhibitors in haemophilia B are significant clinically when they require a change in a patient's treatment regimen. Their persistence may increase morbidity and mortality. Multiple laboratory tests are now available for detecting and understanding inhibitors in haemophilia. Inhibitors are traditionally measured by their interference in clotting or chromogenic factor assays. They may also be detected using immunologic assays, such as enzyme-linked immunosorbent assay or fluorescence immunoassay. Anti-FVIII or anti-FIX antibodies of IgG4 subclass best correlate with the presence of functional inhibitors. Improvements in inhibitor measurement have been recently introduced. Preanalytical heat treatment of patient specimens allows testing of patients without delaying treatment. Use of chromogenic and immunologic assays may aid in identification of false-positive results, which are frequent among low-titre inhibitors. Validated reagent substitutions can be used to reduce assay cost. New methods for defining assay positivity and reporting low-titre inhibitors have been suggested. Challenges remain in the areas of quality control, assay standardization, monitoring of patients undergoing immune tolerance induction therapy and testing in the presence of modified and novel treatment products.