Detection and IgG subclass analysis of antibodies to factor VIII in multitransfused haemophiliacs and healthy individuals

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.

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.

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

Essentials Immunologic methods detect factor VIII (FVIII) antibodies in some inhibitor-negative specimens. Specimens were tested by modified Nijmegen-Bethesda assay (NBA) and fluorescence immunoassay. The NBA with preanalytical heat inactivation detects FVIII inhibitors down to 0.2 NBU. IgG4 frequency validates the established threshold for positivity of ≥ 0.5 NBU for this NBA.

SUMMARY

Background The Bethesda assay for measurement of factor VIII inhibitors called for quantification of positive inhibitors by using dilutions producing 25-75% residual activity (RA), corresponding to 0.4-2.0 Bethesda units, with the use of 'more sensitive methods' for samples with RA closer to 100% being recommended. The Nijmegen modification (Nijmegen-Bethesda assay [NBA]) changed the reagents used but not these calculations. Some specimens negative by the NBA have been shown to have FVIII antibodies detectable with sensitive immunologic methods. Objective To examine the performance at very low inhibitor titers of the Centers for Disease Control and Prevention (CDC)-modified NBA (CDC-NBA), which includes preanalytic heat inactivation to liberate bound anti-FVIII antibodies. Methods Specimens with known inhibitors were tested with the CDC-NBA. IgG4 anti-FVIII antibodies were measured by fluorescence immunoassay (FLI). Results Diluted inhibitors showed linearity below 0.4 Nijmegen-Bethesda units (NBU). With four statistical methods, the limit of detection of the CDC-NBA was determined to be 0.2 NBU. IgG4 anti-FVIII antibodies, which correlate most strongly with functional inhibitors, were present at rates above the background rate of healthy controls in specimens with titers ≥ 0.2 NBU and showed an increase in frequency from 14.3% at 0.4 NBU to 67% at the established threshold for positivity of 0.5 NBU. Conclusions The CDC-NBA can detect inhibitors down to 0.2 NBU. The FLI, which is more sensitive, demonstrates anti-FVIII IgG4 in some patients with negative (< 0.5) NBU. The sharp increase in IgG4 frequency between 0.4 and 0.5 NBU validates the established threshold for positivity of ≥ 0.5 NBU for the CDC-NBA, supporting the need for method-specific thresholds.

Comparative measurement of anti-factor VIII antibody by Bethesda assay and ELISA reveals restricted isotype profile and epitope specificity

Factor VIII (FVIII) inhibitor antibodies are produced against functional epitopes of FVIII in about 30% of severe hemophilia A patients leading to inhibition of its procoagulant activity. The Bethesda assay, the most commonly used method to measure FVIII inhibitors, based on inhibition of coagulant activity of FVIII, is neither able to detect noninhibitory antibodies nor their isotype. In this study we employed an indirect enzyme-linked immunosorbent assay (ELISA) to measure dif ferent isotypes and IgG subclasses of anti-FVIII anti body in the plasma of hemophiliacs (with and without inhibitor) and normal individuals using recombinant FVIII-coated microtiter plates. The results showed a predominance of IgG and IgG4, though IgA was slightly elevated in a few inhibitor-positive patients and IgM was hardly detectable. A highly significant correlation was found between the Bethesda titer and the optical density values of total Ig, IgG and IgG4 anti-FVIII antibodies obtained by ELISA (p<0.0001). These findings suggest a restricted specificity of anti-FVIII response in hemophiliacs towards functional epitopes of the molecule. Furthermore, high specificity and reasonable sensitivity of the ELISA, together with other technical advantages, suggest this method as a suitable supplementary technique for rapid large-scale screening of inhibitor-positive samples, though ELISA-negative samples need to be rechecked by the Bethesda assay to identify patients with a low inhibitor titer.

Characterization of inhibitors to FVIII with an ELISA in congenital and acquired haemophilia A

Different methods can be used for the detection and quantification of inhibitors or antibodies to coagulation factor VIII (FVIII). Traditionally, clotting assays have been used, in particular the Bethesda assay. These assays have, however, several shortcomings, due to the complex reaction kinetics of some inhibitors and a low sensitivity to low-titre antibodies. In addition, a universal FVIII inhibitor standard is lacking. Furthermore, clotting assays do not detect noninhibitory antibodies. Use of ELISAs has been described and FVIII from various commercially available FVIII concentrates has been used as target antigen in the assays. In the present study, we systematically explored the influence of different FVIII concentrates on the performance of an ELISA for detection of FVIII antibodies. The described ELISA was also used for further characterization of FVIII inhibitors in patients with acquired and congenital haemophilia A. We found that the source of FVIII had a substantial impact on the frequency of antibody detection. Albumin-free recombinant FVIII as target antigen gave the highest sensitivity for the assay, whereas plasma-derived concentrates containing a high level of von Willebrand factor (vWF) gave the lowest sensitivity. Presumably vWF interferes with the binding of antibodies to FVIII. We suggest that albumin-free recombinant FVIII should be used as target antigen when ELISAs are used for detection of FVIII antibodies.