Standardisation of ACPA tests: evaluation of a new candidate reference preparation.

INTRODUCTION: Commercial assays measuring antibodies to citrullinated protein/peptide (ACPA) show poor quantitative agreement. The diagnostic industry has never adopted the International Union of Immunological Societies-Centers for Disease Control and Prevention (IUIS-CDC) ACPA reference standard. Recently, the National Institute for Biological Standards and Control (NIBSC) prepared a new candidate ACPA standard (18/204). We evaluated both reference materials using different commercially available ACPA assays. MATERIALS AND METHODS: This is an international study in which the NIBSC candidate ACPA standard and the IUIS-CDC ACPA reference material were analysed together with 398 diagnostic samples from individuals with rheumatoid arthritis (RA) and in 1073 individuals who did not have RA using nine commercial ACPA assays. RESULTS: For both reference materials and samples from individuals with RA and individuals who did not have RA, there were large differences in quantitative ACPA results between assays. For most assays, values for the IUIS-CDC standard were lower than values for NIBSC 18/204 and the IUIS-CDC/NIBSC ratio was comparable for several, but not all assays. When NIBSC 18/204 was used as a calibrator, an improvement in alignment of ACPA results across several of the evaluated assays was obtained. Moreover, NIBSC 18/204 could align clinical interpretation for some but not all assays. CONCLUSION: Adoption of an international standard for ACPA determination is highly desirable. The candidate NIBSC 18/204 standard improved the standardisation and alignment of most ACPA assays and might therefore be recommended to be used as reference in commercial assays.

A first WHO reference reagent for the detection of anti-human platelet antigen-15b.

BACKGROUND AND OBJECTIVES: Alloantibodies to human platelet antigen-15b (anti-HPA-15b) have been detected in mothers with foetal-neonatal alloimmune thrombocytopenia and in multiply transfused patients. Assays used to detect this antibody, which aids in disease diagnosis, can be unreliable and vary in sensitivity. The objective was to generate a stable, lyophilized anti-HPA-15b preparation and evaluate its suitability as a World Health Organization (WHO) reference reagent for use in the quality control of platelet alloantibody detection assays. Results from an international collaborative study to evaluate the preparation were used to assign a minimum potency at which laboratories can be expected to detect the antibody. MATERIALS AND METHODS: Recalcified plasma containing anti-HPA-15b was aliquotted, lyophilized and coded 18/220. Twenty-five laboratories in 16 countries tested doubling dilutions of the reconstituted material in glycoprotein-specific assays such as the monoclonal antibody-specific immobilization of platelet antigen assay and reported the last positive (or endpoint) dilution. RESULTS: Twenty-four laboratories (96%) detected antibodies with HPA-15b specificity in preparation 18/220. Reported endpoint dilutions were normally distributed with a modal dilution of 1 in 16 and ranged from 1 in 2 to 1 in 128. Only two laboratories (8%) failed to detect anti-HPA-15b at 1 in 8 dilution. CONCLUSIONS: When diluted 1 in 8, most laboratories detected anti-HPA-15b in preparation 18/220 using HPA-15bb platelets but not with HPA-15aa platelets. The participants agreed this to be an appropriate dilution for assignment as the minimum potency. In October 2020, the WHO Expert Committee on Biological Standardization approved 18/220 as an International Reference Reagent.

IgA rheumatoid factor in rheumatoid arthritis.

OBJECTIVES: Rheumatoid factor (RF) is a well-established marker for the diagnosis and classification of rheumatoid arthritis (RA). Most studies evaluated IgM RF or isotype-nonspecific total RF assays. We evaluated the added value of IgA RF in this context. METHODS: An international sample cohort consisting of samples from 398 RA patients and 1073 controls was tested for IgA RF with 3 commercial assays. For all RA patients and 100 controls essential clinical and serological data for ACR/EULAR classification were available. RESULTS: The sensitivity of IgA RF for diagnosing RA was lower than the sensitivity of IgM RF. Differences in numerical values between IgA RF assays were observed. With all assays, the highest IgA RF values were found in patients with primary Sjögren's syndrome. Double positivity for IgM RF and IgA RF had a higher specificity for RA than either IgM RF or IgA RF. The sensitivity of double positivity was lower than the sensitivity of either IgA RF or IgM RF. Single positivity for IgA RF was at least as prevalent in controls than in RA patients. Adding IgA RF to IgM RF and anti-citrullinated protein antibodies (ACPA) did not affect RA classification. However, combined positivity for IgA RF, IgM RF and IgG ACPA had a higher specificity and lower sensitivity for RA classification than positivity for either of the antibodies. CONCLUSIONS: IgA RF showed a lower sensitivity than IgM RF. Combining IgA RF with IgM RF and ACPA did not improve sensitivity of RA classification. Combined positivity (IgA-RF/IgM-RF/ACPA) increased specificity.

The third international standard for anti-D immunoglobulin: international collaborative study to evaluate candidate preparations.

BACKGROUND AND OBJECTIVES: The purpose of the study was to evaluate a lyophilized anti-D immunoglobulin preparation to serve as a replacement WHO International Standard for the calibration of potency assays of anti-D immunoglobulin products. Such products are used to prevent haemolytic disease of the foetus and newborn due to maternal alloanti-D. MATERIALS AND METHODS: The candidate 3rd International Standard for anti-D immunoglobulin (16/332) was evaluated and calibrated against the 2nd International Standard for anti-D immunoglobulin (01/572), along with a coded duplicate, a second candidate preparation (16/278) and a comparability sample (16/272) in an international collaborative study. Twenty of 21 laboratories in 15 countries performed one or more of the three European Pharmacopoeia reference methods. RESULTS: The overall geometric mean potency (from all methods) of the candidate 3rd International Standard, 16/332, was 296·6 IU/ampoule, with inter-laboratory variability, expressed as % GCV, of 4·7%. SE-HPLC of the immunoglobulin preparations demonstrated combined monomeric and dimeric IgG peak areas of >95% for all samples. Accelerated stability studies have shown both 16/332 and 16/278 to be very stable for long-term storage at -20°C. CONCLUSIONS: Preparation 16/332 was established by the World Health Organisation Expert Committee on Biological Standardization as the 3rd International Standard for anti-D immunoglobulin with an assigned potency of 297 IU/ampoule.

Multicentre study to improve clinical interpretation of rheumatoid factor and anti-citrullinated protein/peptide antibodies test results.

BACKGROUND: Rheumatoid factor (RF) and anti-citrullinated protein/peptide antibodies (ACPA) are important biomarkers for diagnosis of rheumatoid arthritis (RA). However, there is poor harmonisation of RF and ACPA assays. The aim of this study was to refine RF and ACPA interpretation across commercial assays. MATERIALS AND METHODS: Six total RF isotype-non-specific assays, 3 RF IgM isotype-specific assays and 9 ACPA immunoglobulin G assays of 13 different companies were evaluated using 398 diagnostic samples from patients with RA and 1073 disease controls. RESULTS: Using cut-offs proposed by the manufacturer, there was a large variability in diagnostic sensitivity and specificity between assays. Thresholds of antibody levels were determined based on predefined specificities and used to define test result intervals. Test result interval-specific likelihood ratios (LRs) were concordant across the different RF and ACPA assays. For all assays, the LR for RA increased with increasing antibody level. Higher LRs were found for ACPA than for RF. ACPA levels associated with LRs >80 were found in a substantial fraction (>22%) of patients with RA. CONCLUSION: Defining thresholds for antibody levels and assigning test result interval-specific LRs allows alignment of clinical interpretation for all RF and ACPA assays.

Development and validation of a monocyte activation test for the control/safety testing of an OMV-based meningococcal B vaccine.

It is imperative to ensure biological products are free of contaminating pyrogenic material prior to administration to patients. Historically the rabbit pyrogen test (RPT) was used to screen for such contamination in medicines for intravenous delivery. This test was adapted for use to screen vaccines. However, some, including meningococcal vaccines containing outer membrane vesicles, are intrinsically pyrogenic. Indeed, this is the case for Bexsero which contains relatively high levels of endotoxin and other potential pyrogens such as lipoproteins and porins. The RPT proved a difficult method for measuring the pyrogenic content of Bexsero and differences between laboratories in different countries made repeat testing at the control laboratories problematic resulting in batches being wrongly identified as unsafe. At NIBSC a monocyte activation test (MAT) was adapted and validated as an alternative. This required setting of a specification in-house and deciding on a decisional procedure using multiple donors, allowing batches equally pyrogenic or less, than those batches shown to be safe in a clinical trial, to be certified as safe. The resulting format was a reference comparison method with an upper limit of 1.8 relative pyrogen units (RPU). The batch passed if an initial four donors had a response equal to or less than 1.8 RPU, if one donor is above this limit the batch was tested in a further four donors and seven of the eight must be equal to or below 1.8 RPU. If two donors have a response greater than 1.8 the batch failed.

Evaluation of the monocyte activation test for the safety testing of meningococcal B vaccine Bexsero: A collaborative study.

The aim of this collaborative study was to evaluate the robustness of the monocyte activation test (MAT) for quantifying the pyrogenic content in the outer membrane vesicle (OMV)-containing vaccine Bexsero: the first meningococcal B vaccine to be licenced. We analysed datasets from 9 laboratories covering 15 test systems for 3 batches of Bexsero with higher, equivalent and lower activity relative to a reference lot in the MAT. Activity was measured in terms of relative pyrogen units (RPU) based on European Pharmacopoeia (Ph. Eur.) MAT Chapter 2.6.30 Method C: Reference Lot Comparison Test. We report that all 15 test systems were consistent in that they showed sample A to be the most active in the MAT; that 13 of 15 test systems had an accuracy of more than 80% and an overall geometric mean RPU of 1.03 with lower and upper 95% confidence limits of 0.97 and 1.09 respectively for a sample with an expected value of 1.00 RPU. We also report larger variability in the results for test systems involving cells from individual blood donations for sample A suggesting that there could be donor to donor differences in sensitivity to the vaccine constituents responsible for the higher activity of this batch. Overall, the consistency and accuracy of the MAT was remarkable given the range of test systems used by participants, all of which are permitted by the Ph. Eur. General MAT Chapter. This is important given the limitations of the rabbit pyrogen test for the control of pyrogenicity in general and particularly with products with intrinsic pyrogenicity such as Bexsero.