Development of a certified reference material for anti-β2-glycoprotein I IgG - commutability studies

Toward harmonized interpretation of anticardiolipin and anti-β2-glycoprotein I antibody detection for diagnosis of antiphospholipid syndrome using defined level intervals and likelihood ratios: communication from the ISTH SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies

BACKGROUND

Improving harmonization of the clinical interpretation of anticardiolipin (aCL) and anti-β2-glycoprotein I (aβ2GPI) antibodies immunoglobulin G (IgG)/immunoglobulin M (IgM) in the diagnosis of antiphospholipid syndrome (APS) is desirable. Likelihood ratios (LRs) with corresponding test-result intervals can identify the power of a test to discriminate between a diseased and nondiseased patient and may be useful for the semiquantitative interpretation of aCL/aβ2GPI results.

OBJECTIVES

To determine moderate and high thresholds for aCL and aβ2GPI IgG/IgM measured with chemiluminescent immunoassay, enzyme-linked immunosorbent assay, fluorescence enzyme immunoassay, and multiplex flow immunoassay.

METHODS

aCL and aβ2GPI antibodies IgG/IgM were determined with 4 solid-phase systems in a case-control study population including 381 APS patients and 727 controls. Interval-specific LRs (IS-LR) were calculated for ranges determined by prespecified specificity and sensitivity levels. Three methods were used for determining thresholds that separated low, moderate, and high positive antibody levels. Interassay agreement was checked with Cohen's kappa statistics.

RESULTS

Assay- and antibody-specific thresholds demonstrated increasing IS-LR, reflecting different clinical significance for low, moderate, and high levels, especially for IgG aCL and aβ2GPI and in thrombotic APS. IS-LRs per antibody and unit range were comparable across solid-phase platforms resulting in enhanced harmonization of result interpretation. Agreement between assays for identifying high levels was improved by semiquantitative interpretation compared with that by quantitative reporting.

CONCLUSION

aCL and aβ2GPI IgG/IgM moderate and high thresholds were determined for 4 analytical platforms. Thresholds improve harmonized interpretation of aCL/aβ2GPI levels across platforms. The proposed thresholds should be verified in an independent case-control study to check interlaboratory transferability.

Reporting and Establishment of Reference Intervals for Antiphospholipid Antibody Immunoassays: A Survey of Participants in the College of American Pathologists Proficiency Testing Program

CONTEXT.—

Misdiagnosis of antiphospholipid syndrome can occur owing to the wide diversity of antiphospholipid (aPL) assays and a lack of international calibrators and harmonized reference intervals.

OBJECTIVE.—

To assess laboratory practices regarding reporting and establishing reference intervals for immunoglobulin (Ig) G/IgM anti-cardiolipin (aCL) and anti-beta-2 glycoprotein I (anti-β2GPI) assays.

DESIGN.—

Supplemental questions related to reporting and establishing reference ranges for aPL assays were sent as part of the Antiphospholipid Antibody (ACL)-B 2019 College of American Pathologists (CAP) proficiency testing survey. The response rate and methods assessment details were determined, as well as qualitative and quantitative results for 3 test samples.

RESULTS.—

The number of participants reporting results for IgG aCL (n = 489), IgM aCL (n = 476), IgG anti-β2GPI (n = 354), and IgM anti-β2GPI (n = 331) varied by antibody type. The enzyme-linked immunosorbent assay (ELISA) (up to 58.6%, 260 of 444) was the most used method; others included multiplex (from 18.9% to 23.9%), fluorescence enzyme immunoassay (14.4%-17.6%), and chemiluminescence immunoassay (6.5%-9.0%). More respondents reported quantitative than qualitative results, and manufacturer cutoff ranges were used by 92.9% and 94.2% of respondents for aCL and anti-β2GPI, respectively. Despite variation in the use of semiquantitative ranges, qualitative negative/positive reporting of the test samples achieved almost 100% consensus. Qualitative consensus was met in contrast to the wide range of quantitative results obtained for each analyte across different kits.

CONCLUSIONS.—

ELISA remains the most used method for detecting aPL antibodies, with most laboratories reporting quantitative results based on manufacturers' suggested reference ranges. The categorization of quantitative results as equivocal, weak positive, or positive for responders using kits from the same manufacturer was variable.

Solid Phase Assays for Antiphospholipid Antibodies

The diagnosis of antiphospholipid syndrome (APS) relies on the detection of circulating antiphospholipid antibodies (aPL). Currently, lupus anticoagulant (LA), anticardiolipin (aCL), and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG or IgM are the laboratory criteria if persistently present over time. As aCL and aβ2GPI are two out of the three laboratory criteria, the detection of aPL by solid phase assays is an essential step in the diagnosis of APS. Advancement has been made to resolve some of the methodological challenges of aCL and aβ2GPI assays by providing guidelines how to measure aPL, as well as to gain a better understanding of their diagnostic role. However, solid phase assays for aCL and aβ2GPI still show substantive inter-assay differences, resulting in disagreement concerning positive/negative results, but also differences in titer of antibodies. This hampers the semiquantitative classification into low-medium-high positivity. The non-criteria aPL, such as antibodies against the domain one of β2GPI and anti-phosphatidylserine/prothrombin antibodies (aPS/PT) have roles in confirming the risk in APS, and can be useful, especially in patients with incomplete antibody profiles.

Laboratory Diagnosis of Antiphospholipid Syndrome: Insights and Hindrances

Diagnosis of antiphospholipid syndrome (APS) requires the presence of a clinical criterion (thrombosis and/or pregnancy morbidity), combined with persistently circulating antiphospholipid antibodies (aPL). Currently, laboratory criteria aPL consist of lupus anticoagulant (LAC), anticardiolipin antibodies (aCL) IgG/IgM, and anti-β2 glycoprotein I antibodies (aβ2GPI) IgG/IgM. Diagnosis and risk stratification of APS are complex and efforts to standardize and optimize laboratory tests have been ongoing since the initial description of the syndrome. LAC detection is based on functional coagulation assays, while aCL and aβ2GPI are measured with immunological solid-phase assays. LAC assays are especially prone to interference by anticoagulation therapy, but strategies to circumvent this interference are promising. Alternative techniques such as thrombin generation for LAC detection and to estimate LAC pathogenicity have been suggested, but are not applicable yet in routine setting. For aCL and aβ2GPI, a lot of different assays and detection techniques such as enzyme-linked immunosorbent and chemiluminescent assays are available. Furthermore, a lack of universal calibrators or standards results in high variability between the different solid-phase assays. Other non-criteria aPL such as anti-domain I β2 glycoprotein I and antiphosphatidylserine/prothrombin antibodies have been suggested for risk stratification purposes in APS, while their added value to diagnostic criteria seems limited. In this review, we will describe laboratory assays for diagnostic and risk evaluation in APS, integrating applicable guidelines and classification criteria. Current insights and hindrances are addressed with respect to both laboratory and clinical implications.

Role of antiphospholipid antibodies in the diagnosis of antiphospholipid syndrome

The diagnosis of antiphospholipid syndrome (APS) relies on the detection of antiphospholipid antibodies (aPL). Currently, lupus anticoagulant (LA), anticardiolipin (aCL), and antibeta2-glycoprotein I antibodies (aβ2GPI) IgG or IgM are included as laboratory criteria, if persistently present. LAC measurement remains a complicated procedure with many pitfalls and interfered by anticoagulant therapy. Solid-phase assays for aCL and aβ2GPI show interassay differences. These methodological issues make the laboratory diagnosis of APS challenging. In the interpretation of aPL results, antibody profiles help in identifying patients at risk. Other aPL, such as antibodies against the domain I of beta2-glycoprotein (aDI) and antiphosphatidylserine-prothrombin (aPS/PT) antibodies have been studied in the last years and may be useful in risk stratification of APS patients. Because of the methodological shortcomings of immunological and clotting assays, these non-criteria aPL may be useful in patients with incomplete antibody profiles to confirm or exclude the increased risk profile. This manuscript will focus on the laboratory aspects, the clinical relevance of assays and interpretation of aPL results in the diagnosis of APS.

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The presence of antiphospholipid antibodies (aPL) define the diagnosis of the antiphospholipid syndrome (APS).

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Laboratory criteria are lupus anticoagulant, anticardiolipin (aCL), and antibeta2-glycoprotein I antibodies (aβ2GPI) IgG/IgM.

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Lupus anticoagulant measurement, as well as solid phase assays for aCL and aβ2GPI, show methodological challenges.

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Antibodies against domain I of β2GPI (aDI) and antiphosphatidylserine-prothrombin (aPS/PT) antibodies are non-criteria aPL.

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aDI and aPS/PT may be useful in risk stratification of APS patients.

Antiphospholipid Antibody Assays in 2021: Looking for a Predictive Value in Addition to a Diagnostic One

Antiphospholipid antibodies (aPL) are mandatory for the diagnosis but are also a risk factor for the antiphospholipid syndrome (APS) clinical manifestations. Lupus anticoagulant (LA), anticardiolipin (aCL), and anti-beta2 glycoprotein I (β₂GPI) assays are the formal laboratory classification/diagnostic criteria. Additional nonclassification assays have been suggested; among them, antiphosphatidylserine-prothrombin (aPS/PT) and antidomain 1 β₂GPI antibodies are the most promising ones although not yet formally accepted. aPL represent the example of a laboratory test that moved from dichotomous to quantitative results consistent with the idea that reporting quantitative data offers more diagnostic/prognostic information for both vascular and obstetric manifestations. Although the general rule is that the higher the aPL titer, the higher the test likelihood ratio, there is growing evidence that this is not the case for persistent low titers and obstetric events. LA displays the highest diagnostic/prognostic power, although some isolated LAs are apparently not associated with APS manifestations. Moreover, isotype characterization is also critical since IgG aPL are more diagnostic/prognostic than IgA or IgM. aPL are directed against two main autoantigens: β₂GPI and PT. However, anti-β₂GPI antibodies are more associated with the APS clinical spectrum. In addition, there is evidence that anti-β₂GPI domain 1 antibodies display a stronger diagnostic/prognostic value. This finding supports the view that antigen and even epitope characterization represents a further step for improving the assay value. The strategy to improve aPL laboratory characterization is a lesson that can be translated to other autoantibody assays in order to improve our diagnostic and prognostic power.