Variability among commercial batches of normal pooled plasma in lupus anticoagulant testing

INTRODUCTION

Lupus anticoagulant (LA) testing requires normal pooled plasma (NPP) in performing mixing studies and can be used for normalized ratios of clotting times (CTs). The aims were to demonstrate whether significant differences in clotting times between two batches of a same commercial NPP (CRYOcheck™) directly affect NPP-based cut-off values.

METHODS

Diluted Russell Viper venom time (DRVVT) and activated partial thromboplastin time (aPTT) were used for LA testing. Screening, mixing and confirm tests were performed with Stago® instruments and reagents. Two batches of commercial NPP (A1291 and A1301 from CRYOcheck™; frozen) were compared in the determination of cut-off values. Cut-off values were defined as 99th percentile values of 60 healthy donors and compared with Mann-Whitney U test.

RESULTS

Cut-off values obtained with the two NPP batches were significantly different for DRVVT (screen normalized ratio: 1.09 vs. 1.24, screen mix: 41.9 s vs. 38.9 s; index of circulating anticoagulant: 5.0 vs. 8.4; all had p-value <.001). On the contrary, no significant differences were observed for aPTT (screen normalized ratio: 1.32 vs. 1.34; p-value = .4068, screen mix: 37.8 s vs. 38.1 s; p-value = .1153) except for index of circulating anticoagulant: 9.6 versus 10.4 (p-value <.05).

CONCLUSION

This study demonstrates that differences between two commercial NPP batches produced by a same manufacturer influenced LA cut-off values used for mixing studies and normalized ratios. Adequate cut-off setting, taking into account NPP CTs, is important to provide accurate conclusion about the presence or absence of a LA and avoid potential clinical impact.

Testing for Lupus Anticoagulants

Lupus anticoagulant (LA) is one of the three criteria antiphospholipid antibodies (aPLs) employed in classification, and by default diagnosis, of antiphospholipid syndrome (APS). Detection of LA is not via calibrated assays but is based on functional behavior of the antibodies in a medley of coagulation assays. A prolonged clotting time in a screening test is followed by demonstration of phospholipid dependence and inhibitory properties in confirmatory and mixing tests, respectively, which are modifications of the parent screening test. Complications arise because no single screening test is sensitive to every LA, and no test is specific for LA, because they are prone to interference by other causes of elevated clotting times. Several screening tests are available but the pairing of dilute Russell's viper venom time (dRVVT) with LA-sensitive activated partial thromboplastin time (aPTT) is widely used and recommended because it is proven to have good detection rates. Nonetheless, judicious use of other assays can improve diagnostic performance, such as dilute prothrombin time to find LA unreactive with dRVVT and aPTT, and the recently validated Taipan snake venom time with ecarin time confirmatory test that are unaffected by vitamin K antagonist and direct factor Xa inhibitor anticoagulation. Expert body guidelines and their updates have improved harmonization of laboratory practices, although some issues continue to attract debate, such as the place of mixing tests in the medley hierarchy, and areas of data manipulation such as assay cut-offs and ratio generation. This article reviews current practices and challenges in the laboratory detection of LA.

Mixing studies for lupus anticoagulant: mostly no, sometimes yes

Mixing tests have long been a mainstay in the lupus anticoagulant (LA) testing armoury of screen, mix and confirm assays. If a sample with an elevated screening test does not evidence inhibition in the mixing test, the search for an LA is halted and a different diagnostic pathway embarked upon. Recent years have seen studies evidencing sometimes high frequencies of false-negative mixing tests with perhaps sinister implications for missed diagnoses and skewed patient management. Issues such as the dilution effect, between-reagent sensitivity and specificity differences, variability of normal pooled plasma (NPP) quality and suitability and interpretive inconsistencies all contribute to questioning the reliability of mixing tests and their pivotal place in the LA assay hierarchy. The advent of integrated testing, where phospholipid-dependence is demonstrated or excluded prior to any attempt to evidence inhibitory properties with a fallible analytical principle, provides an alternative path to LA detection. In the absence of other causes of elevated clotting times, LA assay screen and confirm discordance is sufficient to secure a laboratory diagnosis of the presence of an LA, leaving the mixing test in a supplementary yet valuable role when further diagnostic discrimination is required.

Identification of high thrombotic risk triple-positive antiphospholipid syndrome patients is dependent on anti-cardiolipin and anti-β2glycoprotein I antibody detection assays

Essentials Triple-positivity is associated with a high risk for a first thrombotic event and recurrence. Identification of triple-positives is dependent on the solid phase assay used. In triple-positivity, IgM only adds value in thrombotic risk stratification together with IgG. Thrombotic risk in triple-positive patients with IgM only, depends on the platform.

ABSTRACT

Background The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of antiphospholipid antibodies (aPL). Triple-positivity (i.e. positivity for lupus anticoagulant [LAC], anti-cardiolipin [aCL] and anti-β2glycoprotein I [aβ2GPI] antibodies) is associated with a high thrombotic risk. Objectives We investigated the variability in triple-positivity detection by measuring the same samples with four commercially available solid phase assays. In addition, the added clinical value of aPL in LAC-positive patients was investigated, as well as the association of IgM triple-positivity and thrombosis. Patients/Methods We included 851 patients from seven European medical centers. Anti-CL and aβ2GPI IgG/IgM antibodies were determined by four platforms: BioPlex® 2200, ImmunoCap® EliA, ACL AcuStar® and QUANTA Lite ELISA® . Results Triple-positivity detection by solid phase assays varied, ranging from 89 up to 118 in thrombotic APS patients (n = 258), of which 86 were detected independent of the platform. Lupus anticoagulant positivity resulted in an odds ratio (OR) for thrombosis of 3.4; triple-positivity (irrespective of the isotype) increased the OR from 4.3 up to 5.2, dependent on the platform. Triple-positivity solely for the IgM isotype did not increase the OR for thrombosis compared with LAC positivity. The highest OR for thrombosis was reached for positivity for IgG and IgM aβ2GPI and aCL (8.6 up to 28.9). Conclusions Triple-positivity proved to be highly associated with thrombosis, but identification is assay dependent. Within triple-positivity, IgM antibodies only have an added clinical value in patients positive for IgG antibodies.

Newly developed dilute Russell's viper venom reagents for lupus anticoagulant detection with improved specificity

Background Dilute Russell's viper venom time (dRVVT) is indispensible in lupus anticoagulant (LA) detection yet commercial reagents from different suppliers perform variably, no gold standard assays exist and therapeutic anticoagulation interference is problematic. Objective The objective of this study was to compare a new formulation dRVVT with two currently available dRVVTs. Materials and methods Life Diagnostics (LD) dRVVT and Stago PTT-LA were routinely used for lupus anticoagulant detection, plus Taipan snake venom time/ecarin time (TSVT/ET) for patients on warfarin or rivaroxaban. Siemens dRVVT and the new HYPHEN BioMed (HBM) dRVVT were tested with 193 patient samples. Group 1, 59 non-anticoagulated patients (NAPs) LA-positive in LD dRVVT; Group 2, 15 PTT-LA-positive/dRVVT-negative NAPs; Group 3, 24 LA-positive warfarinized patients; Group 4, 13 patients on rivaroxaban; Group 5, 62 LA-negative thrombotic NAPs; Group 6, 20 warfarinized, non-antiphospholipid syndrome patients. Results Accepting that the Life Diagnostics reagents were acting as a pseudo-gold standard, Siemens dRVVT detected 56/59, (95%) Group 1 LA and HBM dRVVT 46/59, (76%), one each from Group 2, and Siemens dRVVT detected one in Group 5. The lower HBM dRVVT detection rate mainly concerned weaker LA, where between-reagent concordance is problematic. All Group 3 patients appeared LA-positive in undiluted plasma with Siemens dRVVT, as did 16/24 (67%) with HBM dRVVT but the fewer LA-positives in mixing tests better mapped to clear LA-positives with LD dRVVT. LD and Siemens dRVVTs exhibited 87% and 95% false-positivity for Group 6 whilst HBM dRVVT had none. Increasing the cut-off improved accuracy. Applying higher cut-offs improved accuracy in Group 4 patients. Conclusion HBM dRVVT exhibited improved specificity, mainly due to less interference by anticoagulation, but reduced sensitivity, compared to the other dRVVTs employed.

Laboratory diagnostic outcome applying detection criteria recommended by the Scientific and Standardization Committee of the ISTH on Lupus Anticoagulant

This study shows the diagnostic outcome of an APTT-based and two dRVVT-based commercial confirmatory integrated tests with the application of the recommendations by the Scientific and Standardization Committee (SSC) on Lupus anticoagulant (LA)/antiphospholipid syndrome (APS) of the International Society on Thrombosis and Haemostasis (ISTH) issued in 2009 concerning the cut-off values for the screening, mixing and confirmatory tests for the detection of LA and the mandatory need to perform mixing tests of patient plasma with pooled normal plasma. The study population included 565 patients collected from a large central coagulation laboratory, for which the attending physicians requested LA detection. One-hundred-six healthy subjects (HS) and 131 selected patients on oral anticoagulant therapy (OAT) were included as negative controls. The results suggest that the performance of mixing tests is indicated for those methods with relatively poor specificity, but is less needed for those methods with high specificity. Furthermore, the SSC recommendation to use normal mid-value (i.e. the 50th percentile of distribution of results from healthy subjects) as the cut-off to interpret results of confirmatory tests, showed a modest increase in LA detection rate (sensitivity) but at the expense of specificity, particularly in methods with low specificity.

Application of different lupus anticoagulant diagnostic algorithms to the same assay data leads to interpretive discrepancies in some samples

BACKGROUND

Gold standard lupus anticoagulant (LA) assays and reference plasmas do not exist and detection is based on inference in a medley of coagulation assays, creating potential for interpretive discrepancies when applying different algorithms.

OBJECTIVES

To investigate discrepancies from applying different algorithms to a common data set.

METHODS

Diagnostic data on 311 non-anticoagulated patients LA-positive by dilute Russell's viper venom time (dRVVT) and/or dilute activated partial thromboplastin time (dAPTT) assays were employed to compare algorithms. Routine testing applied interpretive criteria from guidelines endorsing classification as LA-positive despite negative mixing tests, after exclusion of other clotting abnormalities. Integrated testing without mixing tests, and the classical algorithm where negative mixing tests preclude confirm tests, were then retrospectively applied to those data.

RESULTS

Initial testing showed 92/311 (29.6%) were LA-positive by dRVVT only, 156/311 (50.1%) by dAPTT only, and 63/311 (20.3%) by both assays. All dAPTT-positive plasmas remained positive with integrated testing but eight dRVVT-positives became negative. Other data suggested they were false-negatives. The classical algorithm altered 52/155 (33.5%) dRVVT and 111/219 (50.7%) dAPTT interpretations to LA-negative because of normal mixing tests, most of which were apparently weak LA in undiluted plasma.

CONCLUSIONS

The classical algorithm improves diagnostic specificity and confidence but risks missing some genuine LA due to false-negative mixing tests. Integrated testing can be diagnostically accurate and logistically efficient but oversimplifies complex cases. Performing mix and confirm in response to an elevated screen with their interpretation based on clinical data, coagulation screens and the LA-assay design offers a potentially valuable option.

Current Controversies in Lupus Anticoagulant Detection

Antiphospholipid syndrome is an autoimmune, acquired thrombophilia diagnosed when vascular thrombosis or pregnancy morbidity are accompanied by persistent antiphospholipid antibodies. Lupus anticoagulants (LA) are one of the criteria antibodies but calibration plasmas are unavailable and they are detected by inference based on antibody behaviour in a medley of coagulation-based assays. Elevated screening tests suggest the presence of a LA, which is confirmed with mixing tests to evidence inhibition and confirmatory tests to demonstrate phospholipid-dependence. At least two screening tests of different principle must be used to account for antibody heterogeneity and controversy exists on whether assays, in addition to dilute Russell's viper venom time and activated partial thromboplastin time, should be employed. A variety of approaches to raw data manipulation and interpretation attract debate, as does inclusion or exclusion of mixing studies in circumstances where the presence of a LA is already evident from other results. Therapeutic anticoagulation compromises coagulation-based assays but careful data interpretation and use of alternative reagents can detect or exclude LA in specific circumstances, and this aspect of LA detection continues to evolve. This review focuses on the main areas of debate in LA detection.

The challenges of lupus anticoagulants

The term "lupus anticoagulant" (LA) refers to a heterogeneous group of immunoglobulins behaving as acquired in vitro inhibitors of coagulation. These antibodies, namely anti-β2GPI and anti-prothrombin antibodies, induce the in vitro elongation of clotting time interfering with phospholipid-dependent coagulation cofactors. Positive LA is associated with thrombosis and pregnancy complications, providing one of the three laboratory criteria for the classification of the anti-phospholipid syndrome. LA is the strongest predictor of clinical events, especially when associated with other anti-phospholipid antibodies. Much more controversial is the risk conveyed by isolated and weak LA. LA detection is technically laborious, envisaging screening, mixing and confirming tests. Hopefully critical issues in LA detection, such as the interference of anticoagulants, will be overcome, in the next future.

Mixing studies in lupus anticoagulant testing are required at least in some type of samples

BACKGROUND

According to the ISTH guidelines for lupus anticoagulant (LAC) testing, the second step in the three-step procedure (screening, mixing, and confirmation) is the mixing test, which improves the discrimination between the presence of an inhibitor and coagulation factor deficiencies such as those occurring in patients receiving vitamin K antagonists (VKAs).

OBJECTIVES

From a retrospective analysis of dilute Russell viper venom (dRVVT) results, we evaluated the impact of the mixing test result on the interpretation of LAC positivity.

METHODS

We interpreted the dRVVT clotting times with and without taking into account the results of the mixing test in a patient population with prolonged screening test (n = 267) with special attention to the patients receiving VKAs.

RESULTS AND CONCLUSIONS

The number of samples classified as 'LAC positive' differed substantially depending on the method of interpretation; 170 and 235 of 267 samples were classified as LAC positive with the three- and two-step procedure, respectively. Discrepancy between the two-step (without mixing step) and the three-step procedure was due to not including a mixing test and was more pronounced in the VKA patient population. Screen/confirm ratios carried out on a 1:1 mix of patient and normal pooled plasma (NPP) gave a lower incidence of 59 of 267. We advise continuing to perform mixing test to avoid false-positives. In patients with discrepant results between the two- and three-step dRVVT interpretation, mainly observed in VKA-treated patients, we advise retesting of the patients preferable beyond the period of anticoagulant therapy and additional testing for anti-beta2GPI and/or anti-cardiolipin antibodies.

Antiphospholipid antibody testing and standardization

The laboratory criteria that define patients with antiphospholipid syndrome (APS) include lupus anticoagulant (LAC), anticardiolipin antibodies and anti-β2 glycoprotein I antibodies (aβ2GPI). All assays show methodological shortcomings and the combination of the three tests, each with different sensitivity and specificity, and hence, differences in clinical utility make the laboratory diagnosis of APS challenging. Consensus guidelines and proposals for antiphospholipid antibodies (aPL) testing have been published in the last 20 years and have led to a substantial improvement. Despite efforts so far, standardization is not reached yet, but progress has been made. On-going efforts to reduce the interlaboratory/interassay variations remain important; even an absolute standardization cannot be feasibly achieved. Taking into account the methodological shortcomings of the means we have available, more detailed guidelines may help in adequate performance of aPL testing. This review will focus on the efforts and achievements in standardization and on the weaknesses and strengths of the current available laboratory methods.

14th International Congress on Antiphospholipid Antibodies Task Force. Report on antiphospholipid syndrome laboratory diagnostics and trends

Current classification criteria for definite Antiphospholipid Syndrome (APS) require the use of three laboratory assays to detect antiphospholipid antibodies (aCL, anti-β2GPI and LA) in the presence of at least one of the two major clinical manifestations (i.e. thrombosis or pregnancy morbidity) of the syndrome. However, several other autoantibodies shown to be directed to other proteins or their complex with phospholipids have been proposed to be relevant to APS but their clinical utility and their diagnostic value remains elusive. This report summarizes the findings, conclusions and recommendations of the "APS Task Force 3-Laboratory Diagnostics and Trends" meeting that took place during the 14th International Congress on Antiphospholipid Antibodies (APLA 2013, September 18-21, Rio de Janeiro, RJ, Brazil).

Variability in exposure of epitope G40-R43 of domain i in commercial anti-beta2-glycoprotein I IgG ELISAs

Background

A major problem for diagnosing the antiphospholipid syndrome (APS) is the high variability between commercial anti-β₂glycoprotein I (β₂GPI) assays. Predominantly antibodies reactive against cryptic epitope Glycine40-Arginine43 (G40-R43) in domain I are associated with an increased risk for thrombosis. Upon interaction with anionic surfaces β₂GPI opens up, thereby exposing G40-R43.

Objectives

To examine whether suboptimal exposure of epitope G40-R43 explains the variations in results observed between commercial assays.

Methods

Two patient-derived monoclonal antibodies were tested on neutral versus anionic plates. Antibody P1-117 reacts with G40-R43 in the open conformation while P2-6 recognizes β₂GPI irrespective of its conformation. These antibodies were tested in commercial anti-β₂GPI assays (A–E).

Results

In assay A, both antibodies showed equal reactivity towards β₂GPI, indicating that all the β₂GPI exposes G40-R43. In other assays P1-117 displayed lower reactivity than P2-6, demonstrating reduced G40-R43 availability. To exclude influences of other assay features, reactivity was re-examined on plates of assay A and B using the protocol/reagents from each assay. In all combinations, reactivity of both antibodies on a plate was comparable to results obtained with its own protocol/reagents, suggesting that the coating, rather than other assay components, accounts for the observed differences. In two patient cohorts we demonstrated that a number of domain I-reactive samples are missed in assays characterized by a decreased exposure of epitope G40-R43.

Conclusions

Exposure of epitope G40-R43 on β₂GPI is highly variable between commercial anti-β₂GPI assays. As a consequence, patients can be falsely assigned negative in assays characterized by a reduced exposure of G40-R43.