Variability of cut-off values for the detection of lupus anticoagulants: results of an international multicenter multiplatform study

Lupus Anticoagulant Detection under the Magnifying Glass

Diagnosis of antiphospholipid syndrome (APS) requires the presence of a clinical criterion (thrombosis and/or pregnancy morbidity), combined with persistently circulating antiphospholipid antibodies (aPL). Lupus anticoagulant (LA) is one of the three laboratory parameters (the others being antibodies to either cardiolipin or β2-glycoprotein I) that defines this rare but potentially devastating condition. For the search for aCL and aβ2-GP-I, traditionally measured with immunological solid-phase assays (ELISA), several different assays and detection techniques are currently available, thus making these tests relatively reliable and widespread. On the other hand, LA detection is based on functional coagulation procedures that are characterized by poor standardization, difficulties in interpreting the results, and interference by several drugs commonly used in the clinical settings in which LA search is appropriate. This article aims to review the current state of the art and the challenges that clinicians and laboratories incur in the detection of LA.

Lupus anticoagulant laboratory diagnosis by applying the 2020 ISTH-SSC guidelines

BACKGROUND

The ISTH-SSC guidelines for lupus anticoagulant (LA) testing recommend using in-house determined cut-off values, pooled normal plasma (PNP) for ratio normalization, and a ratio for the mixing test interpretation. They strongly support the mixing step role in the diagnostic process.

OBJECTIVES

To investigate and compare the LA testing results and interpretations obtained following the ISTH-SSC guidelines or the available alternatives.

PATIENTS/METHODS

Blood samples for LA testing from 462 consecutive patients were evaluated for screening, mixing and confirmatory tests. The analysis focused on the interpretation differences between using (1) the in-house cut-off values versus the manufacturer's cut-off values, (2) a normalized ratio calculated using PNP at each run versus the mean of the reference interval, (3) a normalized ratio versus the index of circulating anticoagulant to interpret the mixing step, and (4) a two-step versus three-step procedure.

RESULTS

LA testing outcomes were comparable when using the in-house and manufacturer's cut-off values. More positive dilute Russell's viper venom (DRVV) time results were obtained with the normalized ratio based on PNP than with the mean of the reference interval. Overall, the mixing test results obtained with the normalized ratio and the index of circulating anticoagulant showed a good agreement. Among the 97 DRVV Screen test-positive samples, 33 and 89 were classified as LA-positive with the 3-step and the 2-step procedure, respectively.

CONCLUSIONS

The cut-off value used and the way to normalize ratios had a limited impact. Conversely, it is important to understand the mixing test characteristics to maximize its diagnostic potential.

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.

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.

A multi-laboratory assessment of lupus anticoagulant assays performed on the ACL TOP 50 family for harmonized testing in a large laboratory network

Introduction

Lupus anticoagulant (LA) testing is commonly performed within hemostasis laboratories, and the ACL TOP 50 family of instruments represent a new “single platform” of hemostasis instrumentation. Our aim was to evaluate these instruments and manufacturer reagents or alternatives for utility in LA testing.

Methods

Comparative evaluations of LA testing using newly installed ACL TOPs 550 and 750 as well as comparative assessments with existing “reference,” predominantly Stago, instrumentation, and reagents. Evaluations comprised both dilute Russell viper venom time (dRVVT) and activated partial thromboplastin time (APTT)‐based assays. Establishment of normal reference ranges (NRR).

Results

The HemosIL dRVVT‐based assays showed good comparability with the existing Stago reference method (R > 0.9) and could be considered as verified as fit for purpose. A variety of APTT assays was additionally evaluated for LA utility, and we identified from the assessment good utility of a non‐Werfen solution in Hyphen BioMed Cephen reagents. NRR were established based on ≥120 normal individual plasma samples.

Conclusion

This evaluation of LA reagents on ACL TOP 50 Family instruments identified overall acceptable performance of both dRVVT (Werfen solution) and APTT (non‐Werfen solution) to enable harmonization of LA testing in our large network.

International multicenter, multiplatform study to validate Taipan snake venom time as a lupus anticoagulant screening test with ecarin time as the confirmatory test: Communication from the ISTH SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies

BACKGROUND

Lupus anticoagulant (LA) assays are compromised in anticoagulated patients, and existing strategies to overcome the interferences have limitations. The prothrombin-activating Taipan snake venom time (TSVT) screening test and ecarin time (ET) confirmatory test are innately insensitive to vitamin K antagonists (VKA) and direct factor Xa inhibitors (DFXaI).

OBJECTIVES

Validate standardized TSVT/ET reagents for LA detection, in a multicenter, multiplatform study.

PATIENTS/METHODS

Six centers from four countries analyzed samples with TSVT/ET from 81 nonanticoagulated patients with LA, patients with established antiphospholipid syndrome (APS), and proven persistent LA who were either not anticoagulated (n = 120) or were anticoagulated with VKAs (n = 180) or DFXaIs (n = 71). Additionally, 339 nonanticoagulated LA-negative patients, and 575 anticoagulated non-APS patients (172 VKA, 403 DFXaI) were tested. Anticoagulant spiking experiments were performed and 112 samples containing potential interferences (i.e., direct thrombin inhibitors) were tested. Results were evaluated against locally derived cutoffs. Imprecision was evaluated.

RESULTS

Cutoffs were remarkably similar despite use of different analyzers and donor populations. Cutoffs for TSVT ratio, ET ratio, percent correction, and normalized TSVT ratio/ET ratio ranged between 1.08 and 1.10, 1.09 and 1.12, 9.3% and 14.8%, and 1.10 and 1.15, respectively. Coefficients of variation for TSVT and ET ratios were ≤5.0%. TSVT/ET exhibited sensitivity, specificity, and negative and positive predictive values of 78.2%/95.0%/86.3%/91.5%, respectively, with established APS as the LA-positive population, and 86.9%/95.0%/76.8%/97.4%, respectively, with triple-positive APS. Interference was seen with direct thrombin inhibitors, unfractionated heparin, and low molecular weight heparins, but not VKAs or DFXaIs.

CONCLUSIONS

TSVT/ET are validated for LA detection in nonanticoagulated patients and those on VKAs or DFXaIs.

Diagnostic Challenges on the Laboratory Detection of Lupus Anticoagulant

Lupus anticoagulant (LA) is one of the three laboratory parameters (the others being antibodies to either cardiolipin or β2-glycoprotein I) which defines the rare but potentially devastating condition known as antiphospholipid syndrome (APS). Testing for LA is a challenging task for the clinical laboratory because specific tests for its detection are not available. However, proper LA detection is paramount for patients’ management, as its persistent positivity in the presence of (previous or current) thrombotic events, candidate for long term anticoagulation. Guidelines for LA detection have been established and updated over the last two decades. Implementation of these guidelines across laboratories and participation to external quality assessment schemes are required to help standardize the diagnostic procedures and help clinicians for appropriate management of APS. This article aims to review the current state of the art and the challenges that clinical laboratories incur in the detection of LA.

Current Promising Biomarkers and Methods in the Diagnostics of Antiphospholipid Syndrome: A Review

Antiphospholipid syndrome (APS) is a hypercoagulation condition associated with the incidence of heterogenic antiphospholipid antibodies (aPLs), which non-specifically affect hemostasis processes. APS is clinically manifested by recurrent arterial and venous thromboses and reproduction losses. The aPL antibodies, which may induce clinical manifestations of APS, include criteria antibodies anti-cardiolipin, anti-β2-glycoprotein-I, and lupus anticoagulant, but also non-criteria antibodies, for example anti-β2-glycoprotein-I domain I, anti-phosphatidylserine/prothrombin, anti-annexin V, and many others. APS occurs mostly in patients of younger and middle age, most frequently in females. Laboratory diagnostics of APS are quite difficult, as they include a wide spectrum of examining methods, which are based on various principles of detection and are performed using various laboratory techniques. The objective of the review is to describe the current state of potentially examined biomarkers and methods in APS diagnostics. The aforementioned biomarkers are lupus anticoagulant, anti-β2-glycoprotein-I, anti-cardiolipin, anti-β2-glycoprotein-I domain I, anti-phosphatidylserine/prothrombin, anti-β2-glycoprotein-I IgA, anti-cardiolipin IgA, anti-annexin V and II, anti-prothrombin, anti-cardiolipin/vimentin, anti-protein S/protein C, and antibodies against phospholipid antigens for whose diagnostics we may use some of the methods established for a long time and some of the modern methods—the coagulation method for the determination of lupus anticoagulant (LA), enzyme-linked imunosorbent assay (ELISA), chemiluminescence analysis (CLIA), multiplex fluorescence flow immunoassay (MFFIA), fluorescence enzyme immunoassay (EliA), line immunoassay (LIA), multiline dot assay (MLDA), and thin-layer chromatography (TLC). Conclusion: Antibodies against phosphatidylethanolamine, phosphatidic acid, phosphatidylserine, phosphatidylinositol, cardiolipin/vimentin complex, and annexin V are currently the most studied new markers. However, these assays have not been standardized until now, both from the laboratory and clinical point of view. In this review we summarize the evidence of the most studied aPL markers and their potential clinical significance in seronegative APS (SN-APS).

Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis: Update of the guidelines for lupus anticoagulant detection and interpretation

This guidance focuses on methodological aspects of lupus anticoagulant (LA) testing, as well as interpretation of results for clinicians. The main changes in how to test for LA compared with the International Society on Thrombosis and Haemostasis Scientific and Standardization Committee 2009 guidelines, in the preanalytical phase are more detailed recommendations on how to handle testing in anticoagulated patients, and the timing of testing. Also, routine coagulation tests are advised to obtain more information on the coagulation background of the patient, and when necessary, anti-Xa activity measurement for heparins or specific assays for direct oral anticoagulants should be performed. The three-step procedure with two test systems (diluted Russell's viper venom time and activated partial thromboplastin time [aPTT]) is essentially not changed. Silica remains the preferable activator in the aPTT assays, but ellagic acid is not excluded. We advise simultaneous performance of the mixing and confirmatory step, in each sample with a prolonged screening test. The confirmatory step can also be performed on a mixture of patient plasma and normal pooled plasma. Cutoff values should be established in-house on at least 120 normals, with transference of the manufacturer's cutoffs as an alternative. Reporting of results has not been changed, although more attention is focused on what clinicians should know. Patient selection for LA testing has been expanded.

How to Interpret Antiphospholipid Laboratory Tests

PURPOSE OF THE REVIEW

This review focuses on the laboratory tests necessary for the diagnosis of antiphospholipid syndrome (APS). For the interpretation of the results of the tests for antiphospholipid antibodies (aPL), understanding of all pitfalls and interferences is necessary.

RECENT FINDINGS

Progress has been made on the standardization of aPL tests and current guidelines for detection of lupus anticoagulant (LAC), anticardiolipin antibodies (aCL), and antibeta2-glycoprotein I antibodies (aβ2GPI) are useful tools. LAC measurement remains a complex procedure with many pitfalls and interference by anticoagulant therapy. Solid phase assays for aCL and aβ2GPI still show inter-assay differences. Measuring LAC, aCL, and aβ2GPI allows making antibody profiles that help in identifying patients at risk. Other aPL, such as antibodies against domain I of beta2-glycoprotein I (aDI) and antiphosphatidylserine-prothrombin (aPS/PT) antibodies, may be useful in risk stratification of APS patients, but are not included in the current diagnostic criteria as no added value in the diagnosis of APS has been illustrated so far. The laboratory diagnosis of APS remains challenging. LAC, aCL, aβ2GPI IgG, and IgM should be performed to increase diagnostic efficacy, with an integrated interpretation of all results and an interpretative comment. A close interaction between clinical pathologists and clinicians is mandatory.

Lupus anticoagulant assay cut-offs vary between reagents even when derived from a common set of normal donor plasmas

BACKGROUND

Multicenter studies reveal that diagnostic efficacy of lupus anticoagulant (LA) assays is enhanced if cut-offs are locally generated. However, a potential confounder is the inevitable use of separate normal donor populations.

OBJECTIVES

Generate cut-offs for multiple LA reagents with the same analyzer and normal donor plasmas.

METHODS

Cut-offs for screen ratio, confirm ratio, percent correction of screen ratio by confirm ratio, and normalized screen/confirm ratio (NSCR) were derived from the same 50 normal donor plasmas for screen and confirm pairs for two dilute Russell's viper venom time reagents, LA1/LA2 and HEMOCLOT™ LA-S/LA-C, and two APTTs, Actin FSL/FS and Cephen LS/Cephen. The cut-offs were challenged with plasmas from 20 triple-positive APS patients and 25 plasmas from LA-negative, thrombotic patients.

RESULTS

Cut-offs for screen ratio, confirm ratio, percent correction, and NSCR, respectively, were 1.12/1.08/8.3/1.09 for LA1/LA2; 1.17/1.10/13.6/1.13 for HEMOCLOT™ LA-S/LA-C; 1.12/1.13/9.7/1.10 for Actin FSL/FS; 1.09/1.13/11.0/1.11 for Cephen LS/Cephen. LA1 and LA-S screens were elevated in 19/20 and 16/20 triple-positive plasmas, respectively, while 20/20 were detected with both via integrated interpretation ie, percent correction or NSCR irrespective of screen elevation. Actin FSL and Cephen LS screens were elevated in 17/20 and 19/20 triple-positive plasmas, respectively, while one more LA was detected with Actin FSL via integrated interpretation, but not for Cephen LS. Integrated interpretation suggested 5/25 LA-negative plasmas contained weak LA (two with Actin FSL/FS, two with LA1/LA2, one with LA-S/LA-R).

CONCLUSIONS

Employing the same normal donor plasmas and analytical platform does not compensate for between-reagent differences when generating LA assay cut-offs.

Comparison of real world and core laboratory lupus anticoagulant results from the Antiphospholipid Syndrome Alliance for Clinical Trials and International Networking (APS ACTION) clinical database and repository

BACKGROUND

Variability remains a challenge in lupus anticoagulant (LA) testing.

OBJECTIVE

To validate LA test performance between Antiphospholipid Syndrome Alliance for Clinical Trials and International Networking (APS ACTION) Core laboratories and examine agreement in LA status between Core and local/hospital laboratories contributing patients to this prospective registry.

METHODS

Five Core laboratories used the same reagents, analyzer type, protocols, and characterized samples for LA validation. Non-anticoagulated registry samples were retested at the corresponding regional Core laboratories and anticoagulated samples at a single Core laboratory. Categorical agreement and discrepancies in LA status between Core and local/hospital laboratories were analyzed.

RESULTS

Clotting times for the reference/characterized plasmas used for normalized ratios were similar between Core laboratories (CV <4%); precision and agreement for LA positive/negative plasma were similar (all CV ≤5%) in the four laboratories that completed both parts of the validation exercise; 418 registry samples underwent LA testing. Agreement for LA positive/negative status between Core and local/hospital laboratories was observed in 87% (115/132) non-anticoagulated and 77% (183/237) anticoagulated samples. However, 28.7% (120/418) of samples showed discordance between the Core and local/hospital laboratories or equivocal LA results. Some of the results of the local/hospital laboratories might have been unreliable in 24.7% (41/166) and 23% (58/252) of the total non-anticoagulated and anticoagulated samples, respectively. Equivocal results by the Core laboratory might have also contributed to discordance.

CONCLUSIONS

Laboratories can achieve good agreement in LA performance by use of the same reagents, analyzer type, and protocols. The standardized Core laboratory results underpin accurate interpretation of APS ACTION clinical data.

Influence of anticardiolipin and anti-β2 glycoprotein I antibody cutoff values on antiphospholipid syndrome classification

BACKGROUND

Anticardiolipin (aCL) and anti-β2 glycoprotein I (aβ2GPI) immunoglobulin (Ig) G/IgM antibodies are 2 of the 3 laboratory criteria for classification of antiphospholipid syndrome (APS). The threshold for clinically relevant levels of antiphospholipid antibodies (aPL) for the diagnosis of APS remains a matter of debate. The aim of this study was to evaluate the variation in cutoffs as determined in different clinical laboratories based on the results of a questionnaire as well as to determine the optimal method for cutoff establishment based on a clinical approach.

METHODS

The study included samples from 114 patients with thrombotic APS, 138 patients with non-APS thrombosis, 138 patients with autoimmune disease, and 183 healthy controls. aCL and aβ2GPI IgG/IgM antibodies were measured at 1 laboratory using 4 commercial assays. Assay-specific cutoff values for aPL were obtained by determining 95th and 99th percentiles of 120 compared to 200 normal controls by different statistical methods.

RESULTS

Normal reference value data showed a nonparametric distribution. Higher cutoff values were found when calculated as 99th rather than 95th percentiles. These values also showed a stronger association with thrombosis. The use of 99th percentile cutoffs reduced the chance of false positivity but at the same time reduced sensitivity. The decrease in sensitivity was higher than the gain in specificity when 99th percentiles were calculated by methods wherein no outliers were eliminated.

CONCLUSIONS

We present cutoff values for aPL determined by different statistical methods. The 99th percentile cutoff value seemed more specific. However, our findings indicate the need for standardized statistical criteria to calculate 99th percentile cutoff reference values.

Dilute Russel Viper Venom Time analysis in a Haematology Laboratory: An audit

INTRODUCTION

To determine whether the current set of evaluation criteria used for dilute Russel Viper Venom Time (dRVVT) investigations in the routine laboratory meet expectation and identify possible shortcomings.

METHODS

All dRVVT assays requested from January 2015 to December 2015 were appraised in this cross-sectional study. The raw data panels were compared with the new reference interval, established in 2016, to determine the sequence of assays that should have been performed. The interpretive comments were audited, and false-negative reports identified. Interpretive comments according to three interpretation guidelines were compared. The reagent cost per assay was determined, and reagent cost wastage, due to redundant tests, was calculated.

RESULTS

Only ~9% of dRVVT results authorized during 2015 had an interpretive comment included in the report. ~15% of these results were false-negative interpretations. There is a significant statistical difference in interpretive comments between the three interpretation methods. Redundant mixing tests resulted in R 7477.91 (~11%) reagent cost wastage in 2015.

CONCLUSIONS

We managed to demonstrate very evident deficiencies in our own practice and managed to establish a standardized workflow that will potentially render our service more efficient and cost effective, aiding clinicians in making improved treatment decisions and diagnoses. Furthermore, it is essential that standard operating procedures be kept up to date and executed by all staff in the laboratory.

Antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the presence of antiphospholipid antibodies, such as lupus anticoagulant, anticardiolipin antibodies and anti-β2-glycoprotein 1 antibodies. APS can present with a variety of clinical phenotypes, including thrombosis in the veins, arteries and microvasculature as well as obstetrical complications. The pathophysiological hallmark is thrombosis, but other factors such as complement activation might be important. Prevention of thrombotic manifestations associated with APS includes lifestyle changes and, in individuals at high risk, low-dose aspirin. Prevention and treatment of thrombotic events are dependent mainly on the use of vitamin K antagonists. Immunosuppression and anticomplement therapy have been used anecdotally but have not been adequately tested. Pregnancy morbidity includes unexplained recurrent early miscarriage, fetal death and late obstetrical manifestation such as pre-eclampsia, premature birth or fetal growth restriction associated with placental insufficiency. Current treatment to prevent obstetrical morbidity is based on low-dose aspirin and/or low-molecular-weight heparin and has improved pregnancy outcomes to achieve successful live birth in >70% of pregnancies. Although hydroxychloroquine and pravastatin might further improve pregnancy outcomes, prospective clinical trials are required to confirm these findings.

The impact of antibody profile in thrombosis associated with primary antiphospholipid syndrome

Triple positivity (TP) for antiphospholipid antibodies(aPL) may identify aPL carriers with poorer prognosis. The clinical impact of TP in primary antiphospholipid syndrome(PAPS) remains unclear and further clinical evidences are needed to validate TP as a marker of severity. The aim of this study was to evaluate the impact of TP on the clinical course of PAPS with thrombosis(t-PAPS). We performed a retrospective analysis of a cohort of t-PAPS patients, comparing groups of patients with TP and non-TP profiles according to their demographic, clinical and laboratory features. We included 105 patients with t-PAPS, the median follow-up time of 3.7 years. Twenty-two patients(21%) had TP; the demographic distribution, the presence of cardiovascular risk factors and the site of thrombosis were similar between TP and non-TP patients. The frequency of thrombotic events did not differ between TP and non-TP patients during the study period. Pregnancy morbidities were more frequent in women with t-PAPS and TP than in those with non-TP profile (80% vs. 52.8%, P = 0.05). Patients with t-PAPS and TP presented, at diagnosis, higher dRVVT ratio (median R = 2.44 vs. 1.57, P < 0.0001), higher aCL titer (median = 50UI vs. 35 UI, P < 0.0001), lower C3 levels (median = 1.08 vs. 1.30 mg dL^-1 , P = 0.001), lower C4 levels (median = 0.22 vs. 0.25 mg dL^-1 , P = 0.05) and higher frequency of positive ANA test (50% vs. 20%, P = 0.008) than patients with t-PAPS and non-TP. Lower-than-normal levels of C3 was independently associated with TP (OR = 5.1, P = 0.02). The presence of TP in patients with t-PAPS was associated with immune derangement, with no effect on the clinical course of the disease.

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.