Lupus anticoagulant testing using plasma spiked with monoclonal antibodies: performance in the UK NEQAS proficiency testing programme

Testing for the lupus anticoagulant: the good, the bad, and the ugly

Lupus anticoagulant (LA) represents 1 of the laboratory criteria for classification of patients as having definite antiphospholipid syndrome (APS). The other 2 laboratory criteria are anticardiolipin antibodies and anti-beta2-glycoprotein I antibodies. At least 1 of these antiphospholipid antibody (aPL) tests need to be positive, with evidence of persistence, together with evidence of at least 1 clinical criterion for APS, before a patient can be classified as having definite APS. LA and other aPL assays are also important for diagnosis or exclusion of APS, as well as for risk stratification, with triple-positive patients carrying the greatest risk. Whereas LA is identified through "uncalibrated" clot-based assays, the other aPL assays (anticardiolipin and anti-beta2-glycoprotein I antibodies) represent immunological assays, identified using calibrated solid-phase methods. Because LA is identified using clot-based assays, it is subject to considerable preanalytical and analytical issues that challenge accurate detection or exclusion of LA. In this narrative review, we take a look at the good, the bad, and the ugly of LA testing, primarily focusing on the last 10 years. Although harmonization of LA testing as a result of International Society on Thrombosis and Haemostasis guidance documents and other international activities has led to improvements in LA detection, many challenges remain. In particular, several anticoagulants, especially direct oral anticoagulants and also vitamin K antagonists, given as therapy to treat the pathophysiological consequences of aPL, especially thrombosis, interfere with LA assays and can generate false-positive or false-negative LA findings. Overcoming these diagnostic errors will require a multifaceted approach with clinicians and laboratories working together.

16th International congress on antiphospholipid antibodies task force report on antiphospholipid syndrome laboratory diagnostics and trends

Classification criteria for antiphospholipid syndrome (APS) require IgG or IgM isotypes of the anticardiolipin (aCL) antibodies, anti-β2 glycoprotein I (anti-β2GPI) antibodies, and/or the lupus anticoagulant (LA) to satisfy the laboratory disease definition. Over the past 20 years, non-criteria antiphospholipid antibodies (aPL) directed to other proteins of the coagulation cascade (i.e. prothrombin and/or phosphatidylserine-prothrombin complex) or to some domains of β2GPI have been proposed. This task force concentrated and reviewed the literature on data including aPS/PT, antibodies to domain 4/5 of β2GPI and the newly described antibodies to protein/HLA-DR complex. In addition, we discussed testing of LA in the 'new' oral anticoagulants' era and the value of triple positivity in the risk assessment of aPL. The conclusions were presented at a special session during the 16th International Congress on aPL, Manchester, UK, September 2019.

Variable Performance of Lupus Anticoagulant Testing: The Australasian/Asia-Pacific Experience

Lupus anticoagulant (LA) is one of three tests identified as laboratory criteria for definite antiphospholipid syndrome (APS). The other two tests are anticardiolipin antibody (aCL) and anti-β2-glycoprotein I (aβ2GPI) antibody. The presence of LA is assessed using clot-based tests, while the presence of aCL and aβ2GPI is assessed by immunological assays. Since no test can be considered 100% sensitive or specific for LA, current guidelines recommend using two different clot-based assays reflecting different principles, with the dilute Russell viper venom time (dRVVT) and activated partial thromboplastin time (aPTT) recommended. Initially, LA-sensitive reagents are used to screen for LA, and then, in "screen-positive" samples, LA-"insensitive" reagents are used to confirm LA. Because LA assays are based on clot detection, anything that can interfere with fibrin clot development may affect test results. In particular, in addition to LA, the tests are also sensitive to the presence of a wide range of clinical anticoagulants, reflecting preanalytical issues for testing. We provide updated findings for LA testing in our geographic region, using recent data from the Royal College of Pathologists of Australasia Quality Assurance Programs, an international external quality assessment program with approximately 120 participants. Data show a wide variety of assays in use, especially for aPTT testing, and variable outcomes in reported numerical values with these assays when assessing proficiency samples. dRVVT testing mostly comprised reagents from three main manufacturing suppliers, which also showed differences in numerical values for the same homogeneous tested samples. Nevertheless, despite the use of different test reagents and processes, >98% of participants correctly identified LA-negative samples as LA-negative and LA-positive samples as LA positive. We hope our findings, reflecting on the heterogeneity of test processes and test data, help improve diagnostic testing for LA in the future.

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.

Utility of Lupus Anticoagulant Assays (APTT-LA, KCT, DPT and DRVVT) in Detection of Antiphospholipid Syndrome (APS) in High Risk Pregnancy Cases

Routine investigation for recurrent pregnancy loss includes measurement of antiphospholipid antibodies. The lupus anticoagulant has long been associated with increased risks for thrombosis and adverse obstetric outcomes. But there are some disadvantages with lupus anticoagulant (LAC) tests which includes varied sensitivity of different clot based assays. ISTH recommends only 2 assays (preferably DRVVT and APTT-LA) for the identification of lupus anticoagulant but there are some studies which don't support this contention. Our study analyzed 526 samples from high risk pregnancy cases for APLA by all four LAC tests from tertiary centre of northern India. Among all the cases studies 65 cases were positive for lupus anticoagulant 25 of this became negative after 12 weeks. Among the 40 repeated positive assays, dRVVT could able to diagnose 36 cases followed by APTT-LA which could able to diagnose 28 cases, while KCT could able to diagnose 23 cases and dPT could able to diagnose only 14 cases. There were 12 cases in whom all lupus assays were positive. Our study thus concluded that DRVVT was the most sensitive followed by APPT-LA, KCT, dPT. The combination of dRVVT with APTT-LA or KCT appeared to be superior to other combinations. No individual test per se is 100% sensitive for the diagnosis of APLA in high risk pregnancy cases. Further results confirmed that repeated LAC result is required even in a high-risk setting. Positive LAC assay in majority were not associated with exclusively recurrent pregnancy loss but were associated with sporadic stillbirth and thrombosis.

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.

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.

Lupus anticoagulant testing

Antiphospholipid antibodies are a heterogenous group of autoantibodies directed against glycoproteins in concert with anionic phospholipids. In clinical laboratory practice, antiphospholipid antibody evaluations usually consist of a combination of the following: anticardiolipin antibody assay, anti-beta 2 glycoprotein I assay, and at least two lupus anticoagulant assays with an appropriate confirmatory test. Lupus anticoagulants produce their laboratory effect by prolonging recalcification times in assays within which phospholipid content is limited. Although many assays are available, all are based on the fundamental principle of demonstrating normalization of prolonged recalcification times with the addition of exogenous phospholipid. The antibody specificity of an individual lupus anticoagulant is difficult or impossible to determine; however a small proportion do demonstrate avidity for selected proteins such as prothrombin or beta 2 glycoprotein I. The mechanism by which these antibodies cause their clinical manifestations remains unknown; however their relationship to increased risk of thrombosis, pregnancy loss, and autoimmune thrombocytopenia is undoubted. There is no correlation between the "strength" of lupus anticoagulants and the level of thrombotic risk; thus it is important to identify both "weak" and "strong" lupus anticoagulants.

The lupus anticoagulant: results from 2257 patients attending a high-risk pregnancy clinic

Only 62 (2.7%) of 2257 high-risk patients tested positive for LAC over 6 years; only 5 (0.02%) with early recurrent miscarriage tested positive. The 2 assays recommended by ISTH guidelines were less effective than our 4-assay panel at capturing and describing LAC-positive patients.

Direct Thrombin Inhibitors and Factor Xa Inhibitors Can Influence the Diluted Prothrombin Time Used as the Initial Screen for Lupus Anticoagulant

Context.—Lupus anticoagulant (LA) is a heterogeneous group of antiphospholipid antibodies. Among others, diluted prothrombin time (dPT) is a sensitive screening test for LA; however, the interpretation of LA tests is difficult in patients treated with anticoagulants. The effect of different types of anticoagulants on the result of LA tests, particularly on dPT, has not been studied extensively.

Objective.—To determine whether the direct thrombin inhibitors lepirudin and argatroban and the predominantly factor Xa inhibitors enoxaparin, danaparoid, and fondaparinux could interfere with LA screening based on dPT.

Design.—Each drug was added to normal and LA-positive plasmas in clinically relevant concentrations. Each sample was tested for dPT. Samples with factor Xa inhibitors were investigated before and after addition of heparinase. Mixing and confirmatory tests for LA were not performed.

Results.—In the presence of lepirudin or argatroban, dPT increased notably and the dPT ratio exceeded the cutoff value even at subtherapeutic concentrations resulting in false positivity. With increasing factor Xa inhibitor concentrations, a linear increase of dPT ratios and false-positive results were also demonstrated. Although heparinase could almost completely neutralize the anti-Xa effect of all investigated factor Xa inhibitors, dPT ratio returned to the basal level only in case of enoxaparin.

Conclusions.—Here we provide evidence that both the direct thrombin and indirect factor Xa inhibitors influence dPT assay for LA, causing false positivity. This should be considered when interpreting LA results during anticoagulant therapy. However, dPT seems to be a reliable test for LA screening under enoxaparin therapy after neutralization by heparinase.

Prevalence of heparin in samples submitted for lupus anticoagulant testing

INTRODUCTION

Lupus anticoagulant (LA) testing can be affected by the presence of anticoagulant medications such as heparin, and current guidelines recommend caution when performing and interpreting LA testing for patients who are receiving anticoagulation treatment. We searched our reference laboratory database to determine the prevalence of heparin in samples submitted for LA testing.

METHODS

We reviewed 18,676 LA reflexive testing panels. Heparin was identified by partial thromboplastin time (before and after heparin neutralization), thrombin time, and reptilase time results. Samples containing heparin were subclassified (not significant, significant) according to the degree of thrombin time prolongation.

RESULTS

Of the panels, 1909 panels (10%) were LA positive. We found that 2011 samples (11%) contained some heparin and that 616 samples contained a significant amount of heparin (3% of all samples and 31% of samples containing heparin). LA-positive results were obtained for 80 (13%) of these samples, which represented 4% of the samples containing heparin and 0.4% of all samples.

CONCLUSION

LA-testing guidelines recommend that samples not contain anticoagulant medications. Despite these recommendations, our data show that a significant proportion (11%) of these samples contain heparin. We conclude that LA-testing algorithms should use methods to identify and neutralize heparin and that laboratories should provide education regarding appropriate sample collection for LA testing.

Antiphospholipid antibodies--we are not quite there yet

The diagnosis of antiphospholipid syndrome is predominantly made in the laboratory and depends on the persistent presence of antiphospholipid antibodies in individuals with thrombosis or pregnancy morbidity. Correct diagnosis of the syndrome is imperative to prevent unnecessary long secondary thromboprophylaxis. Three antiphospholipid antibody subtypes are included in the classification criteria of the antiphospholipid syndrome: lupus anticoagulants, anticardiolipin antibodies and anti-β2-glycoprotein I antibodies. Only lupus anticoagulants are undisputedly associated with thrombosis, which is why the serological criteria of the antiphospholipid syndrome are under debate. All of the assays used to detect antiphospholipid antibodies are in need of better standardization, although progress has been made in the detection of lupus anticoagulants. The inconsistent association between both anticardiolipin and anti-β2-glycoprotein I antibodies and thrombosis is a cause for alarm. We are in need of better assays to detect those individuals at risk for thrombosis and population-based prospective studies to provide us with accurate risk assessments.

Risky business: the interpretation, use, and abuse of antiphospholipid antibody tests in clinical practice

Antiphospholipid antibodies (aPL) are best considered as risk factors. aPL are not diagnostic tests and considering them as such can be misleading and may direct attention away from the more important clinical issue of risk modification and management. When considering aPL as risk factors, quantitative aPL tests such enzyme-linked immunosorbent assay (ELISA) for anticardiolipin (aCL) and anti-beta(2)-glycoprotein I (anti-beta(2)GPI) antibodies, should be interpreted carefully. Risk for clinical manifestations appears to be associated with moderate to high levels of these autoantibodies. Lower levels may be statistically abnormal compared with a control population, but may not be associated with the risk of thrombosis or pregnancy loss. Lupus anticoagulants (LA) are generally thought to be more strongly associated with the risk of clinical manifestation of antiphospholipid syndrome (APS) than aCL and anti-beta(2)GPI antibodies. One reason for the stronger association may be related to patients' antibody titers. LA assays are not very analytically sensitive, i.e. a relatively high concentration of antibodies is required to prolong the clotting time in these tests. Thus, the presence of LA indicates a high titer of aPL and this, rather than the intrinsic functional characteristics of LA antibodies, may explain the high risk of clinical manifestations associated with LA.

European attempts for the standardisation of the antiphospholipid antibodies

According to the Sydney criteria, antiphospholipid syndrome (APS) diagnosis is closely related to the demonstration of antiphospholipid antibodies (aPL) in patients sera. For this purpose, three different assays are conventionally accepted: lupus anticoagulant (LA), anticardiolipin (aCL) and anti-β2 glycoprotein I (β2GPI) antibodies. LA, described in the 1950s is a coagulation-based functional assay, which indirectly detects the presence of aPL. The aCL ELISA was developed in 1985; the identification of β2GPI as a major target of aPL, allowed the introduction of anti-β2GPI ELISA. Even if the diagnostic criteria for APS have been well defined, the laboratory detection of aPL is not always reproducible for many reasons. To achieve a univocal diagnostic definition of APS, efforts were made to reduce the inter- and/or intra-laboratory variability of the diagnostic tests. In this article, we analyse the studies performed to standardise aPL assays that were developed within the European Forum on Antiphospholipid Antibodies.

Laboratory identification of lupus anticoagulants

Lupus anticoagulants (LA) are acquired autoantibodies that can cause antiphospholipid syndrome. LAs prolong phospholipid-dependent coagulation tests, acting as nonspecific inhibitors that are neutralized in the presence of excess phospholipid. However, there is no gold standard test and the testing is influenced by a number of variables. This article summarizes laboratory testing for LAs, with particular focus on technical issues and limitations of testing. Am. J. Hematol. 2009. (c) 2009 Wiley-Liss, Inc.

How we diagnose the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an acquired thrombophilia, characterized by the occurrence of venous and arterial events. This article examines the laboratory and key clinical aspects of APS. Particular focus is given to anti–beta 2-glycoprotein I (β2GPI) antibodies in view of their recent inclusion in the APS classification criteria. The clinical utility of using the β2GPI enzyme-linked immunosorbent assay, in conjunction with the established lupus anticoagulant assays and cardiolipin enzyme-linked immunosorbent assay, for diagnosing and risk stratifying patients suspected of having APS is discussed. The relative importance of the various assays in diagnosing obstetric APS (early and late gestation miscarriages) is explored. The implications of recent epidemiologic findings for possibly understanding the underlying pathophysiologic mechanisms of obstetric APS are highlighted. Insights into which patients with obstetric APS may be at most risk of thrombotic complications are presented.

Clinical significance of different antiphospholipid antibodies in the WAPS (warfarin in the antiphospholipid syndrome) study

To assess the clinical significance of lupus anticoagulants (LAs) and antiphospholipid antibodies (aPLs) toward thrombosis and abortions, we measured them in 112 patients whose samples were available at enrollment in the warfarin in the antiphospholipid syndrome (WAPS) study. Enzyme-linked immunosorbent assay (ELISA) and coagulation test values in the highest and lowest tertiles were compared. When considered separately, IgG antibodies to β2-glycoprotein I (aβ2GPI) and prothrombin (aPT) were associated with anamnestic arterial and venous thrombosis, respectively, and those to annexin AV (aAnAV) with abortions. IgM antibodies to protein S and the lupus ratio of the dilute prothrombin time were associated with prospective thrombosis. No other association for IgM antibodies was seen. LA-positive patients who carried aβ2GPI antibodies were at risk of anamnestic arterial and total thrombosis and aPT antibodies to that of anamnestic venous and total thrombosis. LA-positive patients who carried IgG aβ2GPI and aAnAV antibodies were at risk for both anamnestic abortion and prospective thrombosis. Overall, these data support the inclusion of aβ2GPI antibodies in and suggest the removal of anticardiolipin antibodies from the laboratory criteria of the antiphospholipid syndrome. They also suggest that the measurement of aPT and aAnAV antibodies is useful in some selected situations and that there is little role for IgM antibody detection.

Survey of lupus anticoagulant diagnosis by central evaluation of positive plasma samples

OBJECTIVE

To determine whether the diagnosis of lupus anticoagulant (LAC) in a large cohort of positive patients was confirmed at a reference laboratory.

METHODS

Over a 1-year period, each participating center collected samples from LAC-positive patients. Plasma was filtered and kept deep-frozen until it was sent on dry ice to the reference laboratory by express courier. Centers returned detailed laboratory information and clinical data from each patient. The reference laboratory screened plasma samples by diluted Russell viper venom time (dRVVT) and kaolin clotting time (KCT). When these were prolonged, 1:1 mixing studies were carried out, and confirmatory tests were performed as appropriate. Positive samples were further tested by thrombin time (TT). The presence of heparin was checked by measuring antifactor Xa activity when TT was prolonged. Negative samples were tested by activated partial thromboplastin time using hexagonal phospholipids.

RESULTS

Plasma samples from 302 patients from 29 anticoagulation clinics were analyzed. LAC was excluded in 71 samples (24%), because dRVVT and KCT screening test results were normal (34) or reversed to normal by mixing studies (35). The remaining two samples were considered negative because they contained heparin. LAC-negative patients showed different characteristics from those in whom diagnosis was confirmed. They were significantly older (49.7 vs. 45.0 years, P < 0.03), were more often first diagnosed (66% vs. 41%, P < 0.001), and were more frequently judged as mild in LAC potency (60% vs. 25%, P < 0.0001). Moreover, anticardiolipin and anti-beta(2)-glycoprotein I antibody values were more often normal in LAC-negative (82%) than in LAC-positive (42%) samples (P < 0.0001). LAC-positive samples identified by both dRVVT and KCT (146/231, 63%) showed a LAC potency that was significantly stronger than that in samples in which LAC diagnosis was made by a single test.

CONCLUSIONS

A false-positive LAC diagnosis is not uncommon across specialized centers. Patients' characteristics and a complete antiphospholipid antibody profile may help to identify these individuals.

Attempts to make sense of the antiphospholipid syndrome

Many investigators have been intrigued by the paradoxical association of a circulating anticoagulant, first called lupus anticoagulant by Feinstein and Rapaport [1], with a tendency to develop thrombosis, as initially described by Walter Bowie [2]. Work in Leuven on this topic started when Luis Carreras, an Argentinian hematologist, joined the laboratory of blood coagulation at this university in 1979. At that time, the head of the laboratory was Marc Verstraete. Luis had a particular interest in antibody-mediated coagulation disorders, and had prepared reviews on thrombosis and thrombocytopenia induced by heparin [3] and on the lupus inhibitor [4]. In Leuven, he joined Jos Vermylen, senior member of the laboratory, and an internist with particular interest in hemostasis, thrombosis and vascular disease. As such, Professor Vermylen was involved in both laboratory research and patient care.

Prolonged aPTT after kidney transplantation due to transient lupus anticoagulants

BACKGROUND

After kidney transplantation, a renal biopsy may be needed to elucidate the reasons for lack of graft function. If the activated partial thromboplastin time (aPTT) is prolonged, the biopsy will often be postponed, as increased risk of bleeding must be expected. However, aPTT prolongation is not always due to lack of coagulation factors, but can be due to the presence of lupus anticoagulants (LAs). Clinical observations in our department indicated that a large proportion of recently kidney-transplanted patients developed prolonged aPTT values without clinical complications.

METHODS

A prospective study of patients receiving a kidney transplant in 2004 was conducted to investigate the frequency and cause of prolongation of the aPTT.

RESULTS

Twenty-seven patients were included in the study; none had prolonged aPTT or LAs before the transplantation. In the post-transplantation period, 19 patients (70.4%) had a significantly prolonged aPTT. Further investigation showed that for all 19 patients, prolongation was due to acquired antibodies: 13 had developed LAs and six had developed unspecific antibodies. The acquired antibodies were transient and did not affect clinical outcome.

CONCLUSIONS

This is the first study investigating prolonged aPTT in the post-transplantation period. All patients with prolonged aPTT had acquired transient antibodies, i.e. LA or 'LA-like'. If a renal biopsy was requested, 70.4% of the transplanted patients would presumably have their biopsy postponed due to prolonged aPTT, but as LAs do not increase the risk of bleeding, such a delay would be unnecessary. Immediate LA investigation is therefore recommended if a recently transplanted patient requiring surgical procedures has a prolonged aPTT.