Mis-identification of factor inhibitors by diagnostic haemostasis laboratories: recognition of pitfalls and elucidation of strategies. A follow up to a large multicentre evaluation

International Council for Standardization in Haematology Field Study Evaluating Optimal Interpretation Methods for Activated Partial Thromboplastin Time and Prothrombin Time Mixing Studies

CONTEXT.—

The prothrombin time (PT) and activated partial thromboplastin time (APTT) are screening tests used to detect congenital or acquired bleeding disorders. An unexpected PT and/or APTT prolongation is often evaluated using a mixing test with normal plasma. Failure to correct ("noncorrection") prolongation upon mixing is attributed to an inhibitor, whereas "correction" points to factor deficiency(ies).

OBJECTIVE.—

To define an optimal method for determining correction or noncorrection of plasma mixing tests through an international, multisite study that used multiple PT and APTT reagents and well-characterized plasma samples.

DESIGN.—

Each testing site was provided 22 abnormal and 25 normal donor plasma samples, and mixing studies were performed using local PT and APTT reagents. Mixing study results were evaluated using 11 different calculation methods to assess the optimal method based on the expected interpretation for factor deficiencies (correction) and noncorrection (inhibitor effect). Misprediction, which represents the failure of a mixing study interpretation method, was assessed.

RESULTS.—

Percentage correction was the most suitable calculation method for interpreting PT mixing test results for nearly all reagents evaluated. Incubated PT mixing tests should not be performed. For APTT mixing tests, percentage correction should be performed, and if the result indicates a factor deficiency, this should be confirmed with the subtraction III calculation where the normal pooled plasma result (run concurrently) is subtracted from the mixing test result with correction indicated by a result of 0 or less. In general, other calculation methods evaluated that performed well in the identification of factor deficiency tended to have high misprediction rates for inhibitors and vice versa.

CONCLUSIONS.—

No single method of mixing test result calculation was consistently successful in accurately distinguishing factor deficiencies from inhibitors, with between-reagent and between-site variability also identified.

International Council for Standardization in Haematology (ICSH) recommendations for the performance and interpretation of activated partial thromboplastin time and prothrombin time mixing tests

This guidance document has been prepared on behalf of the International Council for Standardization in Haematology (ICSH). The aim of the document is to provide guidance and recommendations for the performance and interpretation of activated partial thromboplastin time (APTT) and prothrombin time (PT) plasma mixing tests in clinical laboratories in all regions of the world. The following areas are included in this document: preanalytical, analytical, postanalytical, and quality assurance considerations as they relate to the proper performance and interpretation of plasma mixing tests. The recommendations are based on good laboratory practice, published data in peer-reviewed literature, and expert opinion.

Evolution of Hemostasis Testing: A Personal Reflection Covering over 40 Years of History

There is no certainty in change, other than change is certain. As Seminars in Thrombosis and Hemostasis celebrates 50 years of publication, I felt it appropriate to reflect on my own 40-year plus scientific career. My career in the thrombosis and hemostasis field did not start until 1987, but the subsequent 35 years reflected a period of significant change in associated disease diagnostics. I started in the Westmead Hospital "coagulation laboratory" when staff were still performing manual clotting tests, using stopwatches, pipettes, test tubes, and a water bath, which we transported to the hospital outpatient department to run our weekly warfarin clinic. Several hemostasis instruments have come and gone, including the Coag-A-Mate X2, the ACL-300R, the MDA-180, the BCS XP, and several StaR Evolution analyzers. Some instruments remain, including the PFA-100, PFA-200, the AggRAM, the CS-5100, an AcuStar, a Hydrasys gel system, and two ACL-TOP 750s. We still have a water bath, but this is primarily used to defrost frozen samples, and manual clotting tests are only used to teach visiting medical students. We have migrated across several methodologies in the 45-year history of the local laboratory. Laurel gel rockets, used for several assays in the 1980s, were replaced with enzyme-linked immunosorbent assay assays and most assays were eventually placed on automated instruments. Radio-isotopic assays, used in the 1980s, were replaced by an alternate safer method or else abandoned. Test numbers have increased markedly over time. The approximately 31,000 hemostasis assays performed at the Westmead-based laboratory in 1983 had become approximately 200,000 in 2022, a sixfold increase. Some 90,000 prothrombin times and activated partial thromboplastic times are now performed at this laboratory per year. Thrombophilia assays were added to the test repertoires over time, as were the tests to measure several anticoagulant drugs, most recently the direct oral anticoagulants. I hope my personal history, reflecting on the changes in hemostasis testing over my career to date in the field, is found to be of interest to the readership, and I hope they forgive any inaccuracies I have introduced in this reflection of the past.

Activated Partial Thromboplastin Time and Prothrombin Time Mixing Studies: Current State of the Art

Mixing studies have long been in the clinical laboratory armamentarium for investigating unexpected, prolonged activated partial thromboplastin time (aPTT) or prothrombin time (PT). The purpose of the mixing study is to identify whether the aPTT/PT prolongation is secondary to a factor deficiency versus an inhibitor, which would present as a "corrected" and "noncorrected" mixing study, respectively. The differentiation between a factor deficiency and inhibitor may likely further direct clinical decisions, including additional diagnostic testing or factor replacement therapy. While aPTT/PT mixing studies are simple tests to perform, there is a lack of standardization for both the testing protocol and the interpretation of what is considered to be a corrected or noncorrected mixing study result. This review will describe the common indications for the mixing test, preanalytic variables that may affect mixing study performance, and describe several methods for interpreting the results of aPTT and PT mixing tests.

International Council for Standardization in Haematology recommendations for laboratory measurement of factor VIII and FIX type I inhibitors

This guidance document has been prepared on behalf of the International Council for Standardisation in Hematology. The aim of the document is to provide guidance and recommendations on the measurement of factor VIII (FVIII) and factor IX (FIX) inhibitors. After an introduction on the clinical background and relevance of factor VIII and factor IX inhibitor testing, the following aspects of laboratory testing are included: screening for inhibitors, assay principle, sample requirements, testing requirements and interpretation, quality assurance, interferences and recent developments. This guidance document focusses on recommendations for a standardised procedure for the laboratory measurement of FVIII and FIX type I inhibitors. The recommendations are based on published data in peer-reviewed literature and expert opinion.

International Council for Standardisation in Haematology (ICSH) recommendations for collection of blood samples for coagulation testing

This guidance document has been prepared on behalf of the International Council for Standardisation in Haematology (ICSH). The aim of the document is to provide guidance and recommendations for collection of blood samples for coagulation tests in clinical laboratories throughout the world. The following processes will be covered: ordering tests, sample collection tube and anticoagulant, patient preparation, sample collection device, venous stasis before sample collection, order of draw when different sample types need to be collected, sample labelling, blood-to-anticoagulant ratio (tube filling) and influence of haematocrit. The following areas are excluded from this document, but are included in an associated ICSH document addressing processing of samples for coagulation tests in clinical laboratories: sample transport and primary tube sample stability; centrifugation; interfering substances including haemolysis, icterus and lipaemia; secondary aliquots-transport and storage; and preanalytical variables for platelet function testing. The recommendations are based on published data in peer-reviewed literature and expert opinion.

Factor VIII/IX inhibitor testing practices in the United Kingdom: Results of a UKHCDO and UKNEQAS national survey

INTRODUCTION

Inhibitor formation is the greatest challenge facing persons with haemophilia treated with factor concentrates. The gold standard testing methodologies are the Nijmegen-Bethesda assay (NBA) for FVIII and Bethesda assay (BA) for FIX inhibitors, which are affected by pre-analytical and inter-laboratory variability.

AIMS

To evaluate inhibitor testing methodology and assess correlation between self-reported and actual methodology.

METHODS

Methodology was evaluated using a survey distributed alongside a UK National External Quality Assessment Service Blood Coagulation external quality assurance (EQA) exercise for FVIII and FIX inhibitor testing.

RESULTS

Seventy four survey and EQA exercise responses were received (response rate 63.2%), with 50 paired survey/EQA results. 47.1% (33/70) reported using the NBA and 42.9% (30/70) the BA for FVIII inhibitor testing. Review of FVIII inhibitor assay methodology demonstrated discrepancy (self-reported to actual) in 64.3% (BA reporting) and 27.6% (NBA reporting). Pre-analytical heat treatment was used by 32.4%, most commonly 56°C for 30 minutes. Assay cut-offs of 0.1-1.0 BU/mL were reported. EQA samples (acquired FVIII and congenital FIX) demonstrated titres and coefficients of variation (CV) of 3.1 BU/mL (0.7-15.4 BU/mL; CV = 43%) and 18.0 BU/mL (0-117 BU/mL; CV = 33%), respectively. No significant assay or laboratory factors were found to explain this variance, which could have resulted in change in management for 6 patients (5 misclassified high-titre FVIII inhibitors and 1 false negative for a FIX inhibitor).

CONCLUSIONS

Heterogeneity was seen at each stage of assay methodology. No assay-related factors were found to explain variation in inhibitor titres. Further standardization is required to improve inhibitor quantification to guide patient care.

Chronometric vs. Structural Hypercoagulability

Prolonged tourniquet stasis induced by venepuncture can lead to the release of the plasma of cell lysis products, as well as tissue factor (TF), impairing the quality of coagulation test results. The accidental presence of TF in vitro can trigger the coagulation mechanism, generating a false decrease in prothrombin time (PT). Background and Objectives: Identification of short PT tests below the normal reference value that could suggest a situation of hypercoagulability. The study aimed to compare the results of the shortened PT tests at their first determination with the eventual correction following duplication of the analysis from the same sample. Materials and methods: Identification of the shortened PT tests has been carried out for a period of 4 months, upon 544 coagulation samples referred to the Hematology department of Sf. Spiridon County Clinical Emergency Hospital from Iasi, Romania. Results: Out of the 544 samples of which the results indicated a state of hypercoagulability, by repeating the determination from the same sample, for 200 (36.76%) PT tests (p = 0.001) the value was corrected, falling within the normal reference range. For 344 (63.24%) tests, the results suggested a situation of hypercoagulability. Conclusions: In order to guarantee the highest quality of the laboratory services, a proper interpretation and report of the patients' results must be congruent and harmoniously associated to the actual clinical condition of the patient. Duplication of the PT determination from the same sample would exclude situations of false hypercoagulability and would provide significant improvement for the patient's safety.

Coagulation mixing studies: Utility, algorithmic strategies and limitations for lupus anticoagulant testing or follow up of abnormal coagulation tests

Coagulation testing underpins the investigation of hemostasis and/or monitoring of anticoagulation therapy for prevention and/or treatment of thrombosis related pathology. Assessment of coagulation results requires comparison against a normal reference range or interval (NRR/NRI). Results flagged as "abnormal" (ie, above the NRR/NRI for patients not on anticoagulant therapy), typically require further evaluation. eg, follow up or reflexive testing is used to identify the reason for prolongation, especially when supported by clinical context (eg, bleeding). Mixing tests may have utility to help identify the pathway of follow-up testing (ie, towards investigation of factor deficiencies, or else inhibitors), and are also useful for investigation of lupus anticoagulants (LA). In general, mixing tests that "correct" tend to suggest the presence of factor deficiencies, where as those that do not correct suggest the presence of "inhibitors". Various approaches can be used to identify correction/non-correction, and all have strengths and limitations. Furthermore, eventual identification of causal factor deficiencies or even "inhibitors" may (eg, factor VIII or IX deficiencies or inhibitors) or may not (eg, factor XII deficiency) be clinically important. Ultimately, mixing studies performed in view of appropriate clinical scenarios (eg, bleeding patient) and for LA investigations in symptomatic patients will have best utility.

Post-analytical Issues in Hemostasis and Thrombosis Testing

Analytical concerns within hemostasis and thrombosis testing are continuously decreasing. This is essentially attributable to modern instrumentation, improvements in test performance and reliability, as well as the application of appropriate internal quality control and external quality assurance measures. Pre-analytical issues are also being dealt with in some newer instrumentation, which are able to detect hemolysis, icteria and lipemia, and, in some cases, other issues related to sample collection such as tube under-filling. Post-analytical issues are generally related to appropriate reporting and interpretation of test results, and these are the focus of the current overview, which provides a brief description of these events, as well as guidance for their prevention or minimization. In particular, we propose several strategies for improved post-analytical reporting of hemostasis assays and advise that this may provide the final opportunity to prevent serious clinical errors in diagnosis.

Diagnostic accuracy study of a factor VIII ELISA for detection of factor VIII antibodies in congenital and acquired haemophilia A

Antibody formation to factor VIII (FVIII) remains the greatest clinical and diagnostic challenge to the haemophilia-treating physician. Current guidance for testing for inhibitory FVIII antibodies (inhibitors) recommends the functional Nijmegen-Bethesda assay (NBA). A FVIII ELISA offers a complementary, immunological approach for FVIII antibody testing. It was the aim of this study to retrospectively evaluate the performance of a FVIII ELISA (index) for detection of FVIII antibodies, compared with the NBA (reference). All samples sent for routine FVIII antibody testing at two haemophilia Comprehensive Care Centres, were tested in parallel using the NBA and a solid-phase, indirect FVIII ELISA kit (Immucor). A total of 497 samples from 239 patients (severe haemophilia A=140, non-severe haemophilia A=85, acquired haemophilia A=14) were available for analysis. Sixty-THree samples tested positive by the NBA (prevalence 12.7 %, 95 % confidence interval [CI], 9.9–15.9 %), with a median inhibitor titre of 1.2 BU/ml (range 0.7–978.0). The FVIII ELISA demonstrated a specificity of 94.0 % (95 %CI, 91.3–96.0), sensitivity of 77.8 % (95 %CI, 65.5–87.3), negative predictive value of 96.7 % (95 %CI, 94.5–98.2), positive predictive value 65.3 % (95 %CI, 53.5–76.0), negative likelihood ratio 0.2 (95 %CI, 0.1–0.4), positive likelihood ratio 13.0 (95 %CI, 8.7–19.3) and a diagnostic odds ratio of 54.9 (95 %CI, 27.0–112.0). Strong positive correlation (r=0.77, p< 0.001) was seen between the results of the NBA (log adjusted) and FVIII ELISA optical density. In conclusion, FVIII ELISA offers a simple, specific, surveillance method enabling batch testing of non-urgent samples for the presence of FVIII antibodies.

Nonneutralizing antibodies against factor VIII and risk of inhibitor development in severe hemophilia A

Nonneutralizing antibodies against FVIII are detected in untreated or minimally treated patients with hemophilia A. The presence of nonneutralizing antibodies is associated with a substantially increased risk of inhibitor development.

Concurrent acquired inhibitors to factor VIII and IX, a laboratory artifact: a case report

Acquired inhibitors to coagulation factors other than factor VIII are extremely rare. We describe a case of a 59-year-old woman with abnormal bleeding, diagnosed with concurrent inhibitor antibodies to factor VIII and IX by Bethesda testing. We demonstrate that anti-FVIII antibodies of a very high titre are capable of disturbing the aPTT-based Bethesda assay, resulting in falsely-positive antibodies to factor IX. The case also illustrates the usefulness of the immunological assay (ELISA) in complementing the inhibitor diagnosis.

Pearls and pitfalls in factor inhibitor assays

The proper performance and interpretation of factor inhibitor assays is a critical role for the hemostasis laboratory. Both false-positive and false-negative inhibitor assays may be reported, leading to serious patient mismanagement. Knowledge and recognition of common causes of both false-positive and negative-results can aid in the identification of these potential pitfalls. Safeguards to reporting accurate factor inhibitor assays include initial characterization of the sample, using the Nijmegen modification, properly performing and interpreting an incubated mixing test in conjunction, and performing two dilutions for each dependent dilution in the factor inhibitor assay.

Acquired hemophilia: a case report and review of the literature

Acquired hemophilia A (AHA) is a rare bleeding disorder caused by autoantibodies against clotting factor VIII (FVIII). FVIII autoantibody is characterized as polyclonal immunoglobulin G directed against the FVIII procoagulant activity. This disease occurs most commonly in the elderly population and with preponderance of men in nonpregnancy-related AHA. There are well-established clinical associations with AHA such as malignancy, other autoimmune diseases and pregnancy. However, up to 50% of reported cases remain idiopathic. The clinical manifestation of AHA includes mostly spontaneous hemorrhages into skin, muscles and soft tissues, or mucous membranes. AHA should be suspected when a patient with no previous history of bleeding presents with bleeding and an unexplained prolonged activated partial thromboplastin time. The diagnosis is confirmed in the laboratory by the subsequent identification of reduced FVIII levels and FVIII inhibitor titration. There is a high mortality, making prompt diagnosis and treatment vitally important. The principles of treatment consist in controlling the bleeding and eradicating the inhibitor. Because of the overall high relapse rate (15-33%), it is also recommended to follow up these patients. The review summarizes what is currently known about the epidemiology, pathogenesis, clinical features, diagnosis, treatment and prognosis of AHA and starts with a case report.

Laboratory identification of factor inhibitors: an update

Coagulation factor inhibitors comprise antibodies that bind to and then neutralise specific pro-coagulant plasma proteins. Coagulation factor inhibitors can develop against any coagulation factor, although the most common are against factor VIII (FVIII). These can develop in individuals with inherited haemophilia A (HA) as an immune response to factor replacement therapy, or as auto-antibodies leading to the condition of acquired HA. Clinical suspicion for inhibitors may arise when individuals present with bleeding symptoms without any prior bleeding diathesis, or when a patient with known mild haemophilia presents with a bleeding diathesis more extreme to their usual presentation, or when there is failure of factor replacement therapy to arrest bleeding in a known haemophiliac. The laboratory identification of factor inhibitors requires a careful and systematic approach that excludes other possible causes of prolonged screening tests, most commonly the activated partial thromboplastin time (APTT), and sometimes prothrombin time (PT). Coagulation factor inhibitor studies, including the Bethesda assay, are then undertaken to measure inhibitor titre, which guides treatment. This paper overviews the laboratory investigation of factor inhibitors, and also briefly reviews recent cross-laboratory inhibitor studies and the most recent evidence related to differential inhibitor formation according to type of therapy.

Laboratory testing for the antiphospholipid syndrome: making sense of antiphospholipid antibody assays

The antiphospholipid syndrome (APS) is an autoimmune condition characterised by a wide range of clinical features (primarily thrombosis and/or obstetric related), associated with the presence of antiphospholipid antibodies (aPL) as detected by a diverse range of laboratory tests. APS remains a significant diagnostic challenge for clinicians across a wide range of specialities, largely due to issues related to laboratory testing as well as the expanding range of reported clinical manifestations of APS. The laboratory issues include limitations in detailed knowledge by both clinical and laboratory personnel regarding the 'complete' range of available aPL tests, as well as ongoing problems with assay reproducibility and standardisation. aPL are identified using diverse laboratory procedures based on one of two distinct test processes, namely solid phase and liquid phase assays. The former includes anticardiolipin antibodies (aCL) and anti-β(2)-glycoprotein I antibodies (aβ(2)GPI). The latter are centred on clot-based tests that are used to identify the so-called lupus anticoagulant (LA). This article will discuss: (i) issues related to laboratory testing for APS in terms of the currently available solid-phase and liquid-phase assays, and identifiable biases resulting from these tests usually being performed in different laboratories; (ii) current problems with calibration, standardisation and reproducibility of these assays; (iii) pre-analytical, analytical and post-analytical considerations and ongoing initiatives for improvement; (iv) issues related to potential combinations/panels of available aPL tests; and (v) the entities of seropositive APS, seronegative APS and non-APS aPL-positivity. In doing so, this review will hopefully help bridge the two disciplines of haematology and immunology ('representing' liquid-phase and solid-phase aPL testing, respectively), by improving the understanding of those working in each of these disciplines of the merits and limitations of the assays performed in the other discipline, and encouraging inter-discipline cooperation in the reporting of aPL test results.

Laboratory reporting of hemostasis assays: the final post-analytical opportunity to reduce errors of clinical diagnosis in hemostasis?

The advent of modern instrumentation, with associated improvements in test performance and reliability, together with appropriate internal quality control (IQC) and external quality assurance (EQA) measures, has led to substantial reductions in analytical errors within hemostasis laboratories. Unfortunately, the reporting of incorrect or inappropriate test results still occurs, perhaps even as frequently as in the past. Many of these cases arise due to a variety of events largely outside the control of the laboratories performing the tests. These events are primarily preanalytical, related to sample collection and processing, but can also include post-analytical events related to the reporting and interpretation of test results. The current report provides an overview of these events, as well as guidance for prevention or minimization. In particular, we propose several strategies for the post-analytical reporting of hemostasis assays, and how this may provide the final opportunity to prevent serious clinical errors in diagnosis. This report should be of interest to both the laboratory scientists working in hemostasis and clinicians that request and attempt to interpret the test results. Laboratory scientists are ultimately responsible for these test results, and there is a duty to provide both accurate and precise results to enable clinicians to manage patients appropriately and to avoid the need to recollect and retest. Also, clinicians will not be in a position to best diagnose and manage their patient unless they gain an appreciation of these issues.