[Acquired hemophilia A and emicizumab for the treatment of bleeding: two case report and a literature review]. / Hémophilie A acquise, le traitement par emicizumab peut relayer les agents by-passant : à propos de deux cas et une revue de la littérature.

Emicizumab is a bispecific antibody that mimics the function of factor VIII (FVIII) and is indicated for prophylactic use in patients with congenital hemophilia A with or without inhibitors. Acquired hemophilia A (AHA) is a rare and severe disorder causes by autoantibodies that inhibit FVIII. In AHA, acute bleeding are managed with bypassing agents but several reports described the off-label use of emicizumab. The aim of this article is to describe two cases of AHA treated with emicizumab and a review of the scientific littérature. Reports indicate that the use of emicizumab is efficacious to treat acute bleeding with less thrombotic events thant with bypassing agents and with a reduced hospitalisation duration. Nevertheless biological monitoring is more complicated with assay interferences and a persistent circulation more than 6 months after the last injection was observed for our two patients.

Acquired Hemophilia A Diagnosed Based on Gross Hematuria: A Case Report and Literature Review.

Acquired hemophilia A (AHA) is an acquired bleeding disorder caused by neutralizing antibodies (inhibitors) against Coagulation Factor VIII (FVIII:C), causing sudden hemorrhagic symptoms (i.e., subcutaneous bleeding, intramuscular bleeding, and hematuria). Herein, this study is aimed at presenting a case of AHA diagnosed based on hematuria and reviewing patients who were diagnosed with AHA due to hematuria. A 67-year-old woman was referred to Atsugi City Hospital with painless gross hematuria that began 4 weeks before presentation. Contrast-enhanced computed tomography (eCT) revealed an approximately 2 cm mass in the right renal pelvis, and the patient's activated partial thromboplastin time (APTT) was elevated (61.4 s). The day after the endoscopic biopsy, the patient was in shock due to a large retroperitoneal hematoma. Although her condition stabilized after intravenous radioembolization, she underwent emergency surgeries several times because of rebleeding within the next 3 weeks. At that time, APTT was more prolonged at 106.4 s, and the FVIII:C level was 2%. Mixing tests showed an upwardly convex curve after 2-h incubation, indicating the presence of an inhibitor. Factor VIII inhibitor titer was ≥5.1 Bethesda unit (BU)/mL. A combined product of Plasma-Derived Factors VIIa and X (pd-FVIIa/FX), as second-line hemostatic therapy, as well as cyclophosphamide (CYP), were administered after Recombinant Activated Factor VIIa (rFVIIa) had been ineffective. Following this, the Factor VIII inhibitor titer was undetectable, FVIII:C levels were restored, and APTT decreased to within the normal range. Gross hematuria was significantly alleviated. However, the patient died of cytomegalovirus and fungal infections due to prolonged immunosuppressive therapy. Although AHA diagnosed based on hematuria may have a better prognosis than others, there have been occasional cases with severe outcomes. APTT, detected upon initial hematological testing in patients with hematuria, may be a potential indicator of an existing AHA.

Efficacy and safety of recombinant porcine factor VIII in Japanese patients with acquired hemophilia A.

Acquired hemophilia A (AHA) is a rare bleeding disorder caused by autoantibodies inhibiting human factor VIII (hFVIII). This phase II/III open-label study evaluated the safety and efficacy of recombinant porcine factor VIII (rpFVIII, susoctocog alfa) in adults with AHA and severe bleeding episodes in Japan (NCT04580407). The initial rpFVIII dose was 200 U/kg, with subsequent doses based on clinical measures including plasma FVIII activity. The primary efficacy endpoint was the proportion of severe bleeding episodes with a positive response to rpFVIII therapy 24 h after treatment initiation. Five patients were eligible for, and completed, rpFVIII treatment (age group: 60s-80s; median hFVIII inhibitor: 52 BU/mL; porcine FVIII [pFVIII] inhibitor: 3/5 patients). The median (range) total dose/patient was 548.4 (198-1803) U/kg with a median 3.0 infusions/patient. All patients responded positively to rpFVIII therapy at 24 h regardless of baseline pFVIII inhibitor status. rpFVIII treatment was well tolerated with no adverse events of special interest such as thromboembolic events or de novo pFVIII inhibitors. This study supports the use of rpFVIII as a novel therapy in the clinical management of patients with AHA in Japan. rpFVIII was approved for treating bleeding episodes in adults with AHA in Japan in 2024.

A Rare Case of Hemophilia: Acquired Factor VIII Deficiency.

Unlike hereditary hemophilia, acquired hemophilia is a rare life-threatening bleeding disorder that occurs in a person who has no personal or family history of bleeding episodes. It usually presents with new-onset subcutaneous/joint/muscle bleeding causing ecchymoses and hematomas, hematuria, GI bleeding, retroperitoneal bleeding, or rarely acute intracranial hemorrhage in elderly individuals. The diagnosis involves assessing prolonged activated partial thromboplastin time (aPTT) and conducting mixing studies after ruling out lupus anticoagulants and interfering substances such as heparins. Management consists of controlling hemostasis using recombinant activated factor VII (rFVIIa) and activated prothrombin complex concentrates (aPCC), along with eradicating autoantibodies against factor VIII from the body system using immunosuppressive therapies. Due to the risk of uncontrolled bleeding in individuals who were previously normal, delayed diagnoses and recurrences are not uncommon, potentially resulting in unfavorable outcomes.

A Rare Case of Acquired Factor VIII Inhibition in an Elderly Female.

Acquired hemophilia A is a rare condition characterized by the development of autoantibodies against coagulation factor VIII. It often initially presents as serious bleeding in the absence of risk factors and carries high morbidity and mortality if not diagnosed early. Due to its rare nature, data is limited, and guidelines are primarily based on expert opinion. Here we present a case of an elderly patient with severe gastrointestinal bleeding found to have activated partial thromboplastin times, plasma mixing studies, and coagulation factor activity levels consistent with acquired hemophilia A. We hope to bring awareness of this rare disease and promote its consideration in the differential of unexpected bleeding to improve safety outcomes.

First-line immunosuppressive therapies for acquired hemophilia A: A 25-year cohort experience and network meta-analysis.

Acquired hemophilia A (AHA) presents a significant bleeding risk. Management involves bleeding control and immunosuppressive therapy (IST) to eliminate inhibitors. This study, encompassing a retrospective cohort of 76 newly diagnosed AHA patients (1997-2022), evaluated IST outcomes such as complete remission (CR), relapse, and mortality rates, alongside influencing factors. Supplementing these findings, a systematic review and network meta-analysis compared CR and relapse rates across ISTs, sourcing from Embase, Scopus, and ScienceDirect up to November 2023. In our cohort, demarcated by a 20 Bethesda-unit titer threshold, cyclophosphamide plus prednisolone (CP; n = 64) was the predominant initial IST. Lower inhibitor levels significantly correlated with higher CR rates (86.8 % vs 62.2 %; P = .014) and showed an odds ratio of 0.26 for CR (P = .021). Median relapse-free survival (RFS) extended to 37.13 months, significantly enhanced by CP (hazard ratio, 0.24; 95 % confidence interval, 0.10-0.60; P = .002). Our network meta-analysis, including 1476 CR and 636 relapse patients, indicated CP and rituximab-based ISTs significantly outperformed steroid monotherapy in terms of CR and lower relapse rates (risk differences of 0.15 and -0.13/-0.15, respectively; P < .05), without significant differences between CP and rituximab. Moreover, adding rituximab to the front-line treatment did not produce superior outcomes compared to the CP regimen alone, positioning CP as a viable first-line choice, particularly where rituximab is less accessible. The consideration of IST toxicity remains critical in treatment decisions.

Retrospective study of the clinical manifestations and efficacy of immunotherapy for patients with acquired hemophilia A.

OBJECTIVE: Raising awareness of acquired hemophilia A (AHA) and early diagnosis is critical to reduce the associated mortality rate. We aimed to characterize acquired hemophilia in Chinese patients and evaluate the effectiveness of immunotherapy. METHODS: The clinical characteristics, laboratory test data, therapeutic approaches, and outcomes of 20 patients with AHA who were admitted to Xi'an Central Hospital between January 2012 and December 2020 were retrospectively studied. RESULTS: Nine of the patients (45%) were treated by single glucocorticoid administration; three (15%) with cyclophosphamide (CP) in combination with a glucocorticoid; four individuals (20%) received a combination therapy of rituximab with CP and glucocorticoid or rituximab with CP, vincristine, and a glucocorticoid; three (15%) by injection of human immunoglobulin in combination with a glucocorticoid; and one (5%) with CP alone. Six patients (30%) achieved total remission and 11 (55%) partial remission (PR), but three (15%) did not enter remission, indicating an objective response rate of 85%. CONCLUSION: Combination therapy with rituximab or intravenous human immunoglobulin achieves superior results in some patients with AHA. Immunosuppression and the administration of coagulation factors can rapidly control the disease and are efficacious, but >50% of patients only achieved PR. These findings suggest that the complete elimination of inhibitors requires prolonged immunosuppression therapy.

Acquired Hemophilia A after SARS-CoV-2 Immunization: A Narrative Review of a Rare Side Effect.

Acquired hemophilia A (AHA) is a rare bleeding disorder (1.4 per million inhabitants per year) caused by neutralizing antibodies against factor VIII. Although uncommon, these autoantibodies can cause a high rate of morbidity and mortality. Several conditions are linked with AHA; based on an EACH2 study, 3.8% of AHA could be connected to infection. In the last four years, most humans have contracted the SARS-CoV-2 infection or have been vaccinated against it. Whether or not COVID-19 immunization might induce AHA remains controversial. This review aims to evaluate the evidence about this possible association. Overall, 18 manuscripts (2 case series and 16 case reports) were included. The anti-SARS-CoV-2 vaccination, as also happens with other vaccines, may stimulate an autoimmune response. However, older individuals with various comorbidities are both at risk of developing AHA and of COVID-19-related morbidity and mortality. Therefore, the COVID-19 vaccine must always be administered because the benefits still outweigh the risks. Yet, we should consider the rare possibility that the activation of an immunological response through vaccination may result in AHA. Detailed registries and prospective studies would be necessary to analyze this post-vaccine acquired bleeding disorder, looking for possible markers and underlying risk factors for developing the disease in association with vaccination.

Emicizumab as first-line therapy in acquired hemophilia A.

Acquired hemophilia A (AHA) is a rare autoimmune disease resulting from the development of autoantibodies directed against endogenous factor (F)VIII, leading to bleeding manifestations that can be life-threatening. The current standard hemostatic treatment involves the use of bypassing agents that circumvent FVIII (recombinant activated FVII, activated prothrombin complex concentrate, and recombinant porcine FVIII) that must be administered intravenously and possess a short half-life. These limitations and the risk of potentially fatal bleeding complications justify the early initiation of immunosuppressive treatment (IST) aimed at promptly eradicating the autoantibodies. IST is not without side effects, sometimes severe and possibly fatal, especially in persons with AHA who are generally older and have multiple comorbidities. Emicizumab, a bispecific antibody that mimics the action of FVIII, has emerged as an effective hemostatic therapy among persons with congenital hemophilia, whether complicated by the presence of anti-FVIII antibodies or not. Numerous arguments from recent clinical experiences suggest positioning emicizumab as a first-line treatment for AHA. This strategy has the potential to reduce bleeding complications and, importantly, the side effects associated with IST, which can be delayed and tailored to each patient.

[Novel treatment strategies for acquired hemophilia A].

Acquired hemophilia A (AHA) is a bleeding disorder caused by autoantibody (inhibitor) production targeting blood coagulation factor VIII (FVIII). It is characterized by sudden onset, and often causes extensive and severe bleeding in soft tissue. Acquired hemophilia A is diagnosed when coagulation tests show normal PT, prolonged APTT, decreased FVIII activity, normal VWF activity, and positive FVIII inhibitor. Hemostatic therapy mainly consists of bypass therapy, which activates the extrinsic coagulation pathway, bypassing the need for FVIII or factor IX. Emicizumab, a bispecific antibody that substitutes for FVIII function, can be used to prevent bleeding. Immunosuppressive therapy is necessary to suppress or eradicate inhibitors. The majority of patients go into remission with treatment, but some die from bleeding symptoms or infections associated with immunosuppressive therapy.

Case Report: Effective management of adalimumab-induced acquired hemophilia A with the CyDRI protocol.

Introduction: Acquired Hemophilia A (AHA) is a rare autoimmune disorder characterized by the emergence of inhibitors that specifically target coagulation Factor VIII, frequently resulting in severe bleeding episodes. Methods: We conducted a retrospective analysis of the medical records of a 68-year-old male patient who presented with adalimumab-induced AHA. Results: The patient received adalimumab, a tumor necrosis factor inhibitor antibody, as part of his treatment for rheumatoid arthritis. The patient's clinical journey, characterized by intense bleeding and coagulopathy, was effectively managed with the application of recombinant Factor VIIa (rFVIIa) and the CyDRi protocol. Discussion: The case emphasizes the importance of prompt coagulation assessment in patients with bleeding symptoms receiving disease-modifying therapy for rheumatoid arthritis that includes adalimumab therapy, considering the rare yet life-threatening nature of AHA. Additionally, this report provides an extensive review of the existing literature on drug-induced AHA, with a special emphasis on cases linked to immunomodulatory medications. Through this two-pronged approach, our report aims to enhance understanding and awareness of this severe complication among healthcare providers, promoting timely diagnosis and intervention.

Inhibitor eradication and treatment for acquired hemophilia A.

INTRODUCTION: Acquired hemophilia A (AHA) is a rare hemorrhagic autoimmune disorder characterized by autoantibodies against coagulation factor VIII (FVIII). In approximately half of the cases AHA does not recognize any cause (idiopathic form), while in the other cases it may be triggered by autoimmune disorders, cancers, drugs, infections, or pregnancy. Besides treating the underlying disorder, specific AHA treatment includes management of bleeding, if necessary, and inhibitor eradication. AREAS COVERED: This narrative review summarizes the main epidemiological, clinical, laboratory, and therapeutic characteristics of AHA. In particular, it is focused on the current therapeutic options for the inhibitor eradication, also showing the latest findings on the innovative therapies. A literature search strategy was performed, without temporal limits, through Medline and PubMed electronic databases. EXPERT OPINION: Various first-line and second-line immunosuppressive agents are currently available for the management of AHA. Among the latter, the anti-CD20 monoclonal antibody rituximab has been the object of intense research during the last years from investigators as innovative promising eradicating therapy for AHA. Preliminary data from the studies support the use of this drug as a first-line option for newly diagnosed AHA cases.

Acquired hemophilia A: a single-center study of 165 patients.

Background: Acquired hemophilia A (AHA) is a rare hemorrhagic disorder caused by factor (F)VIII inhibitors. The diagnosis and management of AHA remains challenging because of its rarity and heterogeneity. Objectives: To analyze the characteristics of AHA to enhance our understanding of this disease and identify effective treatment strategies. To analyze the characteristics of AHA to enhance our understanding of this disease and identify effective treatment strategies. Methods: Clinical features of 165 patients with AHA from a single center between July 1997 and December 2021 were retrospectively analyzed. Results: The median age of patients at diagnosis was 45 years. The median time to diagnosis was 30 days. All 165 patients experienced bleeding, with a median bleeding score (BS) of 4 (range, 2-12). Hemostatic therapy was administered to 129 (78.2%) patients. Bleeding control was achieved in 80.0% of patients who received prothrombin complex concentrate and in 92.3% of patients who were treated with recombinant activated FVII. Of the 163 patients who received immunosuppressive therapy, 80 (49.1%) received rituximab-based therapy with a 93.3% complete remission (CR) rate, 50 (30.7%) received steroids plus cyclophosphamide with an 85.0% CR rate, and 22 (13.5%) received steroids alone with an 82.4% CR rate. Six cases relapsed after a median duration of 330 days. Immunosuppressive therapy-related adverse events were reported in 17 patients. Seven deaths were recorded. FVIII inhibitor titer of ≥15 BU/mL and BS of ≥6 were identified as significantly poor prognostic factors for CR. Conclusion: Immunosuppressive therapies yield remarkably high response rates, with a CR rate exceeding 80%; notably, the regimen containing rituximab exhibits a CR rate of approximately 90%. FVIII inhibitor titer of ≥5 BU/mL and BS of ≥6 were poor predictors of CR in patients with AHA.

Combined life-threatening internal organ bleeding and postpartum hemorrhage associated with acquired hemophilia A.

Key Clinical Message: Acquired hemophilia A (AHA) can present as life-threatening bleeding during the postpartum period. Prompt treatment allows patients with AHA to achieve complete remission and have normal subsequent pregnancies. Abstract: Acquired hemophilia A (AHA) is a rare bleeding disorder caused by the production of autoantibodies against factor VIII (FVIII). AHA can present with severe bleeding, especially in postpartum patient. We report a 38-year-old woman who presented in an emergency department with severe postpartum hemorrhage 2 weeks after cesarean section. Her investigation showed an isolated prolongation of partial thromboplastin time (PTT), low factor VIII assay and a factor VIII inhibitor test, resulting in abnormal Bethesda units which consistent with AHA. This case report highlights the importance of early diagnosis and treatment of AHA. With timely and appropriate management, most patients can achieve a good outcome.

Relapse of Acquired Hemophilia A after COVID-19 Infection.

Acquired hemophilia A (AHA) is a rare disease in which an autoantibody causes bleeding by interacting with and inhibiting the coagulation activity of endogenous factor VIII (FVIII). Most cases of AHA are idiopathic; known causes include autoimmune diseases, malignant tumors, pregnancy, drugs, and viral infections. An 86-year-old man was diagnosed with AHA based on the following results: an activated partial thromboplastin time (aPTT) extension of 130.7 seconds, presence of an inhibitor pattern in a mixing study, an endogenous factor VIII (FVIII) level of <1%, and an FVIII inhibitor titer of >5.1 Bethesda units (BU). The activity of von Willebrand factor (vWF) was diminished (<10%), which was considered a complication of acquired von Willebrand syndrome (AVWS). The patient was started on prednisolone, and the inhibitor level eventually became negative. vWF values also became normal. However, 1 year later, he was hospitalized for treatment of coronavirus disease 2019 (COVID-19). Blood testing showed an aPTT extension of 110.5 seconds, FVIII level of 4%, and FVIII inhibitor titer of 0.8 BU; thus, a relapse of AHA was diagnosed. After administration of corticosteroid and remdesivir, he recovered from COVID-19 and AHA. The inhibitor level became negative on the 9th day of admission. Several studies have implicated COVID-19 infection and vaccination in AHA. We recommend that aPTT be measured when patients with AHA are infected with SARS-CoV2, to confirm AHA relapse.

Characteristics and outcome of a territory-wide cohort study of patients with acquired hemophilia A in Hong Kong.

INTRODUCTION: Acquired hemophilia A (AHA) is a rare bleeding disorder with destruction of factor VIII by autoantibodies. Comprehensive data for Chinese patients are lacking. Predictors of hospital stay have not been investigated. METHODS: A territory-wide review of patients diagnosed with AHA from January 1, 2012, to December 31, 2021 was performed by retrieving patients' information from an electronic database system in Hong Kong. RESULTS: Overall, 165 patients were included in this 10-year study, and the estimated incidence was 2.4 per million/year, which was higher than those reported from Caucasian cohorts. The median age of diagnosis was 80 years old. Patients had a long hospital stay (median: 25 days) and high mortality (55.2 %). The majority of deaths were caused by immunosuppression-related sepsis (49.5 %). Age was an independent predictor of overall survival (Hazard ratio: 1.065, 95 % CI: 1.037-1.093, p < 0.001), complete remission (CR) status (odd ratios (OR): 0.948, 95 % CI: 0.921-0.976, p < 0.001) and time to achieve CR (OR: 1.043, 95 % CI: 1.019-1.067, p < 0.001). Higher hemoglobin level on presentation was associated with shorter time to achieve CR (OR: 0.888, 95 % CI: 0.795-0.993, p = 0.037). Factor VIII level < 1 % normal, high inhibitor titer and intensive immunosuppressive regimen predicted long hospital stay. CONCLUSION: We presented comprehensive data of Chinese patients with AHA which comprised predominantly frail elderly who required long hospital stay and had high sepsis-related mortality. This posed challenges in managing AHA in such patients. Individualized immunosuppressive therapy is needed to balance the benefits and risk of septic complications.

Key Genes of the Immune System and Predisposition to Acquired Hemophilia A: Evidence from a Spanish Cohort of 49 Patients Using Next-Generation Sequencing.

Acquired hemophilia A (AHA) is a rare bleeding disorder caused by the presence of autoantibodies against factor VIII (FVIII). As with other autoimmune diseases, its etiology is complex and its genetic basis is unknown. The aim of this study was to identify the immunogenetic background that predisposes individuals to AHA. HLA and KIR gene clusters, as well as KLRK1, were sequenced using next-generation sequencing in 49 AHA patients. Associations between candidate genes involved in innate and adaptive immune responses and AHA were addressed by comparing the alleles, genotypes, haplotypes, and gene frequencies in the AHA cohort with those in the donors' samples or Spanish population cohort. Two genes of the HLA cluster, as well as rs1049174 in KLRK1, which tags the natural killer (NK) cytotoxic activity haplotype, were found to be linked to AHA. Specifically, A*03:01 (p = 0.024; odds ratio (OR) = 0.26[0.06-0.85]) and DRB1*13:03 (p = 6.8 × 103, OR = 7.56[1.64-51.40]), as well as rs1049174 (p = 0.012), were significantly associated with AHA. In addition, two AHA patients were found to carry one copy each of the low-frequency allele DQB1*03:09 (nallele = 2, 2.04%), which was completely absent in the donors. To the best of our knowledge, this is the first time that the involvement of these specific alleles in the predisposition to AHA has been proposed. Further molecular and functional studies will be needed to unravel their specific contributions. We believe our findings expand the current knowledge on the genetic factors involved in susceptibility to AHA, which will contribute to improving the diagnosis and prognosis of AHA patients.

[Rapid diagnosis of acquired hemophilia A with ClotPro viscoelastometry analyser]. / A szerzett haemophilia A gyors diagnosztikája ClotPro viszkoelasztometriás készülékkel.

Acquired hemophilia A is a rare condition with the capability of bringing about life-threatening bleeding in the perioperative period, posing a significant challenge for the caregiver anesthetist to identify the underlying cause. However, a quick diagnosis might be supported by viscoelastometry by raising the suspicion of severe and isolated deficiency of the intrinsic coagulation pathway, requiring a prompt consultation with a hematology center. Special laboratory tests of hemostasis are helpful in the differential diagnosis of the detected coagulation disorder. Nevertheless, bypassing agents have gained a crucial role in the treatment of major perioperative blood losses by bypassing Factor VIII inactivated by autoantibodies and thus, initiating coagulation. Early goal-directed supplementation of depleted coagulation factors must also be kept in the focus of the therapy. Orv Hetil. 2023; 164(40): 1600-1604.

Acquired Hemophilia A: Rare Cause of Upper Airway Hematoma and a Literature Review.

BACKGROUND: Acquired hemophilia A (AHA) is a rare blood disorder with high morbidity and even mortality as severe bleeding can occur in up to 90% of affected patients. Unlike congenital hemophilia which presents with intra-articular bleeding, acquired hemophilia causes bleeding into the skin, muscle, mucous membranes and soft tissues. CASE PRESENTATION: We report an unusual case of upper airway hematoma in a 61-year-old man who presented with acute onset dysphagia and shortness of breath. There were bruises on his neck and blood clots on the floor of mouth when he was examined. Endoscopic examination revealed an extensive laryngeal hematoma. A prolonged activated partial thromboplastin time (APTT) prompted us to investigate for factor VIII deficiency leading to the diagnosis of AHA. He recovered completely after a treatment regime instituted by the hematology team without suffering any grave debilitating events. CONCLUSION: AHA with laryngeal hematoma is a rare condition with only a handful of cases reported. Although a life-threatening disease, it is easily reversed with early recognition and administration of medical therapy involving the hematology team.

The Nijmegen ultra-sensitive Bethesda Assay detects very low-titer factor VIII inhibitors in patients with congenital and acquired hemophilia A.

BACKGROUND: An inhibitor can develop in congenital hemophilia A (HA) patients against exogenous infused factor (F)VIII, whereas in acquired HA (AHA) inhibitors initially develop against endogenous FVIII. Inhibitors can be detected with the Nijmegen Bethesda Assay (NBA), which has an international cut-off level of 0.60 Nijmegen Bethesda Units/mL (NBU/mL). Thereby, very low-titer inhibitors may remain undetected. AIM: To describe the design and validation of the Nijmegen ultra-sensitive Bethesda Assay (NusBA) for the detection of very low-titer inhibitors. METHODS: The NusBA is a modification of the NBA in which the ratio of patient plasma to normal pooled plasma is changed from 1:1 to 9:1. Analytical validation was performed according to the CLSI EP10 guideline in order to determine trueness and reproducibility. Clinical validation was performed in two cohorts of congenital HA patients (82 adults) with pharmacokinetic data and four AHA patients. The limit of quantitation (LOQ) was determined by measuring plasma samples spiked with inhibitor levels in the low range (0.05-0.80 NBU/mL). RESULTS: The LOQ for the NusBA was 0.10 NusBU/mL, with a coefficient of variation of 24.2 %. Seven (8.5 %) congenital HA patients had a positive NusBA result, of which only one was detected with the NBA. There was no correlation between NusBA and FVIII half-life. In three of the AHA patients the NusBA remained positive, when the NBA became negative. DISCUSSION: The NusBA is able to detect very low-titer FVIII inhibitors of ≥0.10 NBU/mL. Thereby, it may have added value in early inhibitor detection and therapy adjustments in patients with congenital HA and AHA.