The role of the laboratory in diagnosis and management of inhibitory antibodies in haemophilia.

The treatment of patients with haemophilia A has remarkably improved over the years and the journey to a potential cure continues. Replacement therapy has been the cornerstone of treatment, and, despite major advances, the development of neutralizing antibodies, eg inhibitors, has thus far not been possible to avoid. How, and to what extent, the new non-factor-based options will modify treatment strategy and inhibitor risk is unclear and it is essential that every haemophilia treatment centre is linked to an educated and skilled laboratory performing the relevant inhibitor assays. The current review will focus on when to consider and how to perform inhibitor assay(s) in the management of patients with haemophilia A.

Discrepancies between the one-stage clotting assay and the chromogenic assay in haemophilia B.

INTRODUCTION: Assay discrepancy in factor VIII activity between the one-stage and the chromogenic assays has been described in approximately one third of patients with non-severe haemophilia A. Whether assay discrepancy may also occur in patients with haemophilia B remains unknown. AIM: This study compared the results from the one-stage and the chromogenic assays in patients with haemophilia B. METHODS: Plasma samples from patients with haemophilia B attending the haemophilia centre in Malmö, Sweden, were collected after a wash-out period of more than 7 days and analysed with both assays. RESULTS: Fifty samples from 36 patients were analysed. No discrepancy was found in patients with severe haemophilia B. Among the 44 plasma samples from patients with non-severe disease, 15 showed a twofold or greater difference between the results of the two methods, with the chromogenic method presenting the higher value (mean FIX:Cone-stage 0.02 vs. FIX:Cchromo 0.06 IU mL-1 ). Of these 15 samples, 14 were from seven individuals from five families with the same mutated amino acid at the N-terminal cleaving site of the activation peptide (FIX: c.572G>A; p.Arg191His or FIX: c.571C>T; p.Arg191Cys). These mutations were not observed in any patients with non-discrepant results. The reported bleeding frequency for these patients was low and indicative of a mild bleeding phenotype. CONCLUSION: Our findings imply that assay discrepancy occurs for factor IX activity and that both type of assays are needed for a correct diagnosis and classification of haemophilia B. The underlying mechanism by which the mutation influences the assays remains to be determined.

Surgery and survival in birth cohorts with severe haemophilia and differences in access to replacement therapy: The Malmö experience.

BACKGROUND: Persons with severe haemophilia require lifelong replacement therapy, prophylaxis, to prevent bleeding. Data describing long-term outcomes of prophylactic treatment are scarce. The aim of this study was to investigate joint surgery and survival among persons with severe haemophilia with special attention to access to prophylaxis in the early years of life. METHODS: Eligible participants had severe haemophilia A or B and were treated at the Malmö centre from the 1960s onward. Time from birth until joint surgery was analysed for participants negative for factor inhibitor and alive in 2000. We compared survival among the entire cohort with severe haemophilia treated at the Malmö centre with the general male population of Sweden and a sample of persons with severe haemophilia from the United Kingdom (UK). RESULTS: Overall, 167 participants were included, 106 (63.5%) of whom had complete data on joint surgery. Among those born before 1970, 1970-1979 and ≥1980 approximately 37%, 21% and 0% had their first joint surgery by age 30, respectively. There were no second joint surgeries reported in cohorts born ≥1970. Persons with severe haemophilia and negative for HIV treated in Malmö have attained approximately similar survival to that of the general male population in Sweden and live slightly longer than persons with severe haemophilia from the UK. DISCUSSION AND CONCLUSION: Prophylaxis in Sweden, although costly, has markedly improved survival and joint outcomes for persons with severe haemophilia. This study highlights the importance of early start of replacement therapy to prevent or postpone serious joint damage.

Comparative burden of arthropathy in mild haemophilia: a register-based study in Sweden.

INTRODUCTION: Mild haemophilia is a congenital bleeding disorder affecting males. The burden of arthropathy in mild haemophilia has not been comprehensively described. AIM: The aim of this study was to compare the incidence, age at diagnosis and surgery for arthropathy and related hospitalizations between people with mild haemophilia and the general population in Sweden. METHODS: This was a register-based cohort study. Eligible participants were those with mild haemophilia born between 1941 and 2008 and a randomly selected, birthdate and sex-matched comparison group from the general population. Follow-up was from birth (or earliest 1984) until death, emigration or end of the study in 2008. Data on arthropathy were obtained from a national patient register. Negative binomial and competing risk regression and Kaplan-Meier estimate curves were used in the analysis. RESULTS: Overall, 315 people with haemophilia and 1529 people in the comparison group were included. Participants with haemophilia born between 1984 and 2008 had a ninefold (95% CI: 3.3-27.2) and 16-fold (95% CI: 6.7-36.5) increased incidence of arthropathy-related hospital admission and arthropathy diagnosis respectively. None in this cohort underwent surgery. Among participants with haemophilia born prior to 1984, the rates of arthropathy diagnosis and surgery of the index joints (knee, elbow, ankle) were increased twofold (95% CI: 1.0-3.2) and fivefold (95% CI: 1.7-17.8) respectively. CONCLUSION: Our data suggested a higher burden of arthropathy among individuals with mild haemophilia compared to the general population. Further research should investigate the need for targeted joint screening programmes among individuals with mild haemophilia.

Anti-factor VIII antibodies in brothers with haemophilia A share similar characteristics.

INTRODUCTION: The development of neutralizing antibodies (inhibitors) against coagulation factor VIII (FVIII) is currently the most serious complication for patients with haemophilia A undergoing FVIII replacement therapy. Several genetic factors have been acknowledged as risk factors for inhibitor development. AIM: To analyze the influence of genetic factors on the nature of the humoral immune response to FVIII in eight brother pairs with inhibitors. METHODS: The domain specificity of FVIII-specific IgG was analysed by antibody binding to FVIII fragments and homologue-scanning mutagenesis (HSM). The FVIII-specific IgG subclasses were measured by direct ELISA. RESULTS: Of the 16 patient analysed with both methods, 12 had A2- and 13 had C2-specific IgG. The presence of A1-, A3- or C1-specific IgG was identified in nine of 14 patients analysed by HSM. IgG1, IgG2 and IgG4 subclasses contributed to the anti-FVIII IgG response, and the amount of FVIII-specific IgG1 (r = 0.66) and IgG4 (r = 0.69) correlated significantly with inhibitor titres. Patients with high concentrations of total anti-FVIII IgG (r = 0.69) or high inhibitor titres (r = 0.52) had a high proportion of FVIII-specific IgG4. Statistical analysis revealed trends/evidence that the subclass distribution (P = 0.0847) and domain specificity to HC/LC (P = 0.0883) and A2/C2 (P = 0.0011) of anti-FVIII IgG were more similar in brothers compared to unrelated subjects. CONCLUSION: Overall, our data provide a first hint that anti-FVIII IgG characteristics are comparable among haemophilic brothers with inhibitors. Whether genetic factors also influence the nature of patients' antibodies needs to be confirmed in a larger study population.

The association between health utility and joint status among people with severe haemophilia A: findings from the KAPPA register.

INTRODUCTION: People with severe haemophilia A have reportedly impaired health related quality of life (utility) mainly due to recurrent bleeding, arthropathy and treatment burden. AIM: To estimate utilities and evaluate their potential correlates - most importantly the joint status - among people with severe haemophilia A. METHODS: In this cross-sectional study, eligible participants had severe haemophilia A, were aged ≥15, negative for factor VIII inhibitor and included in the KAPPA register of Denmark, Norway and Sweden. Data on demographics, treatment history, haemophilia joint health score, and EQ-5D utility were obtained from the register. We used box plots to present utilities and joint status and ordinary least squares regression to evaluate correlates of utilities. Participants were consecutively enrolled in the KAPPA register between April 2013 and June 2016. RESULTS: Overall, 173 participants with median age of 34 (interquartile range: 25-45) were included. Twelve (6.9%) participants were on episodic treatment while 161 (93.1%) were treated using prophylaxis. Concomitant diseases and positive inhibitor history were reported for 73 (43.2%) and 21 (12.1%) participants, respectively. The highest median utility (1.0) was observed among those aged <29 on prophylaxis and those aged 30-44 who had started prophylaxis by age 3. In the multi-variable regression, joint scores of 16-25 (Coef. -0.18, 95% CI: -0.30, -0.06), 26-35 (Coef. -0.21, 95% CI: -0.36, -0.06) and >35 (Coef. -0.37, 95% CI: -0.52, -0.23) were associated with lower utilities. CONCLUSION: Moderate to severe joint manifestations are associated with reduced utilities among persons with severe haemophilia A.

A longitudinal study of family structure in Swedish persons with haemophilia.

Haemophilia is an X-linked inherited rare bleeding disorder affecting mainly men. The treatment consists of replacement therapy that has been associated with severe side effects, such as blood transmitted viral infections, but has markedly improved over the last decades. The aim of this study was to study family structure over time among Swedish persons with haemophilia (PWH), focusing on children, siblings and marital status. PWH A or B were identified from the haemophilia centres and the national Patient Registry. Each PWH was compared to five age- and gender-matched controls. The national Multi-Generation Registry was used to identify children and siblings. A total of 1365 children with a father suffering from haemophilia A or B and 1938 siblings of the PWH were identified. Having one or more children was significantly less common (P = 0.003) for PWH than for controls. Significantly lower rates of having a child were also found for the subgroups of persons suffering from severe haemophilia and those infected with HIV (P < 0.001). A higher proportion of PWH, with or without HIV and/or viral hepatitis had siblings compared to the controls (P < 0.001). However, the mean number of siblings was significantly lower for persons with severe haemophilia (P = 0.001). The number of marriages and divorces did not differ between PWH and controls. Our data indicate a negative impact of HIV and viral hepatitis on family structure for PWH despite the relatively good access to treatment in Sweden over the last few decades. This was particularly true for those with a severe form of haemophilia.

Managing haemophilia for life: 4th Haemophilia Global Summit.

The 4th Haemophilia Global Summit was held in Potsdam, Germany, in September 2013 and brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. The programme was designed by an independent Scientific Steering Committee of haemophilia experts and explored global perspectives in haemophilia care, discussing practical approaches to the optimal management of haemophilia now and in the future. The topics outlined in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care globally. In this supplement from the meeting, Jan Astermark reviews current understanding of risk factors for the development of inhibitory antibodies and discusses whether this risk can be modulated and minimized. Factors key to the improvement of joint health in people with haemophilia are explored, with Carlo Martinoli and Víctor Jiménez-Yuste discussing the utility of ultrasound for the early detection of haemophilic arthropathy. Other aspects of care necessary for the prevention and management of joint disease in people with haemophilia are outlined by Thomas Hilberg and Sébastian Lobet, who highlight the therapeutic benefits of physiotherapy and sports therapy. Riitta Lassila and Carlo-Federico Perno describe current knowledge surrounding the risk of transmission of infectious agents via clotting factor concentrates. Finally, different types of extended half-life technology are evaluated by Mike Laffan, with a focus on the practicalities and challenges associated with these products.

Switching clotting factor concentrates: considerations in estimating the risk of immunogenicity.

The development of neutralizing antibodies to factor VIII (FVIII) is the most serious complication of therapy for haemophilia A. There is now excellent documentation that a large number of both genetic and environmental factors contribute to the risk of FVIII inhibitor incidence. One of the environmental factors that has been proposed as an influence on this complication is the occurrence of FVIII product switching. There are only a small number of clinical studies that have addressed this question, and thus, the amount of objective information available to assess this association is limited. In this review, in addition to summarizing past evidence pertinent to this subject, we present the results of a complementary strategy, a Delphi analysis, to add to the considerations of product switching and FVIII immunogenicity. With the imminent arrival in the clinic of several new FVIII products, the haemophilia community must be prepared to collect prospectively controlled data to better address this important management issue.

Antibody formation and specificity in Bethesda-negative brother pairs with haemophilia A.

Antibodies directed towards non-neutralizing epitopes on the factor VIII protein (FVIII) may be detected in patients with haemophilia A. We evaluated the prevalence of non-neutralizing antibodies, in 201 inhibitor-negative brother pairs with severe haemophilia A, enrolled in the Malmö International Brother Study and the Haemophilia Inhibitor Genetics Study. To evaluate binding specificity of the antibodies, ELISA plates were coated with two recombinant full-length (FL) FVIII-products and one recombinant B-domain-deleted (BDD) product. Seventy-nine patients (39.3%) had a history of positive inhibitor titre measured by Bethesda assay, and FVIII antibodies were detected in 20 of them (25.3%). Additional 23 samples from subjects without a history of FVIII inhibitors were ELISA-positive corresponding to a frequency of non-neutralizing antibodies of 18.9%. The antibody response towards the different FVIII products was heterogenous, and was raised not only towards the non-functional B-domain but also towards both FL-rFVIII and BDD-rFVIII. In patients considered successfully treated with immune tolerance induction, 25.4% had remaining FVIII antibodies. The number of families with an antibody response in all siblings was increased when the total antibody response was taken into account, further supporting the concept of a genetic predisposition of the immune response. Further studies and careful monitoring over time are required to appreciate the immune response on the risk of inhibitor development or recurrence in the future.

Incidence, mortality rates and causes of deaths in haemophilia patients in Sweden.

Sweden has been a pioneer in the treatment of haemophilia, with the first concentrate available in the 1950s. Treatment has improved over the years to its current state-of-the art. The aim of the current study was to evaluate the long-term outcome of haemophilia in terms of incidence, morbidity and mortality. Patients diagnosed with haemophilia A or B registered at the national haemophilia centres and/or the Patient Registry and born before 2009 and alive in 1968 were enrolled and linked to the Cause of Death-, Migration- and Medical Birth registries. Five age- and sex-matched controls were selected for each patient. A total of 1431 patients with haemophilia A or B were compared with 7150 controls. The 3-year moving average incidence rate per 100,000 population varied between 21 and 36. The hazard ratio for all-cause mortality compared with controls was 2.2, 95% CI: [1.8; 2.7], P < 0.001 for the entire group of patients and 1.7, 95% CI: [1.3; 2.2], P < 0.001 when patients with HIV and/or viral hepatitis were excluded. The corresponding figures for the severe haemophilia subgroup were 6.6, 95% CI: [4.5; 10.0], P < 0.001 and 8.2, 95% CI [3.2; 20.8], P < 0.001 respectively. The most common causes of death were related to malignancies and the haemostatic defect. People with haemophilia were 57% less likely to die from ischaemic heart disease than controls. People with haemophilia in Sweden demonstrate higher mortality over time, independent of HIV and viral hepatitis, despite relatively advantageous access to clotting factor concentrates.

F8 haplotype and inhibitor risk: results from the Hemophilia Inhibitor Genetics Study (HIGS) Combined Cohort.

Ancestral background, specifically African descent, confers higher risk for development of inhibitory antibodies to factor VIII (FVIII) in haemophilia A. It has been suggested that differences in the distribution of FVIII gene (F8) haplotypes, and mismatch between endogenous F8 haplotypes and those comprising products used for treatment could contribute to risk. Data from the Hemophilia Inhibitor Genetics Study (HIGS) Combined Cohort were used to determine the association between F8 haplotype 3 (H3) vs. haplotypes 1 and 2 (H1 + H2) and inhibitor risk among individuals of genetically determined African descent. Other variables known to affect inhibitor risk including type of F8 mutation and human leucocyte antigen (HLA) were included in the analysis. A second research question regarding risk related to mismatch in endogenous F8 haplotype and recombinant FVIII products used for treatment was addressed. Haplotype 3 was associated with higher inhibitor risk among those genetically identified (N = 49) as of African ancestry, but the association did not remain significant after adjustment for F8 mutation type and the HLA variables. Among subjects of all racial ancestries enrolled in HIGS who reported early use of recombinant products (N = 223), mismatch in endogenous haplotype and the FVIII proteins constituting the products used did not confer greater risk for inhibitor development. Haplotype 3 was not an independent predictor of inhibitor risk. Furthermore, our findings did not support a higher risk of inhibitors in the presence of a haplotype mismatch between the FVIII molecule infused and that of the individual.

Prevention and prediction of inhibitor risk.

Studies of determinants of the development of inhibitory antibodies in patients with haemophilia indicate that this is a complex process involving several factors. The foundation is characterized by the T- and B-cell repertoire and the antigen presenting cells and to elicit an immune response to the deficient factor, a pre-disposing foundation is needed. Hence, in the absence of a certain set of circumstances, there will be no risk for development of inhibitors. Conversely, in patients fundamentally at risk, genetic and non-genetic factors might add to the risk. These factors may be additive or interactive, and ultimately promote or counteract the immune reaction by modifying immune regulators and the cytokine profile in an individual. In some subjects, only minor inflammatory signals might be needed, whereas in others a more pronounced pro-inflammatory state will be required. Regarding genetic markers other than the type of mutation and the HLA class II molecules, polymorphisms in various immune regulatory genes have been associated with inhibitor risk. These associations have not, however, been consistent across all patient groups. The reason for this is not clear, but could be related to study design or statistical power, family relationships among those studied, the complexity of interacting molecules and ethnic genomic variation. The Hemophilia Inhibitor Genetics Study (HIGS) has identified additional candidates within the intracellular pathways, all of which require additional evaluation to be fully appreciated. In the case of non-genetic factors, the overall view is that immune system challenges might add to the risk. HIGS data suggest that it will be possible to calculate a genetic score to identify patients at high risk for inhibitor development before the start of treatment. By doing so, it may hopefully be possible in the future to prevent the formation of inhibitors in these patients by offering therapeutic options other than the native factor VIII or IX molecule in an inflammatory setting.

Malignant disease in the haemophilic population: moving towards a management consensus?

The Malignancy in Haemophilia Workshop Group convened a consensus working group of haematologists and oncologists to review topics related to malignancy in haemophilia. The treatment of malignant disease in this population is increasingly relevant as both outcome and lifespan continue to improve. Although adequate guidance exists for control of spontaneous bleeding episodes and of haemostasis in general surgery, information for management of haemostasis in patients with various malignancies is sparse. To date, no clinical guidelines exist for management of complex bleeding problems, diagnosis, therapy and follow-up of malignancies in haemophilia. Furthermore, it remains unclear whether or not morbidity and mortality outcomes associated with malignancies are affected by haemophilia or by its treatment. Through presentation of five malignancies - prostate cancer, colorectal cancer, acute leukaemia, bladder cancer and hepatocellular carcinoma - important issues are highlighted, such as risk from bleeding as a symptom of malignancy; risks from invasive screenings and how these should be handled in haemophilic individuals; the implications of chemotherapy and treatment schedules, bone marrow suppression, radiotherapy, or surgery; and the likelihood of an interaction between treatment for haemophilia and malignancy outcomes. Ultimately, the aim is to establish consensus guidelines to direct and harmonize future treatment policy for malignant disease in the haemophilic population.

Daily dosing prophylaxis for haemophilia: a randomized crossover pilot study evaluating feasibility and efficacy.

Regular replacement therapy (prophylaxis) for haemophilia has been shown to prevent development of disabling arthropathy and to provide a better quality of life compared to treatment on demand; however, at a substantially higher cost. Calculations based on pharmacokinetic principles have shown that shortening dose intervals may reduce cost. The aim of this prospective, randomized, crossover pilot study was to address whether daily dosing is feasible, if it reduces concentrate consumption and is as effective in preventing bleeding as the standard prophylactic dosing regimen. In a 12+12 month crossover study, 13 patients were randomized to start either their own previously prescribed standard dose, or daily dosing adjusted to maintain at least the same trough levels as obtained with the standard dose. Ten patients completed the study. A 30% reduction in cost of factor concentrates was achieved with daily prophylaxis. However, the number of bleeding events increased in some patients in the daily dosing arm and patients reported decreased quality of life during daily prophylaxis. Daily treatment had a greater impact on daily life, and the patients found it more stressful.Prophylaxis with daily dosing may be feasible and efficacious in some patients. A substantial reduction of factor consumption and costs can be realized, but larger studies are needed before the introduction of daily prophylaxis into clinical routine can be recommended.

Recommendations for assessment, monitoring and follow-up of patients with haemophilia.

Over the last few decades, clinical follow-up of patients with haemophilia has become more complex as a result of the introduction of new treatment strategies, the presence of comorbidities related to haemophilia or ageing, as well as the emergence of new tools to evaluate the medical and social consequences of haemophilia. This publication describes the parameters and information that should be documented and the tests, examinations and interventions required for optimal follow-up of a patient with haemophilia. In the absence of formal studies, the present recommendations have been established as result of a series of consensus meetings in the frame of the European Haemophilia Therapy Standardization Board (EHTSB). The following 11 domains were identified: Baseline information, Current status, Treatment, Inhibitor status, Bleeding, Joint status and pain, Comorbidities, Dental care, Physical activities, Social participation and Quality of life. For each domain, details are proposed for the relevant parameters to be captured and monitored as well as the relevant tools that facilitate data collection. Adopting these recommendations should help the individual care of patients and, even though this is not the primary objective of this article, it should also help at national and international level to shape a new approach to haemophilia by working towards a more standardized outcome assessment. Greater standardization should have implications for data collection, improvements in treatment evaluation and optimizing resources.

Treatment of haemophilia A and B and von Willebrand's disease: summary and conclusions of a systematic review as part of a Swedish health-technology assessment.

In an ongoing health-technology assessment of haemophilia treatment in Sweden, performed by the governmental agency Dental and Pharmaceutical Benefits Agency (TLV; tandvårds-och lakemedelsförmånsverket), the Swedish Council on Health Technology Assessment (SBU; statens beredning för medicinsk utvardering) was called upon to evaluate treatment of haemophilia A and B and von Willebrand's disease (VWD) with clotting factor concentrates. To evaluate the following questions: What are the short-term and long-term effects of different treatment strategies? What methods are available to treat haemophilia patients that have developed inhibitors against factor concentrates? Based on the questions addressed by the project, a systematic database search was conducted in PubMed, NHSEED, Cochrane Library, EMBASE and other relevant databases. The literature search covered all studies in the field published from 1985 up to the spring of 2010. In most instances, the scientific evidence is insufficient for the questions raised in the review. Concentrates of coagulation factors have good haemostatic effects on acute bleeding and surgical intervention in haemophilia A and B and VWD, but conclusions cannot be drawn about possible differences in the effects of different dosing strategies for acute bleeding and surgery. Prophylaxis initiated at a young age can prevent future joint damage in persons with haemophilia. The available treatment options for inhibitors have been insufficiently assessed. The economic consequences of various treatment regimens have been insufficiently analysed. Introduction of national and international registries is important.

Bone density and health-related quality of life in adult patients with severe haemophilia.

Severe haemophilia and reduced bone density can negatively influence perception of patient's health-related quality of life (HRQoL), especially considering future aspects, the risk of losing independence or pain suffering. The aim of this study was to assess levels of HRQoL in severe haemophilia patients and to compare HRQoL to those of the general population as well as to determine whether reduced bone density is correlated to the perceived HRQoL. Patients were divided into two groups based on timing of being treated with prophylaxis: Group A (started prophylaxis at age of ≤ 3 years; n = 22); Group B (at age of >3 years; n = 15). The bone mineral density (BMD g cm(-2)) of different measured sites was measured by dual energy X-ray absorptiometry (DXA). HRQoL was assessed using SF-36 questionnaire. Group A have mean BMD T-score >-1.0 (i.e. normal score) at all measured sites, and have almost similar scores in the SF-36 domains compared with the reference population. Group B have mean BMD T-score <-1.0 at hip region, and >-1.0 at lumbar spine and total body, and their scores in the SF-36 domains were lower compared with the reference population. Moreover, significant correlations were found between BMD at femoral neck and total body with physical domains. With adequate long-term prophylaxis since early childhood, adult patients with haemophilia report a comparable BMD and HRQoL to the Swedish reference population. Reduced BMD in group B correlated with impaired physical health, which underscores the importance of early onset of adequate prophylactic treatment.

Inhibitor development: patient-determined risk factors.

The reasons that inhibitory factor VIII antibodies develop in only a fraction of patients with haemophilia A remain unclear, but studies of genetically related subjects have indicated that the immunological outcome of replacement therapy is to a large extent determined by patient-related risk factors. Non-genetic factors will also influence the inhibitor risk, since events challenging the immune system will elicit and stimulate immune regulatory processes with the potential of modifying the immune response. Further insight into the immunological pathways and risk factors involved will be important in order to better predict and prevent this complication. This review will briefly summarize the data obtained to date in unrelated and related subjects in the Malmö International Brother Study (MIBS) regarding genetic factors and discuss how these factors might interact with non-genetically determined factors and events.

Thrombin generation in vitro in the presence of by-passing agents in siblings with severe haemophilia A.

Previous data have shown an inter-individual difference in the thrombin generating capacity in vitro as well as phenotypic bleeding pattern among patients with severe haemophilia A (FVIII:C activity below 1%). The reason for this is not known. In addition, there are no reports on how thrombin generation may correlate between siblings. In this study, we evaluated and compared thrombin generation in vitro using plasma samples in the presence of by-passing agents (FEIBA and NovoSeven) in 21 unrelated brother pairs with and without inhibitors enrolled in the Malmö International Brother Study (MIBS). Mean maximum thrombin formation in patients with a current inhibitor titer was 182.0 +/- 52.8 mmol mL-1 (FEIBA) and 130.7 +/- 54.9 mmol mL-1 (rFVIIa), respectively, and somewhat higher in those without inhibitors, 222.7 +/- 85.5 mmol mL-1 (FEIBA) and 142.8 +/- 53.6 mmol mL-1 (rFVIIa) (P = 0.16 and 0.29). The variance regarding the maximum thrombin production within a family was significantly lower compared with the thrombin production between families (P < 0.001 for both FEIBA and NovoSeven). Our data indicate that genetically determined factors, other than the FVIII:C activity seems to influence the phenotypic variation in thrombin formation in the presence of by-passing agents. The nature of these determinants remains to be identified.