A performance evaluation of a novel human recombinant tissue factor prothrombin time reagent (Revohem™ PT).

INTRODUCTION: A new prothrombin time reagent (Revohem™ PT) based on recombinant human tissue factor produced by the silkworm-baculovirus expression system was tested. The aim of this study was to compare the performance of the new PT reagent with two widely used routine PT reagents. METHODS: All testing was performed on a Sysmex CS-5100 coagulometer. Revohem™ PT was tested for imprecision and stability using normal and abnormal lyophilized commercial control plasmas. Comparability was assessed with two widely used reagents: one containing recombinant human tissue factor (Reagent A) and the other a human placental thromboplastin (Reagent B) using a wide range of normal and abnormal plasmas and analyser-specific ISI values. RESULTS: Excellent between-day imprecision was obtained for Revohem™ PT (CV <1.0%) and acceptable open-vial on-board stability over 7 days. There was good agreement between methods in samples from patients with liver disease and patients receiving warfarin and no significant differences between methods with increasing INR values. Both recombinant reagents suffered less interference from lupus anticoagulant than the placental thromboplastin. Revohem™ PT had similar sensitivity to reagents A and B for FII, V, VII and X deficiency and demonstrated dose responsiveness to dabigatran, apixaban and rivaroxaban with steeper response curves than the comparison reagents. CONCLUSION: Revohem™ PT showed comparable or improved performance relative to two widely used reagents and is suitable for use in warfarin control, detection of inherited factor II, V, VII and X deficiency and assessment of liver disease coagulopathy.

Evaluation of the Sysmex XN-550, a Novel Compact Haematology analyser from the XN-L® series, compared to the XN-20 system.

INTRODUCTION: The XN-550 is a new, automated, compact, haematology analyser designed to generate a full blood count with a standard five-part white blood cell differential and an immature granulocyte count, as well as an optional reticulocyte and optical platelet (PLT) counts. The aim of the study was to evaluate the performance of the XN-550 and compare it to the established XN-20 system. METHODS: We evaluated the basic parameter and special measurement channels of the XN-550, using the XN-20 (which has a similar operating system), as a reference analyser. Precision, carry-over and throughput evaluations were performed. In addition, a total of 202 samples including normal controls and various pathological samples were studied for comparability. RESULTS: Good correlations with the reference analyser were obtained for all parameters except basophils. The XN-550 offers impedance and optical PLT counts and the latter showed a better correlation and less scatter than the impedance count and was comparable to the XN-20 fluorescent count at PLT counts ≤40×109 /L. Precision was good, and no significant carry-over was detected. CONCLUSIONS: The XN-550 was simple and easy to use, while maintaining the good diagnostic sensitivity seen with high-range systems such as the XN-20, making this compact device suitable for near-patient services and smaller satellite laboratories.

ICSH recommendations for modified and alternate methods measuring the erythrocyte sedimentation rate.

INTRODUCTION: The gold standard for the determination of the erythrocyte sedimentation rate (ESR) is the Westergren method. Other methods to measure the ESR have become available. They range from modest modifications of the Westergren method to very different methodologies. The ICSH therefore established a Working Group to investigate these new approaches and compile recommendations for their validation and verification. METHODS: A panel of six experts in laboratory hematology examined the peer-reviewed literature and EQA surveys from over 6000 laboratories on four continents performing ESR testing. This information was used to create lists of ESR instrument manufacturers and their methods. RESULTS: Only 28% of laboratories surveyed used the unmodified Westergren method, while 72% of sites used modified or alternate methods. Results obtained with the new instruments could differ from results obtained with the Westergren method by up to 142%. Different non-Westergren methods showed differences from each other of up to 42%. The new methods were often significantly faster, safer, and less labor-intensive. They reduced costs and often used standard EDTA tubes, eliminating the need for a dedicated ESR tube. CONCLUSION: Based on the consensus of the Working Group, recommendations for manufacturers for the validation of new ESR methods were developed. In addition, a list of recommendations for laboratories that are moving to modified or alternate methods was compiled, addressing instrument performance verification and communications of results to clinical users.

Rivaroxaban limits complement activation compared with warfarin in antiphospholipid syndrome patients with venous thromboembolism.

Essentials Complement activation has a pathogenic role in thrombotic antiphospholipid syndrome (APS). Coagulation proteases such as factor Xa can activate complement proteins. Complement activation markers were elevated in anticoagulated thrombotic APS patients. Complement activation decreased in APS patients switching from warfarin to rivaroxaban. SUMMARY: Background Complement activation may play a major role in the pathogenesis of thrombotic antiphospholipid syndrome (APS). Coagulation proteases such as factor Xa can activate complement proteins. Aims To establish whether rivaroxaban, a direct factor Xa inhibitor, limits complement activation compared with warfarin in APS patients with previous venous thromboembolism (VTE). Methods A total of 111 APS patients with previous VTE, on warfarin target INR 2.5, had blood samples taken at baseline and at day 42 after randomization in the RAPS (Rivaroxaban in Antiphospholipid Syndrome) trial. Fifty-six patients remained on warfarin and 55 switched to rivaroxaban. Fifty-five normal controls (NC) were also studied. Markers of complement activation (C3a, C5a, terminal complement complex [SC5b-9] and Bb fragment) were assessed. Results APS patients had significantly higher complement activation markers compared with NC at both time-points irrespective of the anticoagulant. There were no differences between the two patient groups at baseline, or patients remaining on warfarin at day 42. In 55 patients randomized to rivaroxaban, C3a, C5a and SC5b-9 were lower at day 42 (median (ng mL-1 ) [confidence interval] 64 [29-125] vs. 83 [35-147], 9 [2-15] vs. 12 [4-18] and 171 [56-245] vs. 201 [66-350], respectively) but levels of Bb fragment were unchanged. There were no correlations between rivaroxaban levels and complement activation markers. Conclusions APS patients with previous VTE on warfarin exhibit increased complement activation, which is likely to occur via the classical pathway and is decreased by rivaroxaban administration. Rivaroxaban may therefore potentially provide an additional benefit to its anticoagulant effect in this patient group by limiting complement activation.

Interactions between rivaroxaban and antiphospholipid antibodies in thrombotic antiphospholipid syndrome.

INTRODUCTION: Rivaroxaban can affect lupus anticoagulant (LA) testing and antiphospholipid antibodies (aPL) may interfere with the anticoagulant action of rivaroxaban. AIMS: To establish the influence of rivaroxaban on LA detection and of aPL on the anticoagulant action of rivaroxaban. METHODS: Rivaroxaban and 52 IgG preparations (20 LA+ve, 12 LA-ve thrombotic antiphospholipid syndrome [APS] patients, and 20 normal controls [NC]) were spiked into pooled normal plasma (PNP) for relevant studies. LA detection was also studied in APS patients receiving rivaroxaban 20 mg once daily. RESULTS: In vitro spiking of samples with rivaroxaban showed no false positive LA with Textarin time, Taipan venom time/Ecarin clotting time (TVT/ECT), dilute prothrombin time (dPT) and in-house dilute Russell's viper venom time (DRVVT), but false positives in the majority of NC and LA negative IgG with two commercial DRVVT reagents at 250 ng/mL but not 50 ng/mL rivaroxaban. Ex vivo studies: six LA+ve patients on rivaroxaban remained LA positive with TVT/ECT and DRVVT at peak (162-278 ng/mL) and trough (30-85 ng/mL) rivaroxaban levels. Six LA-ve patients became (apparently) LA+ve with two DRVVT reagents (test/confirm ratio median [confidence interval], 1.6 [1.3-1.8], 1.6 [1.4-1.9]) but not with TVT/ECT at peak rivaroxaban levels, and remained LA-ve with both DRVVT reagents and TVT/ECT at trough levels. aPL positive IgG spiking of PNP had no effect on rivaroxaban's anticoagulant action on thrombin generation or rivaroxaban anti-Xa levels. CONCLUSIONS: The TVT/ECT ratio and Textarin time were not affected even at peak rivaroxaban levels, enabling detection of LA ex vivo. aPL had no effects on rivaroxaban's anticoagulant action in vitro.

ICSH recommendations for the standardization of nomenclature and grading of peripheral blood cell morphological features.

These guidelines provide information on how to reliably and consistently report abnormal red blood cells, white blood cells and platelets using manual microscopy. Grading of abnormal cells, nomenclature and a brief description of the cells are provided. It is important that all countries in the world use consistent reporting of blood cells. An international group of morphology experts have decided on these guidelines using consensus opinion. For some red blood cell abnormalities, it was decided that parameters produced by the automated haematology analyser might be more accurate and less subjective than grading using microscopy or automated image analysis and laboratories might like to investigate this further. A link is provided to show examples of many of the cells discussed in this guideline.

Rivaroxaban and warfarin achieve effective anticoagulation, as assessed by inhibition of TG and in-vivo markers of coagulation activation, in patients with venous thromboembolism.

INTRODUCTION: Rivaroxaban is non-inferior to warfarin for the treatment of venous thromboembolism, with regard to clinical efficacy and safety. The ex-vivo effects of warfarin versus therapeutic dose rivaroxaban on in-vivo markers of coagulation activation and thrombin generation remain undefined. The aim of this study was to compare the effects of warfarin and therapeutic dose rivaroxaban on ex-vivo thrombin generation (TG), and the in-vivo markers of coagulation activation, prothrombin fragment 1.2 (F1.2), thrombin-antithrombin complex (TAT), and D-dimer. METHODS: Eighty-five patients with venous thromboembolism were studied, 45 on warfarin, target INR 2.5 and 40 on rivaroxaban 20mg once daily. RESULTS: Anticoagulation was in therapeutic range in 71% (32/45) warfarin and 65% (26/40) rivaroxaban treated patients. 8 patients on warfarin and 9 patients on rivaroxaban had subtherapeutic INR and rivaroxaban levels respectively. Both rivaroxaban and warfarin reduced endogenous thrombin potential (ETP) and peak thrombin, and prolonged lag time and time to peak, compared to normal controls (p<0.0001). The lag time and time to peak TG were longer, and peak thrombin was lower in patients receiving rivaroxaban (p<0.0001) compared with warfarin, although warfarin-treated patients had lower ETP (p=0.0008). In-vivo coagulation activation markers were within the normal ranges in all rivaroxaban-treated patients (including those with levels considered to be subtherapeutic) and in 37/45 warfarin-treated patients who had an INR≥2.0. The warfarin-treated patients with subtherapeutic INRs exhibited slightly raised F1.2 and/or TAT. CONCLUSION: In conclusion, both rivaroxaban and warfarin provided effective anticoagulation, as assessed by inhibition of TG and makers of in-vivo coagulation activation.

Anti-protein C antibodies are associated with resistance to endogenous protein C activation and a severe thrombotic phenotype in antiphospholipid syndrome.

BACKGROUND: Antiphospholipid antibodies may interfere with the anticoagulant activity of activated protein C (APC) to induce acquired APC resistance (APCr). AIMS: To investigate the frequency and characteristics of APCr by using recombinant human APC (rhAPC) and endogenous protein C activation in antiphospholipid syndrome (APS). METHODS: APCr was assessed in APS and non-APS venous thromboembolism (VTE) patients on warfarin and normal controls with rhAPC or Protac by thrombin generation. IgG anti-protein C and anti-protein S antibodies and avidity were assessed by ELISA. RESULTS: APS patients showed greater resistance to both rhAPC and Protac than non-APS patients and normal controls (median normalized endogenous thrombin potential inhibition): APS patients with rhAPC, 81.3% (95% confidence interval [CI] 75.2-88.3%; non-APS patients with rhAPC, 97.7% (95% CI 93.6-101.8%; APS patients with Protac, 66.0% (95% CI 59.5-72.6%); and non-APS patients with Protac, 80.7 (95% CI 74.2-87.2%). APS patients also had a higher frequency and higher levels of anti-protein C antibodies, with 60% (15/25) high-avidity antibodies. High-avidity anti-protein C antibodies were associated with greater APCr and with a severe thrombotic phenotype (defined as the development of recurrent VTE while patients were receiving therapeutic anticoagulation or both venous and arterial thrombosis). Twelve of 15 (80%) patients with high-avidity anti-protein C antibodies were classified as APS category I. CONCLUSION: Thrombotic APS patients showed greater APCr to both rhAPC and activation of endogenous protein C by Protac. High-avidity anti-protein C antibodies, associated with greater APCr, may provide a marker for a severe thrombotic phenotype in APS. However, in patients with category I APS, it remains to be established whether anti-protein C or anti-ß2 -glycoprotein I antibodies are responsible for APCr.

Complement C5-inhibitor rEV576 (coversin) ameliorates in-vivo effects of antiphospholipid antibodies.

Activation of the complement cascade is an important mechanism for antiphospholipid antibody-mediated thrombosis. We examined the effects of rEV576 (coversin), a recombinant protein inhibitor of complement factor 5 activation, on antiphospholipid antibody-mediated tissue factor up-regulation and thrombosis. Groups of C57BL/6J mice (n=5) received either IgG from a patient with antiphospholipid syndrome (APS) or control IgG from normal human serum (NHS). Each of these groups of mice had IgG administration preceded by either rEV576, or phosphate buffer control. For each of the four treatment groups, the size of induced thrombus, tissue factor activity in carotid homogenates, anticardiolipin and anti-ß2glycoprotein I (anti-ß2GPI) levels were measured 72 h after the first injection. Mice treated with IgG-APS had significantly higher titers of anticardiolipin antibodies and anti-ß2GPI at thrombus induction compared with those treated with IgG-NHS. The IgG-APS/phosphate buffer treatment induced significantly larger thrombi and tissue factor activity compared with other groups. Mice treated with IgG-APS/rEV576 had significantly smaller thrombi and reduced tissue factor activity than those treated with IgG-APS/phosphate buffer. The data confirm involvement of complement activation in antiphospholipid antibody-mediated thrombogenesis and suggest that complement inhibition might ameliorate this effect.

ICSH guidelines for the evaluation of blood cell analysers including those used for differential leucocyte and reticulocyte counting.

This revision is intended to update the 1994 ICSH guidelines. It is based on those guidelines but is updated to include new methods, such as digital image analysis for blood cells, a flow cytometric method intended to replace the reference manual 400 cell differential, and numerous new cell indices not identified morphologically are introduced. Haematology analysers are becoming increasingly complex and with technological advancements in instrumentation with more and more quantitative parameters are being reported in the complete blood count. It is imperative therefore that before an instrument is used for testing patient samples, it must undergo an evaluation by an organization or laboratory independent of the manufacturer. The evaluation should demonstrate the performance, advantages and limitations of instruments and methods. These evaluations may be performed by an accredited haematology laboratory where the results are published in a peer-reviewed journal and compared with the validations performed by the manufacturer. A less extensive validation/transference of the equipment or method should be performed by the local laboratory on instruments prior to reporting of results.

The automation of routine light transmission platelet aggregation.

INTRODUCTION: The investigation of platelet function by aggregometry requires specialist equipment and is labour intensive. We have developed an automated platelet aggregation method on a routine coagulation analyser. METHODS: We used a CS-2000i (Sysmex) with prototype software to perform aggregation in platelet-rich plasma (PRP), using the following agonists: ADP (0.5-10 µm), epinephrine (0.5-10 µm), collagen (0.5-10 mg/µL), ristocetin (0.75-1.25 mg/mL) and arachidonic acid (0.12-1.0 mm). Platelet agonists were from Hyphen Biomed, and an AggRAM aggregometer (Helena Biosciences) was used as the reference instrument. RESULTS: CS-2000i reaction cuvette stirrer speed was found to influence reaction sensitivity and was optimized to 800 rpm. There were no clinically significant changes in aggregation response when the PRP platelet count was 150-480 x 10(9) /L, but below this there were changes in the maximum amplitude (MA) and slope (rate). Dose response with each of the agonists was comparable between CS-2000i and an AggRAM aggregometer and normal subjects receiving antiplatelet drugs. Aggregation imprecision was similar on both the CS-2000i and AggRAM systems, with a cv for 2-5 µm ADP MA and slope varying between 3-12%. CONCLUSION: Our preliminary studies indicated that optimal sensitivity using the CS-2000i was obtained with a reaction cuvette stirrer speed of 800 rpm and a PRP platelet count of 200-300 x 10(9) /L; aggregation with a PRP count <100 x 10(9) /L showed poor sensitivity. Imprecision and detection of antiplatelet drug effects was similar between the CS-2000i and AggRAM. These data demonstrate that CS-2000i is comparable to a stand-alone aggregometer, although CS-2000i has the advantages of walk-away technology and also required a smaller sample volume than the AggRAM (44% less).

Determination of APTT factor sensitivity--the misguiding guideline.

INTRODUCTION: The Clinical and Laboratory Standards Institute (CLSI) has produced a guideline detailing how to determine the activated partial thromboplastin time's (APTT) sensitivity to clotting factor deficiencies, by mixing normal and deficient plasmas. Using the guideline, we determined the factor sensitivity of two APTT reagents. METHODS: APTTs were performed using Actin FS and Actin FSL on a Sysmex CS-5100 analyser. The quality of factor-deficient and reference plasmas from three commercial sources was assessed by assaying each of the clotting factors within the plasmas and by performing thrombin generation tests (TGT). RESULTS: Testing samples from 50 normal healthy subjects gave a two-standard deviation range of 21.8-29.2 s for Actin FS and 23.5-29.3 s for Actin FSL. The upper limits of these ranges were subsequently used to determine APTT factor sensitivity. Assay of factor levels within the deficient plasmas demonstrated that they were specifically deficient in a single factor, with most other factors in the range 50-150 iu/dL (Technoclone factor VII-deficient plasma has 26 iu/dL factor IX). APTTs performed on mixtures of normal and deficient plasmas gave diverse sensitivity to factor deficiencies dependent on the sources of deficient plasma. TGT studies on the deficient plasmas revealed that the potential to generate thrombin was not solely associated with the levels of their component clotting factors. CONCLUSION: Determination of APTT factor sensitivity in accordance with the CLSI guideline can give inconsistent and misleading results.

Rituximab for thrombotic thrombocytopenic purpura: benefit of early administration during acute episodes and use of prophylaxis to prevent relapse.

BACKGROUND: Rituximab has been documented in the treatment of acute (≤ 3 days from admission), relapsed/refractory thrombotic thrombocytopenic purpura (TTP) and given as prophylaxis in selected cases to prevent acute relapse. The precise timing of rituximab in acute TTP has not been determined. OBJECTIVE: To perform a retrospective analysis of rituximab use in a large TTP referral center over an 8-year period. PATIENTS/METHODS: We assessed response to treatment and outcome for all patients treated with rituximab, including 91 patients presenting with 104 episodes of acute TTP and 15 patients given rituximab as prophylaxis to prevent relapse. In the acute TTP group we assessed the benefit of giving early (≤ 3 days from admission) vs. later (> 3 days) rituximab. RESULTS: In acute de novo TTP, previously untreated with rituximab, rituximab was given ≤ 3 days from admission to 54 patients and > 3 days from admission to 32 patients. Earlier administration (≤ 3 days) was associated with faster attainment of remission (12 vs. 20 days, P < 0.001), fewer plasma exchanges (16 vs. 24, P = 0.03) and shorter hospital stay (16 vs. 23 days, P = 0.01). Eighty-two patients (95%) achieved complete remission within 14 days (4-52 days); four patients died acutely. Eleven out of 82 (13.4%) relapsed at a median of 24 months (4-49 months). Rituximab prophylaxis was associated with normalization of ADAMTS13 levels within 3 months in all but one case, with only one acute relapse at follow-up. CONCLUSIONS: Although limited by being retrospective and non-randomized, this study demonstrates the potential benefit of early administration of rituximab in acute TTP, and prophylactic use to prevent acute relapse.

Diagnosis of antiphospholipid syndrome in routine clinical practice.

The updated international consensus criteria for definite antiphospholipid syndrome (APS) are useful for scientific clinical studies. However, there remains a need for diagnostic criteria for routine clinical use. We audited the results of routine antiphospholipid antibodies (aPLs) in a cohort of 193 consecutive patients with aPL positivity-based testing for lupus anticoagulant (LA), IgG and IgM anticardiolipin (aCL) and anti-ß(2)glycoprotein-1 antibodies (aß(2)GPI). Medium/high-titre aCL/aß(2)GPI was defined as >99th percentile. Low-titre aCL/aß(2)GPI positivity (>95(th )< 99(th) percentile) was considered positive for obstetric but not for thrombotic APS. One hundred of the 145 patients fulfilled both clinical and laboratory criteria for definite APS. Twenty-six women with purely obstetric APS had persistent low-titre aCL and/or aß(2)GPI. With the inclusion of these patients, 126 of the 145 patients were considered to have APS. Sixty-seven out of 126 patients were LA-negative, of whom 12 had aCL only, 37 had aß(2)GPI only and 18 positive were for both. The omission of aCL or aß(2)GPI testing from investigation of APS would have led to a failure to diagnose APS in 9.5% and 29.4% of patients, respectively. Our data suggest that LA, aCL and aß(2)GPI testing are all required for the accurate diagnosis of APS and that low-titre antibodies should be included in the diagnosis of obstetric APS.

A comparative evaluation of a new automated assay for von Willebrand factor activity.

The ristocetin cofactor assay (VWF:RCo) is the reference method for assessing von Willebrand factor (VWF) activity in the diagnosis of von Willebrand's Disease (VWD). However, the assay suffers from poor reproducibility and sensitivity at low levels of VWF and is labour intensive. We have undertaken an evaluation of a new immunoturbidimetric VWF activity (VWF:Ac) assay (INNOVANCE(®) VWF Ac. Siemens Healthcare Diagnostics, Marburg, Germany) relative to an established platelet-based VWF:RCo method. Samples from 50 healthy normal subjects, 80 patients with VWD and 50 samples that exhibited 'HIL' (i.e. Haemolysis, Icterus or Lipaemia) were studied. VWF:Ac, VWF:RCo and VWF:Ag were performed on a CS-analyser (Sysmex UK Ltd, Milton Keynes, UK), all reagents were from Siemens Healthcare Diagnostics. The VWF:Ac assay, gave low intra- and inter-assay imprecision (over a 31-day period, n = 200 replicate readings) using commercial normal (Mean 96.2 IU dL(-1), CV < 3.0%) and pathological (Mean 36.1 IU dL(-1), CV < 3.5%) control plasmas. The normal and clinical samples exhibited good correlation between VWF:RCo (range 3-753 IU dL(-1)) and VWF:Ac (rs = 0.97, P < 0.0001), with a mean bias of 5.6 IU dL(-1). Ratios of VWF:Ac and VWF:RCo to VWF:Ag in the VWD samples were comparable, although VWF:Ac had a superior lower level of detection to that of VWF:RCo (3% and 5% respectively). A subset (n = 97) of VWD and HIL samples were analysed for VWF:Ac at two different dilutions to assess the effect on relative potency, no significant difference was observed (P = 0.111). The INNOVANCE(®) VWF Ac assay was shown to be reliable and precise.

A phenotype-genotype correlation of ADAMTS13 mutations in congenital thrombotic thrombocytopenic purpura patients treated in the United Kingdom.

BACKGROUND: ADAMTS13 mutations play a role in thrombotic thrombocytopenic purpura (TTP) pathogenesis. OBJECTIVES: To establish a phenotype-genotype correlation in a cohort of congenital TTP patients. PATIENTS/METHODS: Clinical history and ADAMTS13 activity, antigen and anti-ADAMTS13 antibody assays were used to diagnose congenital TTP, and DNA sequencing and in vitro expression were performed to identify the functional effects of the ADAMTS13 mutations responsible. RESULTS: Seventeen (11 novel) ADAMTS13 mutations were identified in 17 congenital TTP patients. All had severely reduced ADAMTS13 activity and antigen levels at presentation. Six patients with pregnancy-associated TTP and six patients with childhood TTP were homozygous or compound heterozygous for ADAMTS13 mutations located in the metalloprotease (MP), cysteine-rich, spacer and/or distal thrombospondin type 1 domains. The adults had TTP precipitated by pregnancy, and had overall higher antigen levels (median, 30 ng mL(-1) ; range, < 10-57 ng mL(-1) ) than the children (median, 14 ng mL(-1) ; range, < 10-40 ng mL(-1)). Presentation in the neonatal period was associated with more intensive treatment requirements. The two neonates with the most severe phenotype had mutations in the first thrombospondin type 1 motif of ADAMTS13 (p.R398C, p.R409W, and p.Q436H). Using transfected HEK293T cells, we have shown that p.R398C and p.R409W block ADAMTS13 secretion, whereas p.Q436H allows secretion at reduced levels. CONCLUSIONS: This study confirms the heterogeneity of ADAMTS13 defects and an association between ADAMTS13 genotypes and TTP phenotype.

Persistent high factor VIII activity leading to increased thrombin generation - a prospective cohort study.

INTRODUCTION: A persistently elevated level of factor VIII (FVIII) is an independent risk factor for venous thromboembolism (VTE). Although the pathophysiology of VTE is unclear, the involvement of thrombin generation (TG) has been postulated. Consequently this study was designed to (i) investigate the relationships between FVIII, Thrombin generation test (TGT) parameters and D-dimer in VTE patients, (ii) determine whether elevated levels of FVIII and increased TG in these patients are transient or sustained. PATIENTS AND METHODS: After an initial period of anticoagulation had been completed 91 VTE patients and 52 healthy controls were recruited. FVIII levels were determined by one-stage clotting (FVIII:C) and chromogenic (FVIII:Ch) assays. The potential to generate thrombin was measured using the Calibrated Automated Thrombogram (CAT) and D-Dimer was by immuno-turbidometric assay. RESULTS: Patients' FVIII:C levels and FVIII:Ch, exhibited good agreement (rs=0.94; p<0.0001), although FVIII:C exhibited a mean bias of -6%. FVIII:Ch show a significant correlation with TGT Peak Thrombin (rs=0.30; p=0.004) and Peak Thrombin was found to be significantly higher (p=0.04) in patients with FVIII>200 iu/dL. Furthermore elevated levels of FVIII and increased thrombin generation parameters appeared to be consistent over time. CONCLUSION: Our data suggests that high FVIII leading to increased TG confers a significant risk of recurrent VTE and therefore we speculate that these patients may benefit from prolonged anticoagulation therapy.