Factor V inhibitor in thrombosis

Acquired factor V inhibitor: a nation-wide study of 38 patients

Acquired factor V inhibitor (AFVI) is an extremely rare disorder that may cause severe bleeding. To identify factors associated with bleeding risk in AFVI patients, a national, multicentre, retrospective study was made including all AFVI patients followed in 21 centres in France between 1988 and 2015. All patients had an isolated factor V (FV) deficiency <50% associated with inhibitor activity. Patients with constitutional FV deficiency and other causes of acquired coagulation FV deficiencies were excluded. The primary outcome was incident bleeding and factors associated with the primary outcome were identified. Thirty-eight (74 [36-100] years, 42·1% females) patients with AFVI were analysed. Bleeding was reported in 18 (47·4%) patients at diagnosis and in three (7·9%) during follow-up (7 [0·2-48.7] months). At diagnosis, FV was <10% in 31 (81·6%) patients. Bleeding at diagnosis was associated with a prolonged prothrombin time that strongly correlated with the AFVI level measured in plasma {r = 0·63, 95% confidence interval (CI) [0·36-0·80], P < 0·05}. Bleeding onset during follow-up was associated with a slow AFVI clearance (P < 0·001). The corresponding receiver operating characteristics curve showed that AFVI clearance was predictive of bleeding onset with an AFVI clearance of seven months with a sensitivity of 100% (95% CI: 29-100) and a specificity of 86% (95% CI: 57-98, P = 0·02). Kaplan-Meier analysis showed that AFVI clearance >7 months increased the risk of bleeding by 8 (95% CI: [0·67-97], P = 0·075). Prothrombin time at diagnosis and time for clearance of FV inhibitor during follow-up are both associated with bleeding in patients with AFVI.

A high titer of acquired factor V inhibitor in a hemodialysis patient who developed arterial thrombosis

An 87-year-old man with diabetes mellitus was admitted to control recurrent bleeding from hemodialysis puncture sites. He was a smoker and had been diagnosed with arteriosclerosis obliterans. His PT and APTT were markedly prolonged, and all coagulation factors were markedly decreased (factor V [FV] activity < 1%) or below the measurement threshold, with the exception of fibrinogen and factor XIII. Neither PT nor APTT were corrected upon mixing with normal plasma. A high titer of FV inhibitor was found at 415 BU/mL, and anti-FV autoantibody was detected by both immunoblot assay and ELISA. Prednisolone administration and plasma exchange partially improved prolonged PT and APTT and decreased the FV inhibitor level. Five months later, he manifested symptoms of severe ischemia in both legs. Angiography revealed diffuse stenosis downstream of both common iliac arteries. Endovascular therapy was repeated four times, the prednisolone dose was reduced, and low-dose antiplatelet therapy was initiated. After the final successful endovascular therapy, arterial thrombosis was detected using ultrasound and angiography. Aspiration thrombectomy and thrombolytic therapy failed to achieve recanalization, and necrosis of the legs worsened. Despite the severe coagulation abnormalities, vascular interventions should have been performed with regular-dose antiplatelet therapy, as the patient exhibited multiple risk factors for atherothrombosis.

Acquired factor V deficiency in a patient with a urinary tract infection presenting with haematuria followed by multiple haemorrhages with an extremely low level of factor V inhibitor: a case report and review of the literature

: Acquired factor V deficiency (AFVD) is a rare haemostatic disorder that is primarily because of the development of factor V inhibitors. Approximately, 200 cases have been reported and the greatest portion of these cases was because of bovine thrombin exposure. We report a case of a man who presented with haematuria followed by multiple haemorrhages associated with an elevated prothrombin time and an activated partial thromboplastin time. A workup revealed reduced factor V activity and a factor V inhibitor level of 1.9 BU, which were likely secondary to a urinary tract infection. Using corticosteroids, we successfully eliminated the inhibitor and controlled the bleeding. We review the published literature to identify the conditions that are associated with nonbovine thrombin AFVD. We assume that AFVD should be kept in mind for patients who present with multiple haemorrhages.

Factor V Inhibitors: A Diagnostic and Therapeutic Challenge

Historically, inhibitors to coagulation factor V (FV) most often have developed in patients treated with bovine thrombin, a topical hemostatic agent used during surgical procedures. With the advent of newer hemostatic agents, and the concurrent diminished use of bovine thrombin, the incidence of FV inhibitors has fallen. Nevertheless, FV inhibitors are occasionally seen on an idiopathic basis as well as in association with medications, malignancies, autoimmune disorders, pregnancy, and infections. Factor V inhibitors may present with life-threatening bleeding or thrombosis, or they may be discovered incidentally as a coagulation screening test abnormality. Management of patients with FV inhibitors is challenging and consists of control of bleeding and eradication of the inhibitor. In this short overview we review the role of platelet and plasma FV in hemostasis and discuss the unique characteristics, clinical features, diagnosis, treatment, and prognosis associated with FV inhibitors.

Transient cefuroxime/metronidazole treatment induced factor V antibodies

A 29-year-old patient presented with an appendicular infiltrate, initially treated with intravenous antibiotics, but later requiring percutaneous drainage. Both prothrombin time (PT) and activated partial thromboplastin time (aPTT) were prolonged on 3 days of antibiotic treatment and unresponsive to vitamin K or prothrombin complex concentrate. Laboratory investigation ultimately showed reduced factor V activity and factor V antibodies. In contrast to previously described cases of factor V antibodies, PT and aPTT were only mildly prolonged and residual factor V activity was still >20%. Draining of the abscess did not induce significant bleeding. Afterwards, no haemostatic medication was required. The patient was discharged from the hospital without complications. One week after cessation of the antibiotic treatment, PT and aPTT were within normal range again, with a factor V activity level of 36%. In conclusion, we present a patient with transient factor V antibodies, induced by antibiotics, without clinical bleeding tendency.

Acquired factor V inhibitor in a patient with mantle cell lymphoma presenting with hematuria followed by thrombosis: a case report

Acquired factor V inhibitor is a rare hemostatic disorder that presents with hemorrhagic manifestations in the vast majority of patients. Factor V inhibitor may develop through a variety of mechanisms involving development of alloantibodies or autoantibodies specific to Factor V. Autoantibodies, in particular, have been reported in a number of conditions. In this report, we describe a case of acquired factor V inhibitor in a patient with mantle cell lymphoma who presented with hematuria. Seven weeks after diagnosis and successful management, the patient developed deep vein thrombosis in the right lower extremity. The patient’s factor V levels were normalized, and the inhibitor was successfully eradicated using corticosteroids. Here, we discuss this rare disorder, its unusual manifestation, and provide a mini-review of the current literature regarding factor V inhibitors.

Acquired factor V inhibitors: a systematic review

The occurrence of an inhibitor against coagulation factor V (FV) is a rare but challenging condition, which may span from asymptomatic laboratory abnormalities to potentially life-threatening bleeding. The onset of FV inhibitors has been associated most frequently in the past with the patients' exposure to topical bovine thrombin administered during surgery procedures. However, since this preparation is no longer used, in this systematic review we have only focused on non-bovine thrombin-related FV inhibitor cases.

Acquired inhibitor of factor V: first report in China and literature review

We reported a 51-year-old man who developed a severe haematuria with a prolonged prothrombin time (PT) and activated partial thromboplastin time (aPTT). Laboratory evaluation revealed the presence of specific factor V inhibitors. No underlying diseases and associated conditions have been identified. The patient improved after immunosuppressive therapy. However, the FV level remained low, indicating incomplete inhibitor eradication. The clinical course of our patient was consistent with the observations by others that FV inhibitor can be persistent for months or years.

Factor V inhibitors

Acquired deficiencies of, or inhibitors to, factor V are considered rare events. We report a series of 14 acquired factor V deficiencies, 10 of which were confirmed to have inhibitors to factor V, as identified within Australia in the past 5 years following a multi-laboratory investigation. The initial index case seen by one laboratory was followed within 4 months by a separate similar case. This prompted local contact with colleagues (n = 20) working in other haemostasis referral laboratories to identify the current case series. In total, nearly one-half of all haemostasis referral laboratories contacted had seen a case within the past 5 years. Clinical features and the apparent associated risk of bleeding complications generally varied, as did laboratory findings and the likely causal event. There were three females and 11 males. Age ranged from 44 to 95 years (median, 81 years). The level of inhibitor ranged from undetectable to over 250 Bethesda units. The probable cause leading to development of the inhibitors ranged from exposure to bovine thrombin, exposure to antibiotics, surgery and malignancy. Of additional interest was the apparent association of anti-phospholipid antibodies in many of the cases. For example, in the two similar index cases, with factor V inhibitor titres > 200 Bethesda units, high levels of anti-cardiolipin antibodies (> 70 GPL units) were also detected. Although less clear because of inhibitor interference, many of the cases also showed evident co-associated lupus anticoagulant activity. In conclusion, we report a series of factor V inhibitors recently identified within our geographic region that would represent an annual incidence of around 0.29 cases per million Australians. Although considered a rare finding, there is a high likelihood that most haemostasis referral laboratories will see a case every five or so years.

Acquired factor V inhibitor complicated by Hashimoto's thyroditis, primary biliary cirrhosis and membranous nephropathy

A 59-year-old man diagnosed as having Hashimoto's thyroditis, primary biliary cirrhosis (PBC) and membranous nephropathy (MN) showed consciousness disturbance, convulsions of the upper part of his body, and rapid progression of anemia, which seemed to be derived from subdural and retroperitoneal hemorrhage, respectively. He had been diagnosed as having eosinophilia about 6 weeks before the attack. Coagulation tests revealed a prolonged activated partial thromboplastin time and prothrombin time, which could not be normalized by mixing with normal plasma. Factor V (FV) activity was severely decreased and the purified immunoglobulin G of the patient inhibited normal plasma FV activity in a dose-dependent manner, suggesting the presence of antibody-mediated circulating inhibitors specific for FV. Treatment with steroids and azathioprine as well as plasmapheresis led to improvement of his clinical symptoms, normalization of the coagulation tests, and disappearance of eosinophilia. However, the inhibitor reappeared about 7 months later in association with eosinophilia, which was also improved by steroid therapy. To our knowledge, this is the first report of the co-existence of these three kinds of immune-mediated disorders, and the first report concerning the association between acquired FV inhibitors and PBC with MN. A new unknown immune mechanism, which causes eosinophilia, may be involved in the development of the FV inhibitor in this patient.

Isolation and characterization of an antifactor V antibody causing activated protein C resistance from a patient with severe thrombotic manifestations

A 44-year-old woman with a history of severe thrombotic manifestations presented with a markedly reduced activated protein C-sensitivity ratio (APC-SR). DNA sequencing of and around the regions encoding the APC cleavage sites in the factor Va molecule excluded the presence of the factor VLeiden mutation and of other known genetic mutations. No antiphospholipid antibodies were present in the patient's plasma and both prothrombin time and activated partial thromboplastin time were normal. The total immunoglobulin fraction was isolated from the patient's plasma and found to induce severe APC resistance when added to normal plasma and to factor V-deficient plasma supplemented with increasing concentrations of factor V. Immunoblotting and immunoprecipitation experiments with the total immunoglobulin fraction purified from the patient's plasma demonstrated that the antibody recognizes factor V, is polyclonal, and has conformational epitopes on the entire factor V molecule (heavy and light chains, and B region). Thus, the immunoglobulin fraction interferes with the anticoagulant pathway involving factor V. The inhibitor was isolated by sequential affinity chromatography on protein G-Sepharose and factor V-Sepharose. The isolated immunoglobulin fraction inhibited factor Va inactivation by APC because of impaired cleavage at Arg306 and Arg506 of the heavy chain of the cofactor. The isolated immunoglobulin fraction was also found to inhibit the cofactor effect of factor V for the inactivation of factor VIII by the APC/protein S complex. Our data provide for the first time the demonstration of an antifactor V antibody not related to the presence of antiphospholipid antibodies, which is responsible for thrombotic rather than hemorrhagic symptoms.

Acquired FV inhibitors: a needless iatrogenic complication of bovine thrombin exposure

BACKGROUND

FV inhibitors are a largely preventable iatrogenic coagulopathy in which the frequency is increasing in clinical practice.

STUDY DESIGN AND METHODS

Three cases associated with our institution are reported. A systematic review of the MEDLINE database was performed, and reference lists were reviewed to identify relevant publications.

RESULTS

One hundred twenty-six cases of FV inhibitors have been reported in the world's literature. Eighty-seven have been reported in the last decade, of which two thirds are due to exposure to bovine thrombin. Bovine thrombin-associated FV antibodies develop in 40 to 66 percent of cardiac surgery patients and in 20 percent of neurosurgery patients. Thirty-three percent of reported patients developed bleeding complications. Inhibitors persisted on average 2.3 months. Standard coagulation assays do not reliably predict clinical manifestations. Multimodality therapy, including immunosuppression, is useful for treatment of symptomatic patients.

CONCLUSIONS

FV inhibitors are a common complication of bovine thrombin exposure that can have devastating clinical consequences. Transfusion medicine specialists and hematologists can play a critical role in reducing the incidence of FV inhibitors by educating the medical community about safer alternative fibrin sealants.

Acquired hemophilia

Acquired hemophilia is a serious coagulopathy usually affecting the elderly, persons with autoimmune disorders and, infrequently, women in the immediate postpartum period. It is due to autoantibodies directed against specific domains of the factor VIII molecule, leading to inhibition of factor VIII binding to von Willebrand factor, to activated factor IX or to negatively charged phospholipids. This results in bleeding into the skin, muscles, gastrointestinal and genitourinary tracts, and other sites. Mixing patient plasma with normal plasma prolongs the activated partial thromboplastin time of the normal plasma and the Bethesda assay provides a quantitative estimate of the strength of the inhibitor. The selection of therapeutic concentrates for the management of acute bleeding is related to the titer of the inhibitor; if less than 5 Bethesda Units, human factor VIII may be effective, but higher titer inhibitors usually respond only to porcine factor VIII, recombinant factor VIIa or activated prothrombin complex concentrates. Corticosteroid treatment leads to disappearance of the autoantibody in 50% of patients; cyclophosphamide and cyclosporine are effective in many who do not respond to steroids. Occasionally, high dose intravenous immunoglobulin or immunosorbent columns transiently decrease inhibitor titers and enable control of bleeding. Other autoantibodies have been described against factors V, VII, XI and, rarely, factor XIII and prothrombin. New approaches in the management of autoimmune disease and, especially, methods to establish tolerance are in development.

Antibodies to prothrombin, factor V, and beta2-glycoprotein I and vascular access thrombosis

We studied 88 hemodialysis patients for the presence of antibodies to human factor II (hFII), bovine factor V (bFV), and human beta2-glycoprotein 1 (beta2GPI). Forty-one patients had elevated anti-hFII antibodies, 17 had elevated anti-bFV antibodies, and 9 had elevated anti-beta2GPI antibodies. Fifty-two patients had elevated antibodies to one or more protein. Patients with PTFE grafts had elevated antibodies most frequently (21 [75%] vs. 20 fistulas [45%; p = 0.016 compared with PTFE] and 11 tunneled catheters [68.8%]). Twelve of 13 patients (92.3%) with PTFE grafts and thrombosis had elevated antibody levels, compared with 9 of 15 without thrombosis (60%; p = 0.049). The number of thromboses and mean thrombosis rates were significantly higher in PTFE patients with antibodies (1.24 vs. 0.14 thromboses, p < 0.01; 42.67 vs. 6.44 thromboses/100 patient years, p < 0.05). When analyzed individually, thrombotic complications occurred more frequently in patients with PTFE grafts and elevated anti-bFV antibodies (p = 0.016), but did not correlate with anti-hFII or anti-beta2GPI antibodies. Thrombotic complications did not correlate with elevated antibody levels in patients with AV fistulas or cuffed catheters. In conclusion, hemodialysis patients with PTFE grafts frequently have elevated antibodies to FII, FV, and beta2GPI, and the presence of elevated antibody levels to one or more of these proteins is associated with an increased thrombotic risk. Further studies are necessary to determine whether limiting exposure to bovine thrombin preparations will decrease the incidence of these antibodies and PTFE graft thrombosis.

Immunologic impact and clinical outcomes after surgical exposure to bovine thrombin

OBJECTIVE

To determine prospectively the immunologic response and adverse clinical events in surgical patients exposed to bovine thrombin during cardiac surgical procedures.

SUMMARY BACKGROUND DATA

Topical bovine thrombin is used extensively as a hemostatic agent during cardiovascular surgery. Antibodies developing after exposure to bovine thrombin have been anecdotally associated with hemorrhagic complications.

METHODS

One hundred fifty-one patients undergoing cardiac surgical procedures were prospectively recruited for this study before surgical exposure with topical bovine thrombin. Immunoassays were used to determine antibody levels against both bovine and human coagulation proteins before and after exposure to bovine thrombin. Alterations in coagulation assay parameters and adverse clinical events were followed in all patients enrolled in the study.

RESULTS

Baseline elevated antibody levels to one or more bovine coagulation proteins were observed most frequently in patients with a prior history of a surgical procedure during which bovine thrombin is frequently used. More than 95% of patients developed a seropositive response to bovine coagulation proteins, and 51% manifested elevated antibody levels to the corresponding human coagulation proteins after bovine thrombin exposure. Postoperative coagulation abnormalities were more common in patients with antibodies to human coagulation proteins. Patients with multiple elevated antibody levels to bovine proteins before surgery were more likely to sustain an adverse clinical outcome after surgery. Using a logistic regression model, the adjusted odds ratio for sustaining an adverse event with multiple elevated antibody levels to bovine proteins before surgery was 5.40.

CONCLUSIONS

Bovine thrombin preparations are highly immunogenic and appear to be associated with an increased risk for adverse clinical outcomes during subsequent surgical procedures. The clinical safety of these commonly used preparations needs to be reassessed, and reexposure to these agents should likely be avoided.

Antibodies to topical bovine thrombin correlate with access thrombosis

Bovine thrombin is often used topically to promote hemostasis during vascular surgery, including dialysis-access placement. Patients frequently develop antibodies to bovine thrombin preparations, and some may develop antiphospholipid antibodies. We evaluated 88 hemodialysis patients for the presence of antibodies to topical bovine thrombin to determine if elevated antibody levels correlated with vascular access thrombosis. Twenty-seven patients (30.7%) had elevated antibody levels to topical bovine thrombin. More patients with elevated antibody levels had prior vascular access thrombosis than patients with normal antibody levels (13 of 27 versus 5 of 61 patients; P < 0.001). This difference was almost entirely the result of greater levels of thrombosis in patients with polytetrafluoroethylene (PTFE) grafts and elevated antibody levels. In these patients, 11 of 13 patients (84.6%) with elevated antibody levels had a previous thrombosis compared with 2 of 15 patients (13. 3%) with normal antibody levels (P < 0.001). Patients with elevated antibody levels and PTFE grafts also had more prior thromboses (1.92 +/- 1.60 versus 0.133 +/- 0.35 thromboses; P < 0.01) and a greater thrombosis rate (66.89 +/- 63.71 versus 4.65 +/- 12.05 thromboses/100 patient-years; P < 0.01) than patients with normal antibody levels. There were no differences in the frequency of myocardial infarction, coronary artery bypass, access age, presence of diabetes mellitus, platelet counts, anticardiolipin antibody, albumin, lactate dehydrogenase, or C-reactive protein levels. In conclusion, patients with PTFE grafts and elevated antibody levels to topical bovine thrombin had significantly more vascular access thrombosis.

Clinical and laboratory manifestations of anti-factor V antibodies

Factor V is a large, multi-domain glycoprotein that exhibits both procoagulant and anticoagulant activity. Anti-factor V antibodies may develop by several mechanisms and, depending on their epitope specificity, may produce hemorrhagic or thromboembolic complications. The clinical laboratory is an essential component in diagnosing these antibodies, and therapeutic management depends on the predominant clinical manifestations.

Acquired factor V inhibitors

One hundred and five cases of factor V inhibitors were published between 1955 and 1997. According to pathogenesis, factor V inhibitor patients can be divided into five groups: patients exposed to bovine thrombin; patients after surgery without exposure to bovine proteins; miscellaneous associated conditions; 'idiopathic' inhibitors; inhibitors in congenital factor V deficiency. The clinical and biochemical properties are described. The overall prognosis of factor V inhibitors is good, but there are differences among the five groups with the best prognosis in patients exposed to bovine thrombin and the worst prognosis in 'idiopathic' inhibitors. Only a few treatment options are available. Immunoadsorption and plasmapheresis seem to be the most effective methods for therapy of acute bleeding. Many inhibitors disappear spontaneously and it is uncertain whether an immunosuppressive treatment hastens the disappearance of the inhibitor.

Deep-vein thrombosis and coumarin skin necrosis associated with a factor V inhibitor with lupus-like features

We report a 71-year-old man who developed deep-vein thrombosis after major surgery. Coumarin skin necrosis developed after starting oral anticoagulant therapy. An inhibitor to factor V (61 Bethesda units) with lupus-like features was found as well as a low protein C level. The occurrence of these very rare findings indicates that despite profound procoagulant inhibition (factor V inhibition and anticoagulant therapy), hypercoagulation can occur.

Antigenic specificities of antiphospholipid autoantibodies: implications for clinical laboratory testing and diagnosis of the antiphospholipid syndrome

Most autoantibodies associated with the antiphospholipid (aPL) syndrome and detected in standard anticardiolipin and/or lupus anticoagulant assays are directed against beta 2-glycoprotein I (beta 2-GPI) or prothrombin. Recent data indicate that these antibodies can also be detected in immunoassays utilizing purified protein antigens, in the absence of phospholipids. Initial clinical studies suggest that positivity in anti-beta 2-GPI immunoassays is more closely associated with the clinical manifestations of the aPL syndrome than is positivity in conventional anticardiolipin ELISAs. Anti-beta 2-GPI immunoassays may detect certain anti-beta 2-GPI antibodies that are not detectable in conventional anticardiolipin assays, but do not detect authentic (beta 2-GPI-independent) anticardiolipin antibodies. It appears that the former, but not the latter, antibodies are associated with the clinical manifestations of the aPL syndrome. The potential advantages and disadvantages of these new immunoassays in the clinical evaluation of the aPL syndrome are discussed.

Rapid elimination of a high-titer spontaneous factor V antibody by extracorporeal antibody-based immunoadsorption and immunosuppression

We report on the rapid elimination of a potent spontaneous factor V antibody of undetermined etiology by extracorporeal immunoadsorption on sepharose-bound polyclonal sheep antibodies to human immunoglobulins (Ig-Therasorb, Baxter) in combination with immunosuppressive treatment. A 68-year-old woman presented with severe hematuria. Severe factor V deficiency (< 1%) caused by an antibody to factor V (26 BU/ml) was found. Extracorporeal immunoadsorption (8.245 +/- 553 ml plasma processed per session) led to an average reduction of the antibody titer by 75% per session. The procedure was well tolerated without any side effects. Hematuria ceased after three immunoadsorptions and complete elimination of the antibody was achieved after seven sessions (day 15), followed by a rapid increase of the factor V activity to normal levels. Treatment with cyclophosphamide and prednisone was started on day 6 and continued for 2 months. The patient remains in remission at 6 months. Extracorporeal immunoadsorption is a highly effective method for eliminating antibodies to factor V (or other clotting factors) in selected cases, i.e., in patients with severe bleeding tendency, high antibody titer, and low probability of a rapid spontaneous remission.

Factor V inhibitor associated with Sjögren's syndrome

We report an unusual case of a 74-year-old male who developed a serum autoantibody reactive with human coagulation factor V (FV) in an activated form, as demonstrated by coagulation studies and immunoblotting analysis. Despite marked prolongation of a prothrombin time and an activated partial thromboplastin time in this patient, the inhibitor was not associated with clinical bleeding but with multiple cerebral infarctions. The patient had suffered from Sjögren's syndrome with polyclonal hypergammaglobulinaemia. The patient's purified IgG, an immediately acting inhibitor to FV, reacted with a light chain of thrombin-activated FV (FVa) and inhibited the procoagulant activity of FVa without affecting the cleavage of FVa by activated protein C. The FV inhibitor may arise from activation of FV with consequent exposure of neoantigen during the activation of coagulation cascade in the patient with an autoimmune disorder for the background.

[Postoperative disorders of hemostasis caused by antifactor V circulating anticoagulant]

The case of a 34-year-old woman who experienced a biological haemostasis disorder, without clinical manifestations, is reported. The disorder was caused by a factor V inhibitor induced by a prolonged administration of antibiotics required by a postoperative biliary peritonitis. Therapeutic strategies in patients with an acquired factor V inhibitor are reviewed.

Inherited resistance to activated protein C is corrected by anticoagulant cofactor activity found to be a property of factor V

Recently, our laboratory described a defect in anticoagulant response to activated protein C (APC). This response, APC resistance, was shown to be inherited and associated with familial thrombophilia. As other possible mechanisms were excluded, APC resistance was hypothesized to be due to deficiency of a previously unrecognized cofactor of APC. The aim of the present study was to isolate and characterize this factor. Plasma from an individual with pronounced inherited APC resistance was used as test plasma in a biological assay which monitored APC cofactor activity during its isolation from normal plasma. A purification procedure was devised that yielded a protein which was shown to be identical to coagulation factor V. It proved impossible to separate the APC cofactor activity from factor V, even by affinity chromatography using a monoclonal antibody against factor V. The affinity-purified factor V corrected the poor anticoagulant response to APC of APC-resistant plasma in a dose-dependent manner. Because the APC-resistant plasma contained normal levels of factor V procoagulant activity, the results indicated APC resistance to be due to a selective defect in the anticoagulant function of factor V. The present results show factor V not only to express procoagulant properties after its activation by thrombin but also to play an important part in the anticoagulant system as cofactor to APC.