The phenomenon known as acquired activated protein C resistance

Factor V Leiden-independent activated protein C resistance: Communication from the plasma coagulation inhibitors subcommittee of the International Society on Thrombosis and Haemostasis Scientific and Standardisation Committee

Activated protein C resistance (APC-R) due to the single-nucleotide polymorphism factor V Leiden (FVL) is the most common cause of hereditary thrombophilia. It is found predominantly in Caucasians and is uncommon or absent in other populations. Although FVL is responsible for >90% of cases of hereditary APC-R, a number of other F5 variants that also confer various degrees of APC-R and thrombotic risk have been described. Acquired APC-R due to increased levels of coagulation factors, reduced levels of inhibitors, or the presence of autoantibodies occurs in a variety of conditions and is an independent risk factor for thrombosis. It is common for thrombophilia screening protocols to restrict assessment for APC-R to demonstrating the presence or absence of FVL. The aim of this Scientific and Standardisation Committee communication is to detail the causes of FVL-independent APC-R to widen the diagnostic net, particularly in situations in which in vitro APC-R is encountered in the absence of FVL. Predilution clotting assays are not FVL specific and are used to detect clinically significant F5 variants conferring APC-R, whereas different forms of acquired APC-R are preferentially detected using the classical activated partial thromboplastin time-based APC-R assay without predilution and/or endogenous thrombin potential APC-R assays. Resource-specific recommendations are given to guide the detection of FVL-independent APC-R.

Anticoagulant proteins in a population of Mexican mestizo donors

BACKGROUND

To determine the activity of antithrombin (AT), protein C (PC), and protein S (PS), as well as the frequency of deficiencies of these proteins in a population of healthy Mexican mestizo blood donors.

METHODS

AT, PC, and PS were determined from 1,502 plasma samples of healthy blood donors by using commercial kits in a coagulometer 4 STA (Diagnostica Stago, Asnières, France).

RESULTS

A total of 741 women and 761 men were under study. They were divided into age range groups (18-24, 25-34, 35-44, 45-54, and 55-64 years). Activity of AT, PC, and PS was determined. For AT, activity values were specific for each age group according to gender when it had to do with PS, as well as when PC was determined. Frequencies of AT, PC, PS, and activated PC resistance activity deficiencies were obtained from reference levels (RLs) and average levels of this study. Differences were found between both frequencies for AT, PC, and PS, and the average levels obtained were used in this study. The frequencies of the activity deficiencies obtained through the values gotten in this population were: AT, 0.6%; PC, 1.06% (which is higher than the one obtained using the RLs described by commercial kits 0.33% and 0.66%, respectively); and PS, 1% (which is less than 4.5%).

CONCLUSIONS

It is necessary to know the characteristics and biological behavior of the coagulation proteins in the Mexican population because the RLs used have been established for populations that are genetically different.

The influence of maternal Lewis, Secretor and ABO(H) blood groups on fetal growth restriction

BACKGROUND

Fetal growth restriction (FGR) is associated with thrombosis of the placenta and an increased risk of subsequent vascular disease in the mother and fetus. The products of interactions between ABO(H), Lewis and Secretor genes are also associated with thrombosis and vascular disease risk.

OBJECTIVES/METHODS

A prospective case-control study of mothers with a severe FGR pregnancy (cases, n = 128; controls, n = 288) was performed to determine whether FGR is associated with particular maternal blood groups.

RESULTS

No association with ABO(H) status was observed, but FGR was more common in maternal secretors (odds ratio [OR] 1.70, 95% confidence interval [CI] 1.08-2.69) and consequently in those mothers expressing Le(b) on their red cells (OR 1.80, 95% CI 1.15-2.83), with a reduced risk in non-secretors and those expressing Le(a). Given the association between blood groups and both activated protein C resistance (APCR) and von Willebrand factor (VWF) levels, post hoc pilot studies on first-trimester APCR and VWF antigen levels and blood group genotypes were performed. No relationship with Lewis or Secretor was observed. Despite this, lower first-trimester VWF levels were observed in pregnancies subsequently complicated by FGR.

CONCLUSIONS

 This is the first study reporting a relationship between maternal Secretor/Lewis status and FGR. A link between blood groups and FGR is plausible, as both are associated with cardiovascular disease. We observed no relationship between Lewis/Secretor status and VWF or APCR, but this should be confirmed in a larger study. Thus, the mechanism whereby Secretor and/or Lewis influences FGR is unknown.

A moderate aseptic local inflammation does not induce a significant systemic inflammatory response

The aim of this prospective study was to characterize the response of the coagulation system to a defined sterile localized inflammatory process. Tissue cages were implanted subcutaneously in five healthy Beagles. After 9-10 weeks, local inflammation was induced by an injection of 0.5 ml 1% carrageenan. Serial samples of tissue cage fluid (TCF) and blood were collected at 10 time points (0-168 h). Nucleated cells (NC) of TCF were counted automatically to characterize local inflammation. C-reactive protein (CRP), leukocytes and coagulation variables (PT, aPTT, fibrinogen, factor VIII, antithrombin, protein C, protein S, and d-dimers) were determined in blood samples. Carrageenan induced a significant 32-fold increase of NCs in TCF (P<0.0001). A slight increase in leukocytes (P<0.0001) was observed. There was a significant 1.3- to 1.5-fold increase in protein C (P=0.0001) and protein S (P=0.0028). CRP, secondary hemostasis and fibrinolysis did not change. The mild increase from baseline in PC/PS, may reflect a physiological counter reaction.

ABO blood groups and thrombosis: a causal association, but is there value in screening?

ABO(H) antigens are expressed on red cells and on von Willebrand factor. An association between groups other than O and thrombosis exists: an effect that is predominantly mediated by von Willebrand factor. Overall, the risk of venous thrombosis associated with non-O has been estimated at 1.75-fold, with a higher risk (∼2.4-fold) in those with the least O(H) antigen (a combined group of A1A1/A1B/BB). Preliminary evidence also suggests that blood group may influence the venous thromboembolism risk associated with factor V Leiden. Overall, ABO(H) has a more modest effect on arterial disease, with a consistent effect observed in peripheral vascular disease and no influence evident with angina. A modest effect on myocardial infarction and stroke has been reported in some but not all studies. The potential mechanisms whereby blood group influences thrombosis, the limitations of current evidence and the current and future role of blood groups in identifying those at risk of arterial and venous disease is discussed.

Reference Intervals and Method Optimization for Variables Reflecting Hypocoagulatory and Hypercoagulatory States in Dogs using the STA Compact® Automated Analyzer

Reference intervals for coagulation parameters have been rarely determined in dogs for the STA Compact® automated coagulation analyzer, so it is the aim of the current study to validate assays and establish reference ranges for its use in canine specimens. Coagulation parameters were assessed in 56 healthy dogs with a median age of 2 years and evenly distributed sex. The 95% reference intervals were as follows: 1-stage prothrombin time = 5.7–8.0 sec; activated partial thromboplastin time (APTT) = 10.0–14.3 sec; thrombin time (TT) = 11.9–18.3 sec; fibrinogen = 1.3–3.1 g/1; antithrombin (AT) = 107.9–128.0%; D-dimer = 0.023–0.65 μg/ml; anti-factor Xa = 0.04–0.26 IU/1; and activated protein C (APC) ratio = 2.0–3.0. Protein C and S activity was markedly below (<-20%) and factor VIII was 2- to 11-fold above the human calibration standard, so a standard curve had to be prepared from canine pooled plasma. Reference intervals for protein C, protein S, and factor VIII were 75.5–118.9%, 74.4–160.5%, and 70.9–136.4%, respectively, compared with a canine standard curve. Streptokinase-activated plasminogen assay was not suitable for dogs. There was no significant impact of sex on hemostasis test results. Factor VIII activity, AT, protein C, protein S, and APC ratio were overestimated in hemolytic plasma, whereas fibrinogen, TT, and APTT were underestimated. Lipemia resulted only in false-high D-dimers. This study provided useful reference intervals for dogs, but some human tests (i.e., protein C, protein S, factor VIII, and plasminogen) required modification.

Activated Protein C Resistance and Factor V Leiden in Mexico

A common cause of hereditary thrombophilia is activated protein C resistance (APCR), and most cases result from factor V Leiden mutation. An APCR phenotype without association with factor V Leiden has been described. This transversal, observational, nonrandomized study evaluated these 2 phenomena in healthy indigenous and mestizo Mexican subjects (n = 4345), including 600 Mexican natives. No indigenous subjects had APCR, but 82 mestizo subjects did. After retesting, 50 subjects had a negative test. The remaining 32 subjects had factor V Leiden, giving a 0.85% prevalence of factor V Leiden in the mestizo Mexican population. Only 31% of APCR carriers had factor V Leiden. These results show a very low prevalence of APCR and factor V Leiden in Mexico. Except for factor V Leiden, there are no other mutations in the factor V gene responsible for the APCR phenotype. Acquired APCR is nearly twice as prevalent as the inherited variant.

Low Arterial Saturation Is Associated With Increased Sensitivity to Activated Protein C in Children With Congenital Heart Disease

OBJECTIVES

Children with congenital heart disease experience both hemorrhagic and thrombotic complications. In this report the authors test the hypothesis that hypoxemia is associated with altered sensitivity to activated protein C (aPC) in pediatric patients with congenital heart lesions.

DESIGN

A retrospective genetic registry review, with statistical evaluation of factors contributing to the aPC ratio.

SETTING

Large university hospital.

PARTICIPANTS

Cohort of 92 children with congenital heart disease undergoing cardiac catheterization procedures.

INTERVENTIONS

The authors measured the aPC ratio at cardiac catheterization and evaluated the contribution of independent variables using linear regression and classification tree approaches. Independent variables included age, gender, use of aspirin, history of thrombosis, room air arterial saturation, factor VIII:C levels, presence of congestive heart failure, and heterozygosity for factor V Leiden.

MEASUREMENTS AND MAIN RESULTS

At univariate analysis, factor V Leiden, female gender, room air arterial saturation, age greater than 6 months, and plasma factor VIII:C levels were associated with a lower aPC ratio (resistance to aPC). At stepwise linear regression, arterial saturation, factor VIII:C level, female gender, and factor V Leiden were independently associated with a lower aPC ratio, and these variables explained about 49% of the variability in aPC ratio. The classification tree approach confirmed the dependence of aPC ratio on factor V genotype and arterial saturation.

CONCLUSIONS

The aPC ratio in this population is associated with hypoxemia, independent of factors previously observed in adults. Further studies are under way to determine how aPC resistance or sensitivity may independently affect perioperative hemostasis in this population.

Screening for thrombophilia in high-risk situations: a meta-analysis and cost-effectiveness analysis

Laboratory testing for the identification of heritable thrombophilia in high-risk patient groups have become common practice; however, indiscriminate testing of all patients is unjustified. The objective of this study was to evaluate the cost-effectiveness of universal and selective history-based thrombophilia screening relative to no screening, from the perspective of the UK National Health Service, in women prior to prescribing combined oral contraceptives and hormone replacement therapy, women during pregnancy and patients prior to major orthopaedic surgery. A decision analysis model was developed, and data from meta-analysis, the literature and two Delphi studies were incorporated in the model. Incremental cost-effectiveness ratios (ICERs) for screening compared with no screening was calculated for each patient group. Of all the patient groups evaluated, universal screening of women prior to prescribing hormone replacement therapy was the most cost-effective (ICER 6824 pounds). In contrast, universal screening of women prior to prescribing combined oral contraceptives was the least cost-effective strategy (ICER 202,402 pounds). Selective thrombophilia screening based on previous personal and/or family history of venous thromboembolism was more cost-effective than universal screening in all the patient groups evaluated.

New anticoagulants: a pediatric perspective

Unfractionated heparin and vitamin K antagonists such as warfarin have been used as the anticoagulants of choice for over five decades. Subsequently, low molecular weight heparins (LMWHs) became widely available and have provided several advantages, especially in infants and children. The field of anticoagulation, however, has undergone a major revolution with better understanding of the structure of coagulation proteins and the development of a host of new drugs with highly specific actions. Many of these drugs have undergone extensive clinical testing in adults and have been approved for specific indications in adults. Unfortunately, clinical data and the reported use of these drugs in children are extremely limited. A lack of familiarity with the actions and pharmacokinetic properties of these drugs could be a major contributing factor. This review focuses on several of the new anticoagulants, with a special emphasis on those that could be potentially beneficial in pediatric patients with thromboembolic disorders. The need for well-designed trials with large-scale participation by pediatric hematologists in order to improve the antithrombotic care of young infants and children is also emphasized.

Resistance to Activated Protein C Caused by Factor V Leiden Mutation and Orthotopic Liver Transplantation

BACKGROUND

Factor V (FV)-dependent resistance to activated protein C (APCR) is most likely caused by a point mutation in the FV gene, the so-called FV Leiden mutation (FV:Q506), which is a common risk factor predisposing to venous thromboembolism. Little is known about the role of FV:Q506 in recipients of human liver grafts and the significance of acquired APCR caused by orthotopic liver transplantation (OLT).

METHODS

We screened blood samples of 720 patients who underwent OLT by genotyping for FV:Q506 and by testing for APCR with two highly FV-specific tests. Apart from the existing medical records, we obtained clinical data from 551 patients on thromboembolic events (TEs) by means of a questionnaire.

RESULTS

We found 49 (6.8%) heterozygous carriers of FV:Q who did not show APCR after OLT. One patient, heterozygous for FV:Q506, displayed APCR after OLT. In 35 (4.9%) noncarriers of FV:Q we detected APCR after OLT. In comparison with noncarriers, carriers of FV:Q506 demonstrated more TE before transplantation (7% vs. 28%, P<0.0005; relative risk 4.0 [95% confidence interval, 2.3-6.9]); this was also true for Budd-Chiari syndrome (1.8% vs. 10%, P<0.005). At a median follow-up of 5 years (0, 13-12 years), we found a higher incidence of TE after transplantation in patients with "acquired" APCR (16.7% vs. 4,3%; P=0.01; relative risk 3.9 [95% confidence interval, 1.7-9.0]), which included one patient with life-threatening TE during the early postoperative phase.

CONCLUSIONS

APCR caused by FV:Q506 before OLT is a risk factor for TE. OLT-related "acquired" APCR should be considered a risk factor for venous thromboembolism.

Postmenopausal hormone replacement therapy and venous thromboembolism

BACKGROUND

Women taking hormone replacement therapy (HRT) have a 2- to 5-fold increased risk of venous thrombosis compared with nonusers. Increasingly, evidence has suggested that the size of the risk increase varies according to related factors, such as the type of estrogen used, the mode of delivery, and the presence of other predisposing factors.

OBJECTIVE

The aim of this study was to examine the current literature to assess the varying risk of venous thrombosis among women taking HRT.

METHODS

An extensive search was carried out on all major electronic databases including MEDLINE 1995 to October 2005 and BIS (EMBASE) 1980 to October 2005. Relevant keywords relating to thrombosis (venous thromboembolism, venous thrombosis, deep vein thrombosis, and pulmonary embolism) combined with hormones (hormone replacement therapy and estrogen) were used to capture all potentially relevant studies.

RESULTS

The increased risk of a first episode of venous thrombosis in women currently taking HRT compared with nonusers ranged from 1.22 (95% CI, 0.76-1.94) to 4.50 (95% CI, 1.30-15.10). Similar increases in risks for deep vein thrombosis and pulmonary embolism were found. The risk of venous thrombosis is the highest in the first year of therapy, durich which as much as a greater than 6-fold increase was found. Women taking estrogen-progestin HRT had a significantly greater risk of venous thrombosis than those using estrogen-only preparations (odds ratio [OR], 1.60; 95% CI, 1.13-2.26). Studies have also suggested a dose-related effect, suggesting high-dose estrogen therapy is associated with a greater increased risk of venous thrombosis than low-dose preparations. Comparisons between oral and transdermal HRT have shown a significant difference in the relative risk of venous thrombosis (OR, 4.0; 95% CI, 1.9-8.3) favoring the use of transdermal preparations. The presence of thrombophilia, particularly factor V Leiden, further amplifies the risk of venous thrombosis in women using HRT (OR, 13.16; 95% CI, 4.28-40.47). The presence of other risk factors, such as increasing age and being overweight, were all shown to be associated with a further increase in the risk of venous thrombosis.

CONCLUSIONS

Recent studies have confirmed that current users of HRT are at increased risk of venous thrombosis. The increase in risk has been shown to vary according to duration of use, with the risk being greatest during the first year of use. Moreover, the increased risk varies according to the type of preparation and presence of additional risk factors such as increasing age, obesity, cancer, and recent surgery. Few studies have examined the relationship between thrombophilia, HRT and venous thrombosis; thus, more research is required in this area before accurate estimates of the risk can be made.

The response to activated protein C after cardiopulmonary bypass: impact of factor V leiden

Activated protein C (aPC) resistance is a recognized hypercoagulable phenotype that is associated with increased risk for thrombosis in multiple clinical settings. Factor V Leiden (FVL) represents a specific inherited cause of aPC resistance, but the perioperative thrombotic risk of FVL is unclear. In this investigation, we sought to quantify whether cardiopulmonary bypass produces alterations in aPC resistance in FVL carriers and noncarrier controls, testing the hypothesis that FVL is associated with a relatively hypercoagulable postoperative state. Two-hundred-five adult cardiac surgery patients were prospectively enrolled into a genetic registry whose purpose was to study the impact of genetic variables on clinical outcomes. For this study, 8 subjects heterozygous for FVL were identified (group L), as well as 2 control groups: group MC, matched controls, 18 matched subjects without FVL; and group UC, unmatched controls, 11 consecutive subjects without FVL. Plasma was sampled at the beginning of surgery, 10 min after protamine administration, and on postoperative day 1, and assayed for resistance to aPC (normal aPC ratio is >2.0). Both MC and UC groups exhibited normal aPC ratio at baseline (2.40 and 2.36, respectively), which increased significantly (to 2.76 and 2.75, P = 0.007 and 0.021, respectively) on postoperative day 1, indicating increased postoperative sensitivity to aPC. Conversely, group L subjects exhibited aPC resistance at baseline (aPC ratio 1.80), and did not change significantly postoperatively (P = 0.867). Patients without FVL therefore show laboratory evidence consistent with relative protection from postoperative thrombosis, whereas FVL carriers do not. These findings provide mechanistic support for previous speculations of increased postoperative thrombotic risk associated with FVL.

Factor V Leiden and perioperative risk

Factor V Leiden (FVL) is the most common known inherited cause of thrombophilia; it is present in approximately 5% of the Caucasian population. Although the risk of venous thrombosis associated with this polymorphism in various medical settings is well described, its effect on perioperative risk is only beginning to be explored. Specifically, there are few studies addressing the potential risks of FVL in the surgical population, in which both hemorrhagic and thrombotic complications convey substantial clinical and economic significance. There are speculations and unproven hypotheses regarding FVL in this population, and these therefore highlight the need to comprehensively address this issue. This review will describe the physiology of the FVL mutation, briefly clarify its risk in the nonsurgical setting, and assess current data regarding FVL in noncardiac and cardiac surgery. Finally, a summary of current clinical evidence and a plan for more detailed investigation of this potentially significant risk factor will be proposed.

Haemostasis in normal and abnormal pregnancy

Haemostasis is a complex and dynamic equilibrium involving pro-coagulants, the natural anticoagulation system and fibrinolysis. Normal human pregnancy is associated with profound alterations to the process of haemostasis such that the pro-coagulant effect becomes dominant. There are very few studies which have attempted to elucidate the adaptations that take place in the uteroplacental circulation where the haemostatic system faces the conflicting tasks of maintaining blood fluidity during pregnancy while preparing for the haemostatic challenge of delivery. It is hypothesised that excessive thrombosis within the uteroplacental circulation provides the mechanistic basis for the reported associations between the inherited thrombophilias and major pregnancy complications. The evidence underpinning this widely quoted hypothesis is weak.

Activated protein C resistance and pregnancy loss

Activated protein C resistance is a thrombophilia with an established role in producing thrombosis which more recently has been implicated in the pathogenesis of pregnancy loss. This review will analyse recent literature to evaluate this association and address the gestation and type of pregnancy loss.