Nowadays D-dimer testing is frequently applied in the diagnosis of venous thromboembolism. However, the test results of different quantitative D-dimer tests can differ significantly. The background of this variability, which is mainly caused by the variety of fibrin degradation products in plasma, the specificity of D-dimer assays, and the calibrators used in the test, is summarized here. Because D-dimer is not a single entity in plasma but a mixture of heterogeneous fibrin degradation products, method standardization is in principle impossible. Efforts undertaken in the past to standardize D-dimer methods are summarized. All these projects failed and it was concluded that only harmonization of D-dimer test results seems to be feasible. The results of a large field study on which a new approach to the harmonization of quantitative D-dimer methods will be based is summarized in this article. This approach seems to be an adequate solution for overcoming the practical problem of variation of test outcome in different D-dimer assays.
An increased amount of deep abdominal visceral fat has generally been accepted as an important cardiovascular risk factor, and disturbances in hemostasis and fibrinolysis have been suggested to play a role. Fibrinogen and von Willebrand factor, representatives of the hemostatic system, and plasminogen activator inhibitor 1 (PAI-1), as the most important inhibitor of the fibrinolytic system, have been associated with visceral obesity, with the most convincing evidence found for the involvement of PAI-1. The association with fibrinogen and von Willebrand factor has been suggested to be merely a reflection of the association with inflammation and endothelial dysfunction. The fact that PAI-1 is secreted by adipose tissue has attracted much attention. The increase of PAI-1 in visceral obesity could be because visceral adipose tissue produces more PAI-1 compared with subcutaneous abdominal adipose tissue. The contribution of other cell types such as hepatocytes or endothelial cells is probably more important, with stimulation of PAI-1 production by different components of the metabolic syndrome. PAI-1 secretion by adipose tissue has been suggested to have a more local effect, playing a role in tissue remodeling during the development of obesity.
Venous thromboembolism (VTE) requires prolonged treatment to prevent late recurrences. However, the optimal duration of vitamin K antagonist (VKA) therapy is still controversial. More recently, D-dimer (D-d) has emerged as a predictive factor for recurrences. D-d has been evaluated both during and after VKA treatment. Some patients with DVT of the lower limbs have persistently high D-d during anticoagulation and this could reflect insufficient anticoagulation despite apparently adequate antithrombotic treatment. Altered D-d during anticoagulation is more frequent in patients with idiopathic or cancer-associated VTE than in those with secondary VTE. In subjects with an unprovoked VTE event, the time spent at near normal international normalization ratio (INR) values (< 1.5) during the first 3 months of treatment is associated with higher D-d during and after VKA treatment and with a higher risk for late recurrences. Moreover, the combination of altered D-d and inherited thrombophilia, and not residual venous obstruction, is associated with a significantly higher hazard ratio for recurrence. Preliminary results of a management study, the PROLONG study, indicate that subjects with normal D-d at 1 month after VKA withdrawal have a low risk of recurrence, and those with altered D-d have a significantly higher risk and deserve prolonged treatment.
Renal insufficiency is invariably accompanied by elevated plasma concentrations of the sulfur-containing and potentially vasculotoxic amino acid homocysteine. There is a strong relationship between glomerular filtration rate and plasma homocysteine concentration. Unlike creatinine, however, homocysteine is avidly reabsorbed in the renal tubules, and its urinary excretion is minimal. There is no evidence that homocysteine is actively removed by the human kidney. In renal insufficiency, plasma concentrations of S-adenosylmethionine, S-adenosylhomocysteine, cystathionine, cysteine, and sulfate are elevated, pointing to a remethylation or distal transsulfuration/oxidation block as the cause of hyperhomocysteinemia in renal failure. Stable isotope techniques have shown that both whole-body homocysteine remethylation and methionine transmethylation are decreased in renal failure, whereas homocysteine transsulfuration seems intact. Metabolic homocysteine clearance (i.e., transsulfuration relative to plasma homocysteine) is decreased to a major extent. These metabolic disturbances in renal failure can only be partially restored with current treatments. Folic acid treatment lowers plasma homocysteine concentration and increases remethylation and transmethylation rates. Plasma homocysteine, however, is not normalized, and metabolic homocysteine clearance by transsulfuration remains impaired. According to the currently available data, effective normalization of plasma homocysteine can only be obtained when its metabolic clearance through transsulfuration is restored.
Recently, adipocytes have been shown to be endocrine cells that secrete a variety of bioactive substances-the so-called adipocytokines. Among adipocytokines, tumor necrotizing factor alpha, plasminogen activator inhibitor 1, and heparin-binding epidermal growth factor-like growth factor are produced in adipocytes as well as already known organs, and they contribute to the development of vascular diseases. Visfatin is a very recently discovered visceral fat-specific protein that may be related to the development of obesity-related diseases such as diabetes mellitus and cardiovascular disease. In contrast to these adipocytokines, adiponectin, also a newfound adipose tissue-specific collagen-like protein, has been noted recently as an important antiatherogenic as well as antidiabetic protein. The function of adipocytokine secretion might be regulated dynamically by nutritional state. Visceral fat accumulation causes dysfunction of adipocytes including oversecretion of tumor necrotizing factor alpha, plasminogen activator inhibitor 1, and heparin-binding epidermal growth factor-like growth factor, as well as hyposecretion of adiponectin, which results in the development of a variety of metabolic and circulatory diseases. In this review, the importance of adipocytokines, including adiponectin, is discussed with respect to atherosclerosis.
Patients undergoing major lower-extremity orthopedic surgery such as total hip replacement (THR) and total knee replacement (TKR) are at an increased risk of venous thromboembolism (VTE). Routine prophylaxis is necessary to reduce the risk of deep vein thrombosis (DVT), which may progress to potentially fatal pulmonary embolism and secondary complications such as postthrombotic syndrome, recurrent DVT, and chronic pulmonary hypertension. Prophylaxis in patients undergoing TKR, THR, and hip fracture surgery is now standard practice and generally involves anticoagulant treatment with either low-molecular-weight heparin (LMWH) or warfarin for a period of 7 to 10 days, with extended prophylaxis in those with ongoing risk factors such as obesity, cancer, or previous VTE. Data from clinical practice suggest that there is a general trend toward longer postsurgical prophylaxis and shorter hospital stays, making practicality of treatment an important consideration. LMWH is effective for the prophylaxis of VTE, but the parenteral route of administration is not convenient for use in the outpatient setting. Warfarin, on the other hand, can be administered orally but requires the infrastructure for careful patient monitoring and dose adjustments because of its unpredictable dose-response relationship. The development of new anticoagulants has been pursued with the aim of improving efficacy, predictability, consistency of response, safety, and convenience. A recently approved anticoagulant, fondaparinux, has been proven to provide superior efficacy for the prevention of VTE compared with LMWH, but this agent requires parenteral administration and does not overcome the convenience issue. Ximelagatran is the oral form of the direct thrombin inhibitor melagatran, which is available for subcutaneous administration. Ximelagatran has a consistent anticoagulant response allowing fixed oral dosing without the need for coagulation monitoring. The efficacy and safety profile of melagatran/ximelagatran prophylaxis for VTE following THR and TKR has compared favorably with standard LMWH prophylaxis, as seen in the European METHRO II and III trials and EXPRESS trial, and with warfarin prophylaxis, as seen in the North American EXULT A and B trials. Several prophylactic treatment regimens have been evaluated in the European trials to determine the optimal dosing and timing of first dose of melagatran to achieve the best balance of efficacy and safety. Preoperative initiation of melagatran was more effective than when prophylactic treatment was initiated postoperatively, and the lowest rates of bleeding were associated with a postoperative initiation of prophylaxis. Early administration of the first postoperative melagatran dose (4 to 8 hours) was also associated with better prophylactic efficacy relative to a later postoperative start (8 to 12 hours). The results of the comprehensive international clinical trial program and in particular the optimal balance of efficacy/safety data provided by the METHRO III study have led to approval of melagatran/ximelagatran in 2004 in the European Union for the prevention of VTE in patients undergoing elective hip or knee replacement surgery. Ximelagatran has the potential to maximize the use of anticoagulation in patients discharged following major lower-extremity orthopedic surgery.
The oral direct thrombin inhibitor ximelagatran is the first oral anticoagulant since the introduction of the vitamin K antagonists in the early 1940s. A comparison of the discovery and early clinical development of the two classes of oral anticoagulants reveals some similarities but also several differences that illustrate the change in drug discovery over the last half century.
Deep vein thrombosis (DVT) is a disorder frequently affecting the deep veins of the lower limbs; its onset is induced by known risk factors. The main complications of DVT are pulmonary embolism and postthrombotic syndrome (PST). Clinical pulmonary embolism occurs in a high proportion of cases of untreated proximal DVT and is associated with a mortality rate of 11-23% if not treated. PST, however, is a cause of increased morbidity and disability. The natural history of DVT is a dynamic process, with both thrombolysis and thrombus extension occurring after an episode of DVT. With the introduction of duplex scanning, several clinical studies have investigated and tried to clarify the natural history of DVT, the rate of recanalization of the thrombus, and the presence of reflux and its relation to lysis of the thrombus. These and other debated issues associated with PST are reviewed here. Knowledge of the evolution of these processes could result in better understanding of PST and be applied for improvement of medical and surgical management of venous thrombosis and its complications.
Dietary supplementation with B-vitamins that lower plasma total homocysteine concentrations are expected to lower the risk of cardiovascular disease. Folic acid and vitamin B (12) lower blood homocysteine concentrations by about 25-30% in populations without folic acid fortification, but by only 10-15% in populations with fortification. In observational studies, 25% lower homocysteine is associated with about 10% less coronary heart disease (CHD) and about 20% less stroke. This review examines the current status of 12 large-scale randomized trials of B-vitamin supplementation and risk of cardiovascular disease. Seven of these trials are being performed in populations without fortification (five involving participants with prior CHD, two with prior stroke) and five in populations with fortification (two with prior CHD, two with renal disease, and one with prior stroke). Many of these trials may not have included a sufficient number of people or lasted long enough to have adequate power to exclude false-negative results. Taken together, however, these trials involve 32,000 patients with prior vascular disease in unfortified populations and 20,000 (14,000 with vascular disease and 6000 with renal disease) patients in fortified populations. A metaanalysis of these trials should have adequate power to determine whether homocysteine-lowering vitamin supplements can reduce the risk of cardiovascular disease.
The prevention and treatment of age-related cognitive impairment and dementia is one of the greatest and most elusive challenges of our time. The prevalence of dementia increases exponentially with age, as does the prevalence of those with micronutrient deficiency. Several studies have shown that elevated homocysteine is correlated with cognitive decline and with cerebral atrophy and that it predicts the subsequent development of dementia in cognitively intact middle-aged and elderly individuals. If elevated homocysteine promotes cognitive dysfunction, then lowering homocysteine by means of B-vitamin supplementation may protect cognitive function by arresting or slowing the disease process.
Obesity is becoming more prevalent in the developed world because of the abundance of food and the decrease of physical activity. Obesity is a risk factor for a host of diseases from arthritis to cardiovascular disease. The precise mechanisms by which obesity promotes cardiovascular disease are not well understood but are likely to include metabolic and inflammatory responses to the increased amount of stored fat. The endothelium plays a pivotal role in maintaining vascular health. Impaired endothelial function is an independent predictor of cardiovascular disease. Most studies of vascular function in obese subjects have demonstrated impaired endothelial function. This impairment of endothelial function becomes obvious early on, long before any vascular abnormalities become clinically relevant and detectable. Better understanding of the mediators of obesity-induced endothelial dysfunction may lead to the identification of new targets for interventions that may prevent or postpone the development of obesity-related cardiovascular disease.
Given the pivotal role of thrombin in the pathogenesis of acute coronary syndromes (ACS) and its persistent activation at the site of arterial lesions, antithrombin agents are essential for the prevention of coronary events. Antiplatelet agents are used routinely in the prevention of ACS, but their inability to prevent thrombin generation might contribute to the remaining high rates of recurrent ischemic events after intense antithrombotic treatment in the acute phase. Combination treatment with antiplatelet agents and anticoagulants, such as low-molecular-weight heparins (LMWH) and vitamin K antagonists, provides improved efficacy in the secondary prevention of ACS but these agents have limitations that prevent widespread adoption of their use for long-term treatment. Ximelagatran is the first oral agent in the new class of direct thrombin inhibitors (DTIs) and has considerable therapeutic potential in ACS. The DTIs are able to inhibit free and fibrin-bound thrombin by directly binding to the thrombin catalytic site. Furthermore, the oral administration and predictable pharmacokinetics of ximelagatran mean that it can be used at a fixed dose without coagulation monitoring and is convenient for long-term therapy. The efficacy of ximelagatran in the prevention of coronary events has been investigated in patients with recent myocardial infarction (MI) in the phase II Efficacy and Safety of the Oral Direct Thrombin inhibitor Ximelagatran in Patients with Recent Myocardial Damage (ESTEEM) trial. Ximelagatran (24 to 60 mg twice daily) added to aspirin (160 mg once daily) reduced the risk of the composite end point of death, MI, and severe recurrent ischemia by 24% versus aspirin alone, with no significant increase in major bleeding. Elevated serum transaminase enzymes developed during the first 1 to 6 months of treatment in a proportion of patients given ximelagatran. These elevations usually abated without clinical sequelae whether or not treatment was continued. The ESTEEM results highlight the potential for ximelagatran as an efficacious and well-tolerated long-term treatment for the prevention of arterial thrombotic events.
The role of folic acid and homocysteine in pregnancy is becoming clearer. The efforts of many countries to prevent neural tube defects through public awareness of folic acid have been disappointing, but evidence is now emerging that the food fortification programs in the United States and Canada are effective in reducing the numbers of neural tube defects, and there may be additional benefits in terms of other congenital defects such as oral-facial clefts and congenital heart disease. Homocysteine has a significant association with vascular disease in later life, is elevated in preeclampsia, and has been associated with other pregnancy complications such as early pregnancy loss. The data from cohorts of women with a history of preeclampsia during pregnancy indicate that they are at increased risk for cardiovascular and cerebrovascular disease in later life. Elevated homocysteine concentrations may be a common link that accounts for these associations.
Vitamin K antagonists including warfarin are associated with numerous interactions with other drugs and foods. In clinical practice, this complicates the task of maintaining plasma levels of warfarin within a narrow therapeutic window and so maximizing protection against thromboembolic events while minimizing the risk of complications, particularly bleeding. In contrast, ximelagatran has a low potential for pharmacokinetic drug:drug and food interactions. There is no significant metabolism of melagatran, and the main route of elimination of melagatran is renal excretion that appears to occur via glomerular filtration. Most importantly, cytochrome P450 isoenzymes that mediate many drug:drug interactions are not involved in the biotransformation of ximelagatran to melagatran. No significant pharmacokinetic interactions have been observed when oral ximelagatran is administered with a range of agents, including diclofenac, diazepam, nifedipine, digoxin, atorvastatin, or amiodarone. The low potential for drug:drug interactions with ximelagatran is also supported by an analysis of the pharmacokinetic data from clinical studies in patients with atrial fibrillation receiving long-term treatment with oral ximelagatran. Increases of mean melagatran area under the curve and maximum plasma concentration ( Cmax) of up to approximately 80% have been observed when ximelagatran is co-administered with the macrolide antibiotics erythromycin or azithromycin, and the mechanism for this interaction is currently under investigation. The bioavailability of melagatran is not altered by co-administration with food or alcohol. The melagatran-induced prolongation of activated partial thromboplastin time (APTT), an ex vivo coagulation time assay used as a measure of thrombin inhibition, is not altered by other drugs [including digoxin, atorvastatin, acetylsalicylic acid (ASA), and amiodarone], food, or alcohol. The effect of melagatran on capillary bleeding time, which is prolonged as a result of the inhibition of thrombin-induced platelet aggregation, is relatively low and additive to the platelet-inhibitory effect of ASA.
The present issue of Seminars in Vascular Medicine assembles a series of articles that nicely elucidate the current knowledge on the use of fibrin D-dimer testing for venous and arterial thrombotic disease in vascular medicine.
Elevated plasma total homocysteine concentration is a risk factor for venous thrombosis. The association is well established in patients with homocystinuria irrespective of the genetic etiology and metabolic background. Homocystinuria is a human model of chronic exposure to very high concentrations of plasma homocysteine and reflects an abnormal amino acid metabolism. Elevated homocysteine levels in patients with venous thrombosis have attracted considerable interest because homocysteine is a potentially reversible thrombophilic marker for venous thrombosis. In contrast to homocystinuria, hyperhomocysteinemia is mild and reflects environmental and constitutional factors such as age, intake of B-vitamins, derangements of metabolism, and renal impairment. This review examines the evidence for the relationship of homocysteine with risk of venous thrombosis in homocystinuria and in the general population.
Severe hyperhomocysteinemia is typically caused by rare enzymatic defects or by renal failure. In contrast, mild to moderate hyperhomocysteinemia chiefly results from suboptimal status of nutritional factors involved in homocysteine metabolism. Low dietary intake of folate is the most important nutritional cause of elevated homocysteine (tHcy) concentrations. Folic acid is more effective than dietary folate in lowering tHcy concentrations, and a daily dose of 400 mug of folic acid is the minimum daily dose associated with the maximum tHcy-lowering effect ( approximately 20-25% reduction). Mean fasting tHcy concentrations have dropped substantially in populations with mandatory folic acid fortification, and other B-vitamins, such as vitamin B (12), are important determinants of tHcy levels in this setting. Vitamins B (2) and B (6) have little influence on fasting tHcy concentrations, although the former may be relevant in individuals with the MTHFR 677 TT-genotype, and the latter may improve tHcy catabolism in elderly individuals. Betaine and choline can lower fasting tHcy concentrations to a similar extent as folic acid, particularly in the setting of a high intake of methionine. Consumption of tea and coffee increase tHcy concentrations by up to 20%. A high-protein meal also increases tHcy, but these changes are transient, and levels return to normal after an overnight fast. Serine and cystine also influence the methionine-induced postprandial rise in tHcy concentrations. In conclusion, alteration in dietary intake or use of folic acid supplements can substantially lower tHcy concentrations. However, it is not known whether lowering tHcy levels can reduce the risk of cardiovascular disease or cognitive decline or prevent pregnancy complications or osteoporosis.
Homocysteine is derived from the essential amino acid methionine and plays a vital role in cellular homeostasis in man. Homocysteine levels depend on its synthesis, involving methionine adenosyltransferase, S-adenosylmethionine-dependent methyltransferases such as glycine N-methyltransferase, and S-adenosylhomocysteine hydrolase; its remethylation to methionine by methionine synthase, which requires methionine synthase reductase, vitamin B (12), and 5-methyltetrahydrofolate produced by methylenetetrahydrofolate reductase or betaine methyltransferase; and its degradation by transsulfuration involving cystathionine beta-synthase. The control of homocysteine metabolism involves changes of tissue content or inherent kinetic properties of the enzymes. In particular, S-adenosylmethionine acts as a switch between remethylation and transsulfuration through its allosteric inhibition of methylenetetrahydrofolate reductase and activation of cystathionine beta-synthase. Mutant alleles of genes for these enzymes can lead to severe loss of function and varying severity of disease. Several defects lead to severe hyperhomocysteinemia, the most common form being cystathionine beta-synthase deficiency, with more than a hundred reported mutations. Less severe elevations of plasma homocysteine are caused by folate and vitamin B (12) deficiency, and renal disease and moderate hyperhomocysteinemia are associated with several common disease states such as cardiovascular disease. Homocysteine toxicity is likely direct or caused by disturbed levels of associated metabolites; for example, methylation reactions through elevated S-adenosylhomocysteine.
Assays for D-dimer antigen are based on monoclonal antibodies reactive with epitopes found on fibrin fragment D-dimer but not on fibrinogen fragment D, other fibrinogen degradation products, or native fibrinogen. The antibodies react with conformational epitopes generated by factor XIII-induced linkage of the C-terminal appendages of the fibrin gamma-chains of adjacent D-domains within a fibrin polymer. For some monoclonal antibodies, degradation of the cross-linked fibrin compound by plasmin is an additional requirement for the generation of the epitope. In clinical plasma samples, D-dimer antigen assays detect an array of fibrin compounds of different molecular weights, including fibrin fragment D-dimer as well as higher-molecular-weight fibrin degradation products and fibrin X-oligomers. Most D-dimer antigen represents cross-linked soluble fibrin present in circulation rather than degradation products from particulate clots. Due to differences in epitope reactivity, harmonization of D-dimer antigen assays can only be achieved with standard preparations containing a similar variety of cross-linked fibrin compounds. Assay technologies include manual latex agglutination assays, automated latex-enhanced light-scattering immunoassays, enzyme-linked immunoassays, and others.
