Investigations for thrombotic tendencies

Summary

New thrombophilia factors have recently been discovered. They explain the majority of cases of familial or recurrent venous thrombosis and some cases of atherothrombosis. They include activated protein C resistance (Factor V Leiden), prothrombin gene mutation, mild hyperhomocysteinaemia and antiphospholipid antibodies. These factors can identify patients at higher risk of thrombosis who may benefit from prevention and improved treatment strategies. However, these factors are also common in the general population (approximately 1 in 20 people) so a thorough understanding of their significance and clinical management is important.

Introduction

Thrombophilia can be defined as an increased tendency to develop arterial or venous thrombosis which is recurrent, familial, or presents at an unusual site or at a young age. The thrombosis can be catastrophic leading to death, permanent disability, prolonged periods of hospitalisation or chronic symptoms of lower limb venous insufficiency. The results of appropriate laboratory investigations can help us develop strategies that will either prevent the occurrence of thrombosis or assist with decisions about effective antithrombotic treatment. Until recently, the aetiology of most cases of thrombophilia was largely unknown. This situation has dramatically changed because we now know that over 80% of patients with thrombophilia and venous thrombosis have an abnormality of the natural anticoagulant system.

Clinical assessment is important

Most cases of deep venous thrombosis can be explained by the interaction of plasma coagulation `thrombophilia' abnormalities and the well-recognised `clinical' risk factors (such as obesity, major abdominal or orthopaedic surgery, oral contraceptive pill, pregnancy, malignancy or immobility). The larger the number of either clinical and laboratory risk factors, the greater the chance of deep venous thrombosis.

Accurate clinical assessment is important to establish the diagnosis, identify family members at risk, prevent recurrence and rule out other occult disease. Particular emphasis is made on the site and severity of thrombosis, whether or not it occurred spontaneously or postoperatively and if the event is associated with a well-identified precipitating factor such as oestrogen therapy or plane travel. A previously undiagnosed malignancy (particularly a mucin secreting adenocarcinoma) or a myeloproliferative disorder may present with a similar picture of unusual thrombosis. These conditions should be considered before testing for thrombophilia.

Newly described thrombophilia factors

The frequency and the relative risk of venous thrombosis for the thrombophilia factors are found in Table 1.¹

Activated protein C resistance

Activated protein C (APC) resistance is a hereditary defect of the protein C natural anticoagulant pathway (Fig. 1). APC normally acts as a natural anticoagulant. It down regulates the intensity of the clotting cascade by neutralising activated coagulation Factor V. This process is inefficient in people with APC resistance. They do not have the crucial APC cleavage site on the Factor V molecule because of a point mutation (Factor V Leiden).

This mutation is common, occurring in 4% of randomly selected healthy people in Australia.² It can be found in up to 50% of patients with recurrent familial venous thrombosis. The mutation causes an 8-fold increased risk of thrombosis compared to normal. The homozygous form is not infrequent and substantially increases the risk of thrombosis to 100 times normal. The inheritance pattern is autosomal dominant so there is a one in two chance that other family members may have a similar predisposition to thrombosis.¹ APC resistance is usually detected by a sensitive and specific clotting assay and confirmed by molecular analysis for the Factor V Leiden mutation.

Women are especially at risk because oestrogens and pregnancy combined with APC resistance substantially increase the likelihood of thrombosis. The use of a combined oral contraceptive increases the relative risk of thrombosis to 35-fold in those with APC resistance.³ However, the absolute increased risk for thrombosis is small and is estimated to be 3% over a 10-year period of oral contraceptive use.¹

In general, APC resistance is not associated with ischaemic heart disease² or stroke. However, subgroup analysis reveals a 30-fold increased risk of acute myocardial infarction in young women who smoke or who are obese. There are reports of young women who are homozygous for the Factor V Leiden presenting with acute myocardial infarction but normal coronary arteries.

Fig. 1 Regulation of coagulation activation and thrombophilia abnormalities. Activation of the blood coagulation system generates large amounts of thrombin which produce cleavage of fibrinogen to form a stable fibrin clot. Natural anticoagulants down regulate the coagulation cascade. Deficiency of these natural anticoagulants, resistant Factor V (Factor V Leiden) and increased prothrombin levels impair down regulation. This predisposes the patient to thrombosis.
Key	Activation
	Inhibition
Sites of abnormalities * - Factor V Leiden + - prothrombin gene mutation ++ - antithrombin III deficiency s - protein C deficiency II - protein S deficiency

Prothrombin gene mutation

A point mutation in the prothrombin molecule causes an increase in its circulating plasma level predisposing to thrombus formation. The mutation is common in the healthy Australian population (3.3%)⁴ and it occurs in up to 15% of patients with thrombophilia. It is estimated to increase the thrombotic risk by 4-fold compared to those without the mutation.¹ The mutation can only be detected by DNA polymerase chain reaction (PCR). The inheritance is autosomal dominant.

High homocysteine levels

A high concentration of homocysteine is an independent risk factor for atherothrombosis and venous thrombosis.⁵ Although severe hyperhomocysteinaemia is rare, mild hyperhomocysteinaemia is common occurring in about 5% of the general population, increasing to 15% in patients with venous thrombosis and up to 40% in patients with all forms of premature vascular disease. Mild hyperhomocysteinaemia increases the relative risk of coronary artery disease 24-fold and there is a 3-fold increase in the relative risk of venous thrombosis. There appears to be a graded rather than a threshold relationship between plasma homocysteine and the risk of vascular disease and mortality.

Mild elevations of homocysteine concentrations are predominantly caused by subclinical deficiency of folic acid, vitamin B₁₂ or pyridoxine, particularly when also associated with genetic mutations in the enzymes that control the metabolism of methionine (methylene tetrahydrofolate reductase and cystathionine beta synthase). Other causes include chronic renal failure, malignancy, hypothyroidism, cigarette smoking and drugs (methotrexate, phenytoin and theophylline).

Treatment varies with the underlying cause, but correcting vitamin deficiency is generally effective in reducing the homocysteine concentration. Even when there is no detectable vitamin deficiency, folic acid taken in doses of 2-5 mg daily will normalise the homocysteine level. However, randomised clinical trials will be necessary to show that lowering homocysteine levels will have an impact on cardiovascular or thrombotic events. This form of therapy is relatively safe and inexpensive and it would not be unreasonable, whilst awaiting the results of the prospective clinical trials, to give vitamins to patients with atherothrombosis and hyperhomocysteinaemia.⁶

Antiphospholipid antibodies

Auto-antibodies against phospholipid and other molecules on the platelet surface can be associated with atherothrombosis. The antibody type, class, strength and target antigen(s) are extremely varied, not only amongst individual patients but also within the same patient at varying times. The two common laboratory methods used to detect these antibodies are the lupus anticoagulant and the anticardiolipin antibody assays. The lupus anticoagulants are antiphospholipid antibodies detected by clotting methods. Anticardiolipin antibodies are detected by serological methods. Each of these auto-antibodies against phospholipids and other coagulation molecules is associated with both arterial and venous thrombosis. Although most of the time both tests are simultaneously abnormal, only one test may be abnormal in one-third of cases despite identical clinical conditions. These antibodies are common in patients with thrombosis. Their detection is important because there may be a substantial increase in the risk of recurrent thrombosis. However, low titres of auto-antibodies are frequently found in the normal population, are of uncertain clinical significance and may be transient, particularly in response to infection.

Prevention of recurrent thrombosis (especially arterial thrombosis) in patients with persistently high antibody titres may require long-term warfarin therapy. This is set at a higher target INR range (3-4.5).⁷ Aspirin appears to be less effective in the prevention of recurrent thrombosis than warfarin. Patients with recurrent spontaneous abortions may have antiphospholipid antibodies. The combination of low-dose aspirin (100 mg) and standard heparin (5000 IU twice daily) substantially reduces fetal loss in any subsequent pregnancy. The detection of antiphospholipid antibodies in patients with thrombosis is frequently the only manifestation of the auto-immune disease (known as the primary antiphospholipid antibody syndrome), but it can also be associated with other auto-antibody syndromes such as systemic lupus erythematosus.

Co-existing thrombophilia factors increase thrombosis risk

As thrombophilia factors are relatively frequent, compound heterozygotes (such as Factor V Leiden together with the prothrombin gene mutation) are common. This substantially increases the risk of venous thrombosis compared to having only one abnormality.¹ The thrombophilia genes are independently inherited in thrombosis-prone families so they may be absent, or one or other gene may be found, or both abnormalities may co-exist. This may explain why in some families the thrombosis phenotype is variable. The risk of thrombosis may be further increased by the co-existence of antiphospholipid antibodies or high levels of homocysteine.

Laboratory testing

A diagnosis of thrombophilia should ensure appropriate prophylaxis is given in high-risk situations such as after surgery, immobility, long plane flights, during pregnancy and postpartum. Prevention includes measures such as anticoagulant therapy (standard heparin or low molecular weight heparin), early mobilisation and increasing lower limb venous blood flow (compression stocking and plantar plexus foot pump). The intensity of prophylaxis against venous thrombosis depends on the perceived hazard in each case versus the risk reduction by intervention. At the very least, diagnosis of a thrombophilia factor should ensure preventive measures are considered even in family members without a history of thrombosis.

Who to test?

Any patient with spontaneous, unusual, recurrent or a strong family history of venous thrombosis or evidence of premature arterial occlusion should be tested. It is reasonable to test first-degree relatives for an identified hereditary thrombophilia factor because half the family members will inherit the mutation. Finding the abnormality will provide an opportunity to modify other risk factors and ensure appropriate prophylaxis against venous thrombosis in high-risk situations.

Which test and when

Patients can be tested (Table 2) either at the presentation of thrombosis or after finishing anticoagulation. Abnormal results should always be confirmed by repeat testing. This is because the levels of the natural anticoagulant factors may be altered by consumption in the clotting process, blood collection artefacts and standardisation and reproducibility problems inherent in most of the clotting-based laboratory techniques. Testing other family members without thrombosis to confirm the hereditary nature of the problem is often helpful for a patient with uncertain results due to acute thrombosis or anticoagulation.

Molecular confirmation of the Factor V Leiden mutation is recommended not only to confirm the abnormal clotting result, but also to differentiate clearly a homozygote and heterozygote carrier. Protein C and S are vitamin K dependent anticoagulant factors and so deficiency cannot be diagnosed while the patient is on warfarin. However, all the thrombophilia factors can usually be tested while the patient is on therapeutic heparin or low molecular weight heparin. Routine homocysteine testing is only now entering clinical practice and it is uncertain if fasting levels or a methionine load test (analogous to a glucose tolerance test for diabetes) will be required.

Conclusion

Thrombophilia factors are frequently found in the majority of patients with recurrent or familial venous thrombosis. Obtaining a familial history of thrombosis is now analogous to a bleeding history (particularly if the patient has a convincing family history of deep venous thrombosis and is considering surgery or the oral contraceptive pill). Laboratory testing can be used to see if these patients have a higher risk of thrombosis. This information leads to better informed choices and the development of strategies to prevent thrombosis. However, most patients with these thrombophilia factors will never develop thrombosis in their lifetime. The occurrence of thrombosis is explained in people who are at high risk because of the accumulation of an increasing number of either clinical and/or thrombophilia factors.