Laboratory Studies
The decision to initiate a laboratory workup for thrombophilia is complex. [38, 39, 40, 41] A workup for thrombophilia is usually indicated only in patients with one or more of the following risk factors:
-
Recurrent thromboembolic episodes
-
Thromboembolism at a young age (ie, < 40 y)
-
A family history for thromboembolism
-
Thrombosis in an unusual site
Of patients with idiopathic venous thrombosis, which is defined as venous thromboembolism without any obvious risk factor, about 50% have an underlying thrombophilia. Therefore, some authors have recommended performing a thrombophilia workup in patients with idiopathic venous thrombosis.
American Society of Hematology guidelines on thrombophilia testing, published in 2023, conditionally recommend thrombophilia testing in the following scenarios [42] :
-
Patients with venous thromboembolism (VTE) associated with non-surgical major transient or hormonal risk factors
-
Patients with cerebral or splanchnic venous thrombosis, in settings where anticoagulation would otherwise be discontinued
-
Individuals with a family history of antithrombin, protein C, or protein S deficiency, when considering thromboprophylaxis for minor provoking risk factors, and for guidance to avoid combined oral contraceptives or hormone replacement therapy
-
Pregnant women with a family history of high-risk thrombophilia types
-
Patients with cancer who are at low or intermediate risk of thrombosis and have a family history of VTE
The decision to order a thrombophilia workup can be difficult, because the identification of an underlying thrombophilia might not affect therapeutic strategy. If thrombophilia is detected in a patient with no history of thromboembolism, anticoagulation is usually not necessary. Conversely, patients with recurrent thromboembolic events should be anticoagulated even if their workup uncovers no evidence of an underlying thrombophilia or a lupus anticoagulant.
Inherited thrombophilia is often a consideration in children who develop thrombotic disease. Although identification of an inherited disorder is unlikely to influence acute management of the thrombotic event or the duration of anticoagulation, it may lead to diagnosis of the disorder in affected family members, who can then be counseled regarding their thrombotic risk. [43]
Some benefits may exist with testing patients for thrombophilia and antiphospholipid syndrome (lupus anticoagulants).The presence of more than one risk factor results in an incidence rate of venous thrombosis that is greater than the sum of the individual risks. For example, women with a prothrombotic mutation who are taking estrogen have a 25-fold greater risk of venous thrombosis than women without the mutation who are on estrogen. [44]
Patients who have an identifiable thrombophilic risk factor should be advised to have blood relatives tested, as this information would be important for their physicians, in deciding whether to recommend oral contraception or hormone replacement therapy. Also, this will help determine whether these relatives should receive anticoagulation during surgery or immobilization.
In addition to the challenge of deciding who should be tested, clinicians need to be aware of when to test—or more precisely, when not to. Many of the tests (coagulation-based studies) should not be done while patients are on anticoagulants or during active thrombosis.
A full thrombophilia panel consists of simultaneous tests for the following [42] :
-
Factor V Leiden
-
Prothrombin 20210A gene mutation
-
Antithrombin deficiency
-
Protein C deficiency
-
Protein S deficiency
-
Antiphospholipid antibodies compatible with antiphospholipid syndrome
Numerous tests are available for each of the hypercoagulable disorders or thrombophilia. A D-dimer assay is useful as a general test for verifying the presence of thrombosis. Testing for specific disorders is described below.
Antiphospholipid syndrome
Antiphospholipid syndrome (lupus anticoagulant) [20, 21] screening tests are as follows:
-
Activated partial thromboplastin time (aPTT)
-
Prothrombin time (PT)
-
Mixing studies
Confirmation of the diagnosis is based on the following study results:
-
Prolonged phospholipid-dependent dilute Russell viper venom time (dRVVT)
-
Increased titers of anticardiolipin antibodies (IgG and IgM)
-
Increased titers of anti-β(2)glycoprotein 1 antibodies (IgG and IgM)
Confirmatory test results should be positive on two occasions 12 weeks apart. Efforts to standardize these tests have been made, to enable reliable diagnosis of lupus anticoagulants and prediction of the risk for thrombosis. It has been suggested that the risk of thrombosis is greater when results from several of these tests are positive. [45]
For full discussion, see Antiphospholipid Syndrome and Antiphospholipid Antibody Syndrome and Pregnancy
Factor V Leiden
In patients with suspected factor V Leiden, coagulation testing with an activated protein C (APC) resistance assay should be done first, because a small fraction of APC resistance disorders are due to mutations other that factor V Leiden. In this assay, the ratio of aPTT testing performed with and without added APC is reported as the APC resistance (or sensitivity) ratio. [27]
A ratio of < 2.3 suggests abnormal resistance to APC of hereditary origin. In patients who carry the factor V Leiden mutation, those who are homozygous have a very low APC resistance ratio, typically 1.1 to 1.4, while in heterozygous carriers the ratio is usually 1.5 to 1.8. [27]
Patients with an abnormally low APC resistance assay result should undergo genetic testing for factor V Leiden. This test, which is a polymerase chain reaction (PCR) assay, can identify carriers and determine whether they are heterozygous or homozygous.
Antithrombin and prothrombin abnormalities
Available studies for antithrombin deficiency include both functional and antigenic assays. Functional studies should always be performed, because some cases of antithrombin deficiency may be associated with normal antigen levels. The functional study is a chromogenic heparin cofactor assay, which measures the ability of antithrombin to bind heparin and neutralize thrombin or factor Xa. Antithrombin deficiency can be acquired or represent either of two major hereditary types, and further immunologic assessment or DNA sequencing can be done to characterize the specific defect present.
For full discussion, see Antithrombin Deficiency.
Prothrombin (factor II) deficiency can be acquired (eg, due to severe liver disease, vitamin K deficiency, or development of an anti-prothrombin antibody) or hereditary. PCR testing can identify the prothrombin G20210A mutation. For more information, see Factor II.
Protein C deficiency
Although both functional (amidolytic) and antigen assays for protein C are available, functional (amidolytic) studies should always be performed to diagnose protein C deficiency, because some cases of protein C deficiency may be associated with normal antigen levels. For full discussion, see Protein C Deficiency
Protein S deficiency
For protein S deficiency, free antigen, total antigen, and functional assays are available. All three should be performed, because variants of protein S deficiency include the following:
-
Low total protein S, normal free protein S
-
Low-normal total protein S, low free protein S
-
Normal total and free protein S, functionally abnormal protein S
For full discussion, see Protein S Deficiency.
Precautions
Tests for hypercoagulability are affected by a number of conditions. Therefore, precautions are important when ordering laboratory studies to rule out an underlying thrombophilia. Limitations on studies that can be performed while patients are undergoing anticoagulation therapy include the following:
-
Testing for antithrombin functional activity should not be done while the patient is on unfractionated heparin or low molecular weight heparin (LMWH)
-
Testing for protein C or S functional activity should not be done while patients are on warfarin, since protein C and protein S are vitamin K–dependent proteins
-
Testing for APC resistance should be deferred when patients are on anticoagulant therapy, since this test is a coagulation assay; however, genetic tests of factor V Leiden can be ordered
-
DRRVT and phospholipid dependence for confirming lupus anticoagulants should not be done while the patient is being anticoagulated, since they are coagulation-based tests, but testing for anticardiolipid antibodies or anti-β(2)glycoprotein 1 antibodies can be performed during anticoagulation
-
Antithrombin, protein S, and protein C levels may be decreased during acute thromboembolism; therefore, both protein assays and functional assays of these proteins could be inaccurate during the acute phase of thromboembolic disease
The tests should be performed in laboratories that specialize in testing for thrombophilia. In addition, the results can be difficult to interpret, so interpretation is best done by a physician with considerable experience with thrombophilias.
Tables
Condition |
Prevalence in General Population (%) |
Relative Risk of VTE (%) |
Relative Risk of Recurrent VTE (%) |
Factor V Leiden (heterozygous) |
3-7 |
4.3 |
1.3 |
Prothrombin 20210A (heterozygous) |
1-3 |
1.9 |
1.4 |
Protein C deficiency (heterozygous) |
0.02-0.05 |
11.3 |
2.5 |
Protein S deficiency (heterozygous) |
0.01-1 |
32.4 |
2.5 |
Antithrombin deficiency (heterozygous) |
0.02-0.04 |
17.5 |
2.5 |
VTE = Venous thromboembolism |
|||
