Thrombophilia laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Asiri Ediriwickrema, M.D., M.H.S. [2] Jaspinder Kaur, MBBS[3]
Overview
Laboratory findings consistent with the diagnosis of inherited thrombophilias vary based on the etiology of the thrombus.[1]
Laboratory Findings
Initial assessment
- Aim: Determining whether a blood clot classifies as provoked (most common) vs. unprovoked, and whether it is the first episode vs. subsequent are critical aspects of the initial evaluation that can guide further workup and treatment.
- The initial assessment involves a carefully taken clinical history and physical examination in addition to performing appropriate laboratory, imaging and other relevant investigations.
- Depending on the initial assessment and the clinical management decisions to be addressed, laboratory testing for heritable thrombophilia can be considered.
- NICE guidelines:
- Provoked VTE: The most recent guidelines recommend against offering inherited thrombophilia testing to patients presenting with a provoked VTE in any clinical setting.
- Unprovoked VTE: Testing should not be considered unless a first degree relative with a history of VTE exists.
- The NICE guidelines in accordance with the American Society of Hematology’s Choosing Wisely recommendations also recommend against routinely offering thrombophilia testing to asymptomatic first-degree relatives of patients with a history of VTE or known inherited thrombophilia as there is no evidence to support thromboprophylaxis in this setting.
- Hospitalized patients:
- Its been recommended that the clinicians should avoid ordering thrombophilia testing for hospitalized patients with unprovoked VTE because of the following:
- Many thrombophilia tests are inaccurate in the setting of acute VTE and/or anticoagulation
- Results of testing often do not influence management
- Testing is not cost-effective
- A positive test result may lead to unnecessary patient anxiety
- Testing may result in inappropriately prolonged anticoagulation courses or unnecessary involvement of inpatient consultants.
- Its been recommended that the clinicians should avoid ordering thrombophilia testing for hospitalized patients with unprovoked VTE because of the following:
- Individualized approach: Depending on the underlying inherited condition and expression of the genetic abnormality, the relative risk of VTE in patients with inherited thrombophilia is 3- to 20-fold greater than that of the general population which further states that testing for inherited thrombophilia might be clinically useful. However, the testing process may divert attention away from the management of more prevalent and potentially modifiable risk factors such as immobility, oral contraceptive use, or malignancy which are also associated with recurrent VTE. However, the evidence for doing so is very limited. Hence, testing should only be considered using an individualized approach in the outpatient setting with appropriate genetic counseling.
Indications for thrombohilia screening and testing
- Indiscriminate application of laboratory investigations is clinically inappropriate, wastage of scarce resources, creates unnecessary anxiety and can be misleading as diagnostic uncertainty is frequent.
- Hence, its been advisable to take hematologists consult on the selection of patients for testing as the testing of unselected patients is inappropriate and should be avoided.
Table 1: Thrombophilia testing may be indicated in the following scenarios:
| Indications for Thrombophilia testing |
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Common thrombophilia tests
- Table 2: Diagnostic workup for thrombophilia testing
| Diagnostic tests |
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- Following tests are not indicated commonly:
- MTHFR (methylenetetrahydrofolate reductase) genetic test
- Factor VIII level
- tPA (tissue plasminogen activator) blood levels
- PAI-1 (plasminogen activator inhibitor type 1) blood levels
- Genetic tests
Recommendations for laboratory tests and interpretation
- When testing is indicated, it should include assays for heritable defects: deficiency of antithrombin, protein C or protein S, factor V Leiden and prothrombin G20210A mutations and for antiphospholipid antibodies.
- The activated partial thromboplastin time (APTT), prothrombin time and thrombin clotting time should be incorporated in the initial screening. The APTT may identify some patients with antiphospholipid antibodies (depending on the sensitivity of the APTT reagent used), but is not sufficient alone to exclude antiphospholipid antibodies. The thrombin clotting time will allow identification of dysfibrinogenaemia and heparin contamination. The prothrombin time is useful in the interpretation of low protein C or protein S results.
- Functional assays should be used to determine antithrombin and protein C levels
- Chromogenic assays of protein C activity are less subject to interference than clotting assays and are preferable.
- Immunoreactive assays of protein S antigen are preferable to functional assays. If a protein S activity assay is used in the initial screen, low results should be further investigated with an immunoreactive assay of free protein S.
- The modified APC:SR test (predilution of the test sample in factor V-deficient plasma), as opposed to the original APC:SR test, should be used as a phenotypic test for the factor V Leiden mutation.
- PCR-based testing for prothrombin G20210A is required, as there is no screening test
- Laboratories must establish their own reference ranges for the assays and tests that they use, including antithrombin, protein C and protein S and modified APC:SR
- Comprehensive assays for antiphospholipid antibodies (both lupus inhibitors and anticardiolipin antibodies) should also be performed.
- Rigorous internal quality assurance and participation in accredited external quality assessment schemes are mandatory.
- The interpretation of thrombophilia test results is difficult and fraught with pitfalls, which occasionally lead to underdiagnosis and frequently to overdiagnosis of defects.
It is strongly recommended that thrombophilia testing is supervised by and results are interpreted by an experienced clinician who is aware of all relevant factors that may influence individual test results in each individual.
Factors affecting the accuracy of the tests
- Warfarin decreases protein C and protein S levels.
- Heparin decreases antithrombin activity and lead to false positive results for lupus anticoagulant.
- Direct acting oral anticoagulants can interfere with evaluation for lupus anticoagulant, antithrombin activity, protein C, and S levels.
- Acute thrombosis decreases antithrombin, protein C, and protein S levels.
- Pregnancy, cirrhosis, nephrotic syndrome, chemotherapies (l-asparaginase), anticoagulants, disseminated intravascular coagulation, and systemic illness can alter coagulation factor levels.
- Acute inflammation or consumption of coagulation factors due to acute thrombosis can lead to falsely low levels of some coagulation factors.
- For antiphospholipid antibody testing, many medications (including several anticoagulants), as well as underlying connective tissue diseases such as SLE, can affect the testing result and interpretation of the lupus anticoagulant.
- Timing of testing for thrombophilia is important because results can be falsely abnormal at the time of the initial clotting episode or while on a blood-thinner.
When to collect samples for thrombophilia testing
- Some tests for heritable thrombophilia (for example, assays of antithrombin, protein C and protein S) are affected by the acute post-thrombotic state and by anticoagulant use. Also, finding a thrombophilic abnormality almost never influences the management of an acute thrombotic event. There is little point in striving to obtain samples for tests for heritable thrombophilia when the patient presents with an acute thrombotic event. Testing is usually best delayed until at least 1 month after completion of a course of anticoagulation. If possible, testing for heritable thrombophilia should be avoided during intercurrent illness, pregnancy, use of a combined oral contraceptive pill or hormone replacement therapy. If this is impossible, then it is essential that the individual interpreting the screen is aware of the presence and potential influence of these various acquired factors on the components of the test results. PCR-based tests for FV Leiden and the prothrombin 20210A allele are unaffected by the above factors.
- Given the variability discussed above, timing is important to accurately evaluate for certain thrombophilic states.[2][3]
- Acute thrombosis can affect levels of coagulation factors, therefore, testing should be delayed for approximately six months.
- Anticoagulation can affect certain tests, and should be completed approximately four weeks following completion of anticoagulation.
- Avoid testing during severe illness.
- Pregnancy, oral contraceptives, hormone replacement therapy, and cancer chemotherapy may also affect some tests.
- Factor V Leiden and Prothrombin mutation can be done in patients on anticoagulants and even in acute phase, as these are genetic tests.
- When antiphospholipid antibody testing is indicated, the recommendation is that the tests be repeated 12 weeks apart. As such, clinicians should reserve such hypercoagulable testing to carefully selected patients (e.g., young patients with unprovoked thrombosis or in those with confirmed or strong family history thrombophilia). This testing is often performed in the outpatient setting after the acute phase of the thrombosis has resolved and under the guidance of a hematologist consultant.
Emergency room assessment and screening
- When patients present to the emergency department with signs and symptoms suggestive of possible venous thrombosis (see previous section H&P findings), a well-validated scale known as the modified Wells' criteria is applied to help guide further diagnostic studies. For patients with high Wells score, a serum D-dimer should be checked. The D-dimer is a fibrin degradation product that is present in the blood after fibrinolysis. Its elevation is very sensitive (though less specific) to detect venous thrombosis. It is important to note that a D-dimer could also be elevated in other patients such as pregnant and post-surgical patients, or those with underlying malignancy. However, it aids clinicians in deciding whether to pursue further diagnostic imaging. A negative D-dimer result helps to rule out a clot and avoid unnecessary imaging studies or anticoagulation initiation. The pulmonary embolism rule-out criteria (PERC) is also occasionally applied to help decide whether the patient has developed an acute pulmonary embolism (PE), though not applicable for DVT. The modified Wells score can also be used if physicians suspect an acute DVT. When there is a high pretest probability for PE or DVT, imaging studies should be completed immediately without regard to D-dimer levels. For pulmonary embolism, the recommended imaging studies are CT angiography and ventilation/perfusion imaging (V/Q scan). The V/Q scan is sometimes preferred over CTPA to avoid radiation exposure or intravenous contrast in those with underlying renal impairment. However, not all facilities have V/Q scanning capabilities or expertise at interpreting the results, so CTPA is often used. Often, pulmonary emboli result from fragmentation of preexisting thrombosis in an extremity (i.e., DVT). Hence, compression sonography (Duplex US) of lower and/or upper extremities is also often performed to evaluate for concurrent DVT. This is especially important if a provoking catheter-related thrombosis is suspected, as the catheter may require eventual removal.
Protein C deficiency
- Diagnostic testing for protein C deficiency is performed using functional assays including clotting assays, enzyme-linked immunosorbent assays (ELISA) and chromogenic tests to determine levels of protein C activity. Mutational analysis of the PROC gene is also available.
Malignancy
- Malignancy can be an underlying provoking factor for thrombosis (both venous and arterial). In general, it is not recommended for patients to have an extensive malignancy workup in the absence of any clinical factors to suggest underlying malignancy as the provoking cause. Age-appropriate cancer screening is recommended for all patients. In carefully selected patients, especially in those over the age of 50 with seemingly unprovoked thrombosis, in whom malignancy is a possibility, further evaluation to search for occult malignancy as the culprit may be indicated.
- Occult cancer should be considered, although invasive investigation is not routinely indicated. A full blood count should be performed to exclude myeloproliferative disorders.
References
- ↑ Seligsohn U, Lubetsky A (2001). "Genetic susceptibility to venous thrombosis". N Engl J Med. 344 (16): 1222–31. doi:10.1056/NEJM200104193441607. PMID 11309638.
- ↑ Cohoon KP, Heit JA (2014). "Inherited and secondary thrombophilia". Circulation. 129 (2): 254–7. doi:10.1161/CIRCULATIONAHA.113.001943. PMC 3979345. PMID 24421360.
- ↑ Stevens SM, Woller SC, Bauer KA, Kasthuri R, Cushman M, Streiff M; et al. (2016). "Guidance for the evaluation and treatment of hereditary and acquired thrombophilia". J Thromb Thrombolysis. 41 (1): 154–64. doi:10.1007/s11239-015-1316-1. PMC 4715840. PMID 26780744.