Which laboratory tests should be ordered to evaluate for inherited and acquired hypercoagulable disorders?

Laboratory Evaluation for Hypercoagulable Disorders

Initial First-Line Testing

For suspected hypercoagulable states, order the following initial panel: complete blood count (CBC), prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen level. 1, 2 These tests screen for coagulation factor deficiencies, thrombocytopenia, and baseline hemostatic function before proceeding to specialized thrombophilia testing. 1

Basic Coagulation Screen

• CBC with platelet count to identify thrombocytopenia or thrombocytosis that may contribute to bleeding or clotting risk 1, 2
• PT to assess the extrinsic pathway (factors II, V, VII, X) 1, 3
• aPTT to evaluate the intrinsic pathway (factors VIII, IX, XI, XII) 1, 3
• Fibrinogen level (Clauss method) as 97% of clinicians include this in first or second-line testing 1

Inherited Thrombophilia Testing

For inherited hypercoagulable disorders, test for factor V Leiden mutation, prothrombin G20210A gene mutation, antithrombin deficiency, protein C deficiency, and protein S deficiency. 4, 5, 6 These represent the most common inherited causes of venous thromboembolism and are inherited in an autosomal dominant pattern. 4

Specific Inherited Disorder Tests

• Factor V Leiden mutation (activated protein C resistance testing) - the most common inherited thrombophilia 1, 4, 6
• Prothrombin G20210A gene mutation - second most common genetic cause 4, 6
• Antithrombin III level - deficiency causes severe thrombotic risk 4, 5, 6
• Protein C activity assay - deficiency predisposes to venous thrombosis 4, 5, 6
• Protein S activity assay (free and total) - deficiency increases clotting risk 4, 5, 6

Activated Protein C Resistance Testing Approach

Use a coagulation-based aPTT assay with and without added activated protein C (APC) to screen for factor V Leiden. 1 The ratio of clotting times (with APC/without APC) identifies resistance, with ratios typically <2.0 indicating factor V Leiden. 1 Predilution assays (1:10 to 1:40) reduce interference from lupus anticoagulants and elevated factor VIII. 1

Acquired Thrombophilia Testing

For acquired hypercoagulable states, test for antiphospholipid antibodies (lupus anticoagulant, anticardiolipin antibodies, anti-β2-glycoprotein I antibodies) and consider homocysteine levels. 5, 6, 7

Antiphospholipid Syndrome Panel

• Lupus anticoagulant testing using dilute Russell viper venom time or aPTT-based methods 1, 5
• Anticardiolipin antibodies (IgG and IgM) 5, 6
• Anti-β2-glycoprotein I antibodies (IgG and IgM) 5
• Homocysteine level - elevated levels firmly established as risk factor for recurrent venous thromboembolism 6

Second-Line Specialized Testing

If initial testing is normal but clinical suspicion remains high, proceed to expanded coagulation factor assays and fibrinolytic testing. 1

Extended Factor Panel

• Factor II, V, VII, X assays - ordered by 52-55% of specialists as second-line tests 1
• Factor XIII assay - performed by 60% as second-line testing 1
• Plasminogen level and plasminogen activator assays to evaluate fibrinolytic system 5, 6
• Fibrinolysis assays - used by 38% of specialists for unexplained cases 1

Critical Timing and Sample Considerations

Collect samples for inherited thrombophilia testing when the patient is NOT on anticoagulation and at least 2-4 weeks after an acute thrombotic event. 6, 7 Acute thrombosis and anticoagulant therapy can falsely lower protein C, protein S, and antithrombin levels, leading to misdiagnosis. 6, 7

Sample Handling Requirements

• Transport samples at room temperature and separate from blood cells promptly 2
• Freeze plasma at ≤-40°C if testing is delayed 2
• Avoid testing during acute illness, pregnancy, or oral contraceptive use as these elevate clotting factors 2, 6

Common Pitfalls to Avoid

Do not test for thrombophilia during acute thrombosis or while on anticoagulation - protein C, protein S, and antithrombin levels are consumed during active clotting and depleted by warfarin. 6, 7 Wait until anticoagulation is completed or test genetic mutations (factor V Leiden, prothrombin G20210A) which are unaffected by anticoagulation. 6

Check baseline clotting times in activated protein C resistance assays - elevated or reduced baseline times invalidate calculated ratios, particularly in undiluted assays. 1 This is essential even when preliminary screening tests are normal. 1

Consider that approximately 50% of venous thromboembolism episodes in inherited thrombophilia occur with precipitating factors such as surgery, pregnancy (especially postpartum), or immobilization. 6 The presence of a trigger does not exclude underlying inherited thrombophilia. 6

When to Refer for Subspecialty Consultation

Refer to hematology when initial testing shows abnormal results, when clinical suspicion remains high despite normal testing, or when genetic counseling is needed for familial disorders. 3, 7 Approximately 80-90% of hypercoagulability disorders can be correctly diagnosed with current testing methods. 7