What is the diagnostic workup for acquired thrombophilias?

Diagnostic Workup for Acquired Thrombophilias

The diagnostic workup for acquired thrombophilias centers on antiphospholipid syndrome (APS), which is the only genuine acquired thrombophilic state, requiring specific laboratory testing for lupus anticoagulant and antiphospholipid antibodies with confirmatory testing at least 12 weeks apart. 1, 2

Initial Laboratory Evaluation

When to Suspect Acquired Thrombophilia:

• Unexplained prolonged aPTT in a patient with thrombosis, particularly if the mixing study fails to correct 3, 4
• Venous thromboembolism at unusual sites (cerebral, mesenteric, hepatic veins) 1
• Arterial thrombosis in young patients without traditional cardiovascular risk factors 2
• Recurrent pregnancy loss or obstetric complications 2

Core Diagnostic Tests for Antiphospholipid Syndrome

Lupus Anticoagulant Testing:

• Perform a combination of clot-based assays (dilute Russell viper venom time and aPTT-based tests) to demonstrate LA activity 2
• Must exclude heparin contamination by checking thrombin time before interpretation 3, 4
• If patient is on warfarin, defer testing until INR <1.5 or wait 1-2 weeks after discontinuation 4, 5
• Confirmatory testing with high phospholipid concentration (integrated tests) is required if initial screening is positive 3

Antiphospholipid Antibody Testing:

• Measure IgG and IgM anticardiolipin antibodies using solid-phase immunoassays 2
• Measure IgG and IgM anti-β2-glycoprotein-I antibodies 2
• Critical requirement: All positive tests must be repeated at least 12 weeks apart to confirm persistence before diagnosing APS 2

Mixing Study Interpretation for Acquired Inhibitors

When aPTT is Prolonged:

• Perform 1:1 mixing study (patient plasma with normal plasma) immediately and after 2-hour incubation 4, 5
• Calculate Rosner index: ≥11% indicates inhibitor presence (acquired hemophilia A or lupus anticoagulant), while <11% suggests factor deficiency 3, 4
• Failure to correct on mixing study distinguishes acquired hemophilia A from congenital hemophilia 3

If Mixing Study Fails to Correct:

• Measure Factor VIII activity level as the most critical next step 3, 5
• If Factor VIII is low (<40% activity), perform Bethesda assay to quantify inhibitor titer 3, 5
• If Factor VIII is normal and lupus anticoagulant is positive, proceed with complete antiphospholipid antibody profile 3

Critical Timing Considerations

When NOT to Test:

• Do not perform thrombophilia testing during acute thrombosis or during the initial 3-month period of anticoagulation 6
• Avoid testing for natural anticoagulant deficiencies (antithrombin, protein C, protein S) during acute thrombosis, pregnancy, or while on anticoagulant therapy, as these conditions cause acquired deficiencies that will confound results 2
• All abnormal results must be confirmed with repeat testing when the patient is not on anticoagulation to avoid misclassification 2

Specific Clinical Scenarios

Elderly or Postpartum Patients with Prolonged aPTT:

• These populations are at highest risk for acquired hemophilia A 4
• Never dismiss isolated prolonged aPTT without bleeding as benign until acquired hemophilia A is definitively excluded, as bleeding can develop suddenly with high mortality 4
• Proceed immediately with mixing study and Factor VIII activity measurement 4, 5

Patients with Malignancy:

• Consider cancer-associated thrombosis as a distinct acquired thrombophilic state requiring special management 7
• Recent trials suggest low molecular weight heparins are more effective than warfarin in cancer-associated thrombosis 7

Common Pitfalls to Avoid

• Do not assume immediate mixing study correction excludes all inhibitors—proceed with Factor VIII inhibitor testing if clinical presentation suggests acquired hemophilia A with bleeding 4
• Do not interpret low factor levels at face value if lupus anticoagulant is present, as this causes artifactual lowering through phospholipid inhibition in the assay; repeat at higher dilutions 4
• Do not diagnose inhibitor deficiency based on a single abnormal test result—repetitive testing is mandatory 2
• Pre-analytical errors and acquired causes of low natural anticoagulant levels are considerably more common than hereditary deficiencies 2

When Testing is NOT Indicated

Thrombophilia testing should not be performed in most situations and should be used only when the information will influence a decision important to the patient and outweigh the potential risks of testing 6. Specifically:

• Do not test after provoked venous thromboembolism where the provoking factor has been removed 6
• Testing usually does not provide information that impacts management and may result in harm 6