What is the laboratory workup for intrinsic blood clotting disorders?

Laboratory Workup for Intrinsic Blood Clotting Disorders

Begin with activated partial thromboplastin time (aPTT), prothrombin time (PT), and complete blood count (CBC) as your initial screening tests, then proceed to specific factor assays (FVIII, FIX, FXI) and von Willebrand factor testing based on these results. 1

First-Line Laboratory Testing

Essential Screening Tests

• aPTT (100% of experts perform this first-line): Evaluates the intrinsic/contact pathway and is sensitive to deficiencies in factors VIII, IX, XI, XII, prekallikrein, and high-molecular-weight kininogen 1, 2
• PT (100% of experts perform this first-line): Assesses the extrinsic pathway and helps differentiate intrinsic from extrinsic pathway defects 1
• Complete blood count with platelet count: Confirms true thrombocytopenia versus platelet dysfunction and evaluates for other cytopenias 1, 3

Specific Factor Testing (First-Line)

• Intrinsic pathway factors (FVIII, FIX, FXI): Performed by 62% of experts as first-line testing when evaluating bleeding disorders 1
• Von Willebrand factor panel (performed first-line by 84% of experts): 1
  • VWF antigen (VWF:Ag)
  • VWF ristocetin cofactor activity (VWF:RCo)
  • Factor VIII coagulant activity (FVIII)
• Fibrinogen level (Clauss method): Performed first-line by 90% of experts 1

Supportive Tests

• Full blood count: Performed by 65% of experts to assess for anemia from bleeding 1
• Iron studies: Performed by 69% of experts to evaluate chronic blood loss 1
• ABO blood group: Performed by 70% of experts, as blood type affects VWF levels 1

Second-Line Testing (When First-Line Tests Are Normal)

Additional Factor Assays

• Factor XIII: Tested by 60% of experts in second-line workup 1
• Factors II, V, VII, X: Each tested by 52-55% of experts when initial screening is unrevealing 1

Platelet Function Assessment

• Light transmission aggregometry (with ADP, collagen, epinephrine, ristocetin): Most frequently performed second-line test (60% of experts) 1, 4
• Platelet function analyzer (PFA-100/200): Performed by 37% as first-line or 60% as second-line 1
• Flow cytometry for platelet glycoproteins (GPIIb/IIIa, GPIb, GPIb/IX): Performed by 42% in second-line to detect qualitative platelet disorders 1, 4

Specialized Testing

• VWF multimer analysis: Only if VWF:RCo is low or VWF:RCo/VWF:Ag ratio is <0.5-0.7, as this test is technically complex and requires abnormal initial results to justify 1
• Genetic testing: Performed by 48% as second-line when phenotypic testing suggests inherited disorder 1
• Fibrinolysis assays: Performed by 38% in second-line evaluation 1

Critical Sample Handling Requirements

Proper sample collection is essential to avoid false results:

• Collect blood in 3.2% (0.105-0.109 mol/L) trisodium citrate at 1:10 ratio 1
• Double-centrifuge to achieve platelet count <10 × 10⁹/L to prevent platelet contamination affecting results 1
• Test within 4 hours of collection, or freeze at -20°C for ≤2 weeks or -70°C for longer storage 1
• Rapidly thaw frozen samples at 37°C and mix thoroughly before testing 1

Common Pitfalls to Avoid

• Do not rely on aPTT alone: While aPTT detects intrinsic pathway defects, it has variable sensitivity to factor levels and may miss mild deficiencies 2, 5
• Avoid testing during acute illness or anticoagulation: Factor levels and VWF are acute phase reactants that can be falsely elevated during inflammation 1
• Do not skip peripheral blood smear review: This confirms true thrombocytopenia and can reveal platelet size abnormalities (small in Wiskott-Aldrich syndrome, large in Bernard-Soulier) 4, 3
• Recognize contact factor deficiencies (FXII, prekallikrein, high-molecular-weight kininogen) prolong aPTT but do not cause clinical bleeding 2, 5

Interpretation Algorithm

If aPTT is prolonged and PT is normal:

• Suspect intrinsic pathway defect (FVIII, FIX, FXI deficiency) or von Willebrand disease 2, 6
• Proceed to specific factor assays and VWF testing 1

If both aPTT and PT are prolonged:

• Consider common pathway defects (factors II, V, X), fibrinogen deficiency, or vitamin K deficiency 6, 7
• May also indicate liver disease or disseminated intravascular coagulation 2, 6

If all coagulation tests are normal but bleeding history is convincing:

• Proceed to platelet function testing and consider factor XIII assay 1
• 75% of experts formally register these patients even without laboratory abnormalities 1