What is the recommended approach for diagnostic testing of antithrombin deficiency?

Diagnostic Testing for Antithrombin Deficiency

The recommended approach for diagnosing antithrombin deficiency is to begin with a functional antithrombin activity assay, followed by antigen testing when activity levels are low to distinguish between type I and type II deficiency. 1

Initial Testing Strategy

• A functional antithrombin activity assay should be used as the initial screening test for antithrombin deficiency 1, 2
• Testing should be performed on properly collected citrated plasma (3.2% or 0.105-0.109 mol/L trisodium citrate) that has been double-centrifuged to ensure a platelet count <10 × 10^9/L 3
• Plasma samples should be tested within 4 hours of collection or properly frozen (at -20°C for ≤2 weeks or at -70°C for longer storage) 3
• Repeat testing on a second sample is necessary to confirm the diagnosis and rule out pre-analytical errors 1

Follow-up Testing

• When antithrombin activity is low, an antigen assay should be performed to differentiate between type I (quantitative) and type II (qualitative) deficiency 1, 4
• The activity-to-antigen ratio helps classify the deficiency type:
  • Type I: both activity and antigen levels are reduced
  • Type II: normal antigen but reduced activity 1, 4
• Further specialized tests can help subclassify type II deficiencies (IIa, IIb, IIc), though this is typically not performed for routine clinical purposes 5

Interpretation Considerations

• Pediatric reference ranges should be used for children under 6 months of age 1
• Acquired causes of low antithrombin must be excluded before diagnosing hereditary deficiency:
  • Liver dysfunction
  • Proteinuria/nephrotic syndrome
  • Heparin therapy
  • Disseminated intravascular coagulation (DIC)
  • Acute thrombosis
  • Recent surgery 1, 2
• Be aware of medications that can cause falsely normal or elevated results:
  • Direct thrombin inhibitors (argatroban, bivalirudin, dabigatran) in factor IIa-based assays
  • Direct factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) in Xa-based assays 1, 2

Advanced Testing

• Molecular genetic testing is not routinely performed for clinical purposes but may be helpful in:
  • Determining thrombosis risk (which varies among different mutations)
  • Identifying mutations that might be missed by traditional activity assays 1
• DNA testing is not recommended as an initial test due to the vast array of possible mutations that would require sequencing the entire gene 4

Testing in Special Populations

• For pregnant women with suspected antithrombin deficiency, testing should be performed as part of a thorough VTE risk assessment, especially in those with prior history of thrombosis 6
• When evaluating patients for thrombophilia, consider testing for other heritable thrombophilic factors alongside antithrombin testing, particularly:
  • Factor V Leiden
  • Prothrombin 20210A mutation
  • Protein C deficiency
  • Protein S deficiency 3

Common Pitfalls to Avoid

• Failure to repeat testing to confirm diagnosis 1
• Not considering acquired causes of low antithrombin levels 2
• Testing during acute thrombosis or while on anticoagulant therapy, which can affect results 1
• Misinterpreting results without considering medication effects on specific assay types 1, 2
• Relying solely on antigen levels without functional activity testing 4