In a 44-year-old man with acute pulmonary embolism who is currently taking a direct oral anticoagulant (DOAC), how should factor V Leiden DNA testing and functional assays for protein C, protein S, and antithrombin be interpreted when the blood sample was drawn while on the DOAC?

Interpreting Thrombophilia Testing While on DOACs

Factor V Leiden DNA testing remains fully reliable and interpretable while on DOACs, but functional assays for protein C, protein S, and antithrombin should ideally be deferred until after DOAC discontinuation or performed with DOAC removal agents, as these functional tests can be significantly affected by anticoagulation.

Test-by-Test Interpretation

Factor V Leiden (DNA Testing)

• Completely unaffected by DOACs 1
• DNA-based genotyping detects the R506Q mutation regardless of anticoagulation status
• This is the preferred method when testing must occur during DOAC therapy 1
• Results are definitive and require no special interpretation adjustments

Protein C, Protein S, and Antithrombin (Functional Assays)

These are problematic during DOAC therapy:

• DOACs interfere with functional coagulation assays used to measure these natural anticoagulants 2
• Direct FXa inhibitors (rivaroxaban, apixaban, edoxaban) and direct thrombin inhibitors (dabigatran) can falsely prolong clotting times 3, 4
• This interference can produce false-positive results (appearing deficient when not truly deficient)
• The degree of interference is both drug-level dependent and reagent-dependent 3

Practical Management Algorithm

If Testing Already Performed on DOACs:

1. Factor V Leiden DNA result: Accept and interpret normally

2. Protein C/S/Antithrombin results showing deficiency:

   • Consider these unreliable and potentially false-positive
   • Repeat testing is mandatory after DOAC discontinuation (minimum 48-72 hours for most DOACs, longer with renal impairment)
   • Do not make definitive diagnoses of hereditary deficiencies based on these results

3. Protein C/S/Antithrombin results showing normal levels:

   • More reassuring but still not definitive
   • If clinical suspicion remains high (strong family history, recurrent unprovoked VTE), consider repeat testing off anticoagulation

Alternative Strategies When Testing Cannot Be Delayed:

DOAC Removal Agents 2:

• Commercial DOAC removal agents can be applied to plasma samples before testing
• Critical limitation: Removal may be incomplete, and residual DOAC interference cannot be ruled out if results remain abnormal
• Compare pre- and post-removal results to assess the effect of the removal agent itself
• If clotting times remain prolonged after DOAC removal, measure residual DOAC levels to confirm adequate removal

Trough Sampling 2:

• Draw blood at trough (just before next DOAC dose)
• Even at trough, there is no guarantee of lack of DOAC effect on functional assays 2
• Less reliable than complete discontinuation

Critical Pitfalls to Avoid

1. Do not diagnose hereditary protein C, protein S, or antithrombin deficiency based on functional assays performed during DOAC therapy - this is the most common error

2. Do not assume trough levels eliminate interference - functional assays remain unreliable even at low DOAC concentrations 2

3. Recognize that "normal" results on DOACs are more trustworthy than "abnormal" results - DOACs typically cause false-positive deficiencies, not false-negative results

4. DOAC removal agents are not universally effective - incomplete removal is common, and the removal process itself can alter baseline clotting times 2

Optimal Testing Timing

For this 44-year-old man with acute PE on a DOAC:

• Proceed immediately with Factor V Leiden DNA testing - no need to wait 1
• Defer protein C, protein S, and antithrombin functional assays until:
  • At least 3 months after the acute event (to avoid acute phase effects)
  • After DOAC discontinuation for 48-72 hours minimum (longer if renal impairment)
  • Consider testing for prothrombin G20210A mutation simultaneously with Factor V Leiden, as this is also DNA-based and unaffected by anticoagulation 1

The decision to test for hereditary thrombophilia in this patient should be guided by whether results would change management - at age 44 with unprovoked PE, he likely requires extended anticoagulation regardless of thrombophilia status 5.