What do silica clotting time and Activated Partial Thromboplastin Time (APTT) mixing study look for?

Silica Clotting Time and APTT Mixing Studies: Diagnostic Applications

Silica Clotting Time (SCT) and Activated Partial Thromboplastin Time (APTT) mixing studies are essential diagnostic tools for detecting lupus anticoagulant and differentiating between factor deficiencies and inhibitors in coagulation disorders.

Understanding Silica Clotting Time (SCT)

SCT is a specialized coagulation test primarily used for detecting lupus anticoagulant (LA), a type of antiphospholipid antibody that paradoxically prolongs clotting times in laboratory tests but increases thrombosis risk in patients.

• Principle: SCT uses silica as an activator with low phospholipid concentration to enhance sensitivity for lupus anticoagulant detection 1
• Clinical application: SCT is particularly useful in diagnosing antiphospholipid syndrome (APS), which increases risk of thrombosis and pregnancy complications
• Interpretation: Results are typically expressed as a normalized ratio comparing patient results to normal controls

APTT Mixing Studies

APTT mixing studies are performed to determine whether a prolonged APTT is due to factor deficiency or the presence of an inhibitor (such as lupus anticoagulant).

Methodology:

1. Basic procedure: Mix patient plasma with normal pooled plasma in 1:1 proportion 1
2. Immediate testing: Perform testing within 30 minutes of mixing without pre-incubation
3. Calculation: Calculate Index of Circulating Anticoagulant (ICA) or percent correction

Interpretation:

• Factor deficiency: The mixing study will show correction of the prolonged APTT
• Inhibitor present: The mixing study will show persistent prolongation of APTT
• Calculation method: Using the estimated factor correction (EFC) method provides superior accuracy in differentiating between deficiencies and inhibitors 2

Diagnostic Algorithm for Coagulation Disorders

1. Initial screening:

   • Perform APTT and PT tests
   • If prolonged, proceed to mixing studies

2. Mixing study protocol:

   • Mix patient plasma with normal plasma (1:1)
   • Test immediately and after incubation (37°C for 1-2 hours)
   • Calculate correction using EFC method

3. Interpretation:

   • Immediate correction + delayed prolongation: Suggests time-dependent inhibitor
   • No correction at any time: Suggests immediate-acting inhibitor (like LA)
   • Complete correction at all times: Suggests factor deficiency

4. Confirmatory testing:

   • For suspected LA: Perform phospholipid-dependent confirmatory test
   • For suspected factor deficiency: Perform specific factor assays
   • Calculate percent correction: [(screen - confirm)/screen] × 100 1

Clinical Applications

Lupus Anticoagulant Detection:

• The International Society on Thrombosis and Haemostasis recommends using two parallel tests for LA detection 3:

  • dRVVT (dilute Russell's Viper Venom Time)
  • LA-sensitive APTT with silica as activator

• Testing should be performed in patients with:

  • Unprovoked venous or arterial thrombosis
  • Thrombosis at unusual sites
  • Recurrent pregnancy loss
  • Late pregnancy complications 1

Factor Deficiency Assessment:

• APTT mixing studies help identify deficiencies in the intrinsic pathway factors (VIII, IX, XI, XII)
• Particularly useful in diagnosing hemophilia and other bleeding disorders

Important Considerations and Pitfalls

• Anticoagulant interference: Heparin, DOACs, and VKAs can interfere with test results 3

  • For patients on heparin: Some PTT-LA reagents contain neutralizers for heparin up to 0.8 U/mL
  • For patients on DOACs: Consider DOAC removal agents before testing

• Sample preparation: Proper preparation of platelet-poor plasma is essential

  • Double centrifugation to ensure minimal platelet contamination (<10^7/mL)
  • Testing should be performed within 4 hours or samples frozen at -70°C 1

• Reagent selection: Not all APTT reagents have equal sensitivity to LA

  • Colorless APTT reagents without opacity are recommended for CWA (Clot Waveform Analysis) 3
  • Among tested reagents, Thrombocheck APTT-SLA for MDA-II/CS series and HemosIL SynthASil for ACL-Top series are suitable 3

• Timing of testing: Avoid testing during acute thrombosis or pregnancy

  • Repeat positive tests after >12 weeks to confirm persistence 1

Advanced Analysis: Clot Waveform Analysis

Clot waveform analysis provides additional information beyond standard clotting times:

• Parameters measured:

  • First derivative: Reflects coagulation velocity
  • Second derivative: Reflects coagulation acceleration
  • Common parameters: CT (clotting time), Min1 (max velocity), Min2 (max acceleration), Max2 (max deceleration) 3

• Clinical value: Min2 (maximum coagulation acceleration) correlates with clotting function in hemophilia and can discriminate between different levels of factor VIII:C below 1.0 IU/dL 3

By systematically applying these tests and understanding their interpretation, clinicians can accurately diagnose coagulation disorders and provide appropriate treatment to reduce morbidity and mortality associated with thrombotic and bleeding complications.