Why is a mixing study performed?

Why Mixing Studies Are Performed

A mixing study is performed to differentiate between a factor deficiency and an inhibitor as the cause of a prolonged coagulation test (aPTT or PT). This distinction is critical because it fundamentally changes the diagnostic pathway and clinical management—factor deficiencies may require replacement therapy, while inhibitors (particularly lupus anticoagulant) indicate thrombotic risk rather than bleeding risk 1.

Primary Purpose and Clinical Utility

The mixing study is a mandatory second step in lupus anticoagulant (LA) testing after an initial prolonged screening test (dRVVT or aPTT), forming part of the essential 3-step diagnostic algorithm: screening → mixing → confirmation 2, 1.

Key Diagnostic Distinctions:

• "Corrected" mixing study (clotting time normalizes when patient plasma is mixed 1:1 with pooled normal plasma) → suggests factor deficiency

  • Directs toward specific factor assays (e.g., Factor VIII, IX, XII for aPTT; Factor VII for PT)
  • Associated with bleeding risk in symptomatic patients 3, 4

• "Non-corrected" mixing study (clotting time remains prolonged despite mixing) → suggests inhibitor present

  • Most commonly lupus anticoagulant (associated with thrombosis, not bleeding)
  • Less commonly: specific factor inhibitors (e.g., acquired Factor VIII inhibitor)
  • May indicate heparin contamination or other interference 1

Technical Methodology

The standard mixing test protocol per ISTH guidelines 1:

• 1:1 ratio of patient plasma to pooled normal plasma (PNP)
• No preincubation (test within 30 minutes)
• PNP should be platelet-poor (<10⁷/mL platelets) and contain ~100% activity of all clotting factors
• Results expressed as ratio of patient-to-PNP for all procedures
• Cutoff determined at 99th percentile of healthy donor distribution

Critical Interpretation Caveats

Common Pitfalls to Avoid:

1. Anticoagulant interference can cause false-positive non-correction:

   • Heparin (unfractionated or LMWH) particularly affects aPTT
   • VKA therapy causes both false positives and false negatives 2, 1
   • DOACs require removal agents or trough-level sampling 2
   • Always check thrombin time (TT) to identify heparin contamination 1

2. Factor VIII elevation during acute thrombotic events or pregnancy can mask LA 1

3. Mixing studies during VKA therapy are particularly problematic—ideally test when INR <1.5 or 1-2 weeks after discontinuation with LMWH bridging 1

4. Integrated tests (simultaneous screen/confirm at different phospholipid concentrations) may not require mixing studies, though this approach needs further validation 1

Clinical Context Matters

The mixing study has optimal utility when:

• Performed in symptomatic patients (bleeding or thrombosis)
• Part of structured LA testing for antiphospholipid syndrome diagnosis
• Preoperative evaluation with unexplained prolonged aPTT/PT 5

Limited utility when:

• Isolated abnormal screening test without clinical context
• Patient on anticoagulation without proper sample preparation
• Factor XII deficiency (clinically insignificant despite abnormal tests) 4

Algorithmic Approach

1. Prolonged aPTT or PT detected → Perform mixing study
2. Check TT simultaneously → If prolonged, suspect heparin
3. Mixing corrects → Proceed to specific factor assays based on which test prolonged
4. Mixing doesn't correct → Proceed to LA confirmation testing (phospholipid-dependent) OR specific inhibitor assays (if bleeding history)
5. Borderline results → Repeat in 1 week rather than reporting as "dubious" 1

The mixing study serves as a cost-effective triage tool that prevents unnecessary expensive testing and guides appropriate follow-up investigations 3, 6. However, lack of standardization in interpretation criteria (various formulas like Index of Circulating Anticoagulant with different cutoffs) remains a limitation, emphasizing the need for validated local protocols 3, 6, 5.