Which coagulation tests can be performed on whole‑blood specimens, and why is plasma unsuitable for these assays?

Whole Blood Coagulation Testing

Viscoelastic hemostatic assays—specifically thromboelastography (TEG) and rotational thromboelastometry (ROTEM)—are the primary tests performed on whole blood coagulation specimens, and plasma is unacceptable because it omits the critical contributions of cellular components (platelets, red blood cells, and leukocytes) that fundamentally influence clot formation and fibrinolysis 1.

Tests Performed on Whole Blood Specimens

The following assays require whole blood:

• TEG (Thromboelastography) - Measures global hemostasis from initial clot formation through fibrinolysis, providing parameters including:

  • R time (reaction time to initial clot formation)
  • Alpha angle (rate of clot strengthening)
  • MA (maximum amplitude - clot strength)
  • LY30 (percent clot degradation at 30 minutes post-MA) 2, 1

• ROTEM (Rotational Thromboelastometry) - Similar viscoelastic assessment with parameters including:

  • CT (clotting time)
  • MCF (maximum clot firmness)
  • Fibrinolysis measures 2, 1

• Emerging whole blood assays (not yet clinically available):

  • Thrombolysis plate assay
  • Global fibrinolytic capacity assays 1

Why Plasma is Unacceptable

The fundamental limitation of plasma-based testing is the complete absence of cellular elements that are essential to physiologic hemostasis 1, 3, 4:

Critical Missing Components in Plasma:

1. Platelets - Essential for the amplification and propagation phases of thrombin generation. Without platelets, plasma assays completely miss these critical hemostatic events 4

2. Red Blood Cells - Influence fibrinolysis and clot structure through effects on blood viscosity and mechanical properties 1

3. Leukocytes - Contribute to inflammatory modulation of coagulation 1

Clinical Consequences of Using Plasma:

• Standard plasma tests (PT/PTT) fail to detect hypercoagulable states that whole blood assays readily identify 3, 4

• Plasma assays cannot reflect therapeutic effects of certain agents like recombinant FVIIa, which depend on cellular interactions 4

• Loss of global hemostatic assessment - Plasma testing evaluates isolated components rather than the integrated coagulation system 3, 4

Important Caveats for Whole Blood Testing

TEG/ROTEM Limitations to Consider:

• Hematocrit effects: Low hematocrit paradoxically produces hypercoagulable-appearing results on TEG despite clinical bleeding tendency, due to decreased blood viscosity 2

• High variability: Coefficients of variance range from 7.1% to 83.6% for various parameters, requiring rigorous quality control and trained operators 2

• Device-specific results: TEG and ROTEM are not interchangeable; different devices (e.g., ROTEM Delta vs. Sigma) yield different values requiring device-specific interpretation 2

• Temperature limitation: Tests run at 37°C and cannot assess hypothermia effects 2

Clinical Applications Where Whole Blood Testing Excels:

• Hyperfibrinolysis detection: TEG LY30 values of 3-7.5% identify patients at increased risk for massive transfusion and mortality 1

• Trauma and surgical bleeding management: Point-of-care assessment within 60 minutes guides transfusion decisions 1

• Hypercoagulability screening: Detects prothrombotic states missed by conventional plasma tests 3

The requirement for whole blood reflects modern understanding that hemostasis is a cell-based process, not merely a cascade of plasma proteins—plasma testing represents an outdated model that fails to capture the cellular amplification and propagation phases essential to physiologic coagulation 4.