Thromboelastography

Overview of Thromboelastography (TEG)

Definition: Thromboelastography (TEG) and rotational thromboelastometry (ROTEM) are viscoelastic hemostatic assays that provide a comprehensive assessment of hemostasis, including:
- Coagulation: The formation of fibrin clot
- Platelet Function: The contribution of platelets to clot strength
- Fibrinolysis: The breakdown of the clot
Principle:
- Whole Blood Analysis: TEG analyzes the entire coagulation process in whole blood, providing a more comprehensive assessment of hemostasis than traditional plasma-based coagulation tests (PT and aPTT)
- Viscoelastic Measurements: Measures the viscoelastic properties of the developing clot over time, including:
  - Clot Strength
  - Clotting Time
  - Fibrinolysis
- Dynamic Assessment: Provides a dynamic assessment of clot formation and breakdown, reflecting the interactions between coagulation factors, platelets, and fibrinolytic proteins
Clinical Significance:
- Assessment of Bleeding Risk: Helps identify patients at risk of bleeding during surgery or other invasive procedures
- Goal-Directed Hemostatic Therapy: Guides the use of blood products and hemostatic agents in bleeding patients
- Monitoring Anticoagulation: Can be used to monitor the effects of anticoagulants
- Diagnosis of Coagulation Disorders: Can help identify inherited or acquired coagulation disorders
- Assessment of Trauma-Induced Coagulopathy: Useful for assessing coagulopathy in trauma patients
Instruments:
- Thromboelastograph (TEG): Older technology, measures the movement of a pin suspended in a cup of whole blood
- Rotational Thromboelastometry (ROTEM): Newer technology, measures the rotation of a pin suspended in a cup of whole blood

Components of the TEG/ROTEM System

Instrumentation:
- TEG or ROTEM Analyzer: The instrument that performs the viscoelastic measurements
- Cups and Pins: Disposable cups and pins that are used to hold the blood sample and measure clot formation
- Pipettes: Accurate pipettes for measuring reagents and samples
- Temperature Control: System to maintain the sample at a constant temperature (typically 37°C)
Reagents:
- Activators: Reagents that activate the coagulation cascade
  - Kaolin: A contact activator that initiates the intrinsic pathway
  - Tissue Factor (TF): Activates the extrinsic pathway
  - Ellagic Acid: Activates the contact pathway
- Inhibitors (Optional): Used to assess specific components of the coagulation system
  - Cytochalasin D: Inhibits platelet function
  - Aprotinin: Inhibits fibrinolysis
- Quality Control Materials: Commercially available control materials are used to monitor the performance of the instrument
Sample Preparation:
- Whole Blood Collection:
  - Collect whole blood in a specialized tube recommended by the manufacturer (typically a citrate tube with a specific formulation)
  - Ensure that the tube is filled to the correct volume to maintain the proper blood-to-anticoagulant ratio
  - Gentle Mixing: Gently invert the collection tube 5-10 times to mix the blood with the anticoagulant
- Timely Analysis:
  - Test the sample within the timeframe recommended by the manufacturer (typically within 1-4 hours of collection)
  - Store the sample at room temperature (22-25°C) until testing

Procedure for Performing TEG/ROTEM

Sample Preparation:
- Follow the instrument manufacturer’s instructions for sample preparation
- Ensure that the blood sample is well-mixed and free of clots
Reagent Preparation:
- Prepare the reagents according to the manufacturer’s instructions
- Ensure that the reagents are properly stored and are not expired
Instrument Setup:
- Turn on the instrument and allow it to warm up
- Load the appropriate cups and pins into the instrument
- Enter the patient information and test order into the instrument software
Running the Assay:
- Pipette a specific volume of the whole blood sample into the cup
- Add the appropriate activator (e.g., kaolin or tissue factor) to the sample
- Start the test and allow the instrument to perform the analysis
- The instrument will continuously monitor the viscoelastic properties of the developing clot and generate a TEG or ROTEM tracing

Interpreting TEG/ROTEM Results

Key Parameters:
- R (Reaction Time):
  - The time from the start of the test until the first evidence of clot formation (initial fibrin formation)
  - Represents the activity of the coagulation factors
  - Prolonged R: Indicates a deficiency or inhibition of coagulation factors
- K (Clot Formation Time):
  - The time from the end of R until the clot reaches a certain level of strength
  - Represents the rate of fibrin clot formation
  - Prolonged K: Suggests a deficiency or dysfunction of fibrinogen
- α (Alpha Angle):
  - The angle between the R and K parameters
  - Reflects the speed of clot formation
  - Decreased Alpha Angle: Suggests a deficiency or dysfunction of fibrinogen or platelets
- MA (Maximum Amplitude):
  - The maximum strength of the clot, reflecting the interaction of platelets and fibrin
  - Represents the contribution of platelets to clot strength
  - Low MA: Suggests thrombocytopenia or platelet dysfunction
- CI (Clotting Index):
  - A global measure of hemostasis
- LY30 (Lysis at 30 Minutes):
  - The percentage decrease in clot amplitude 30 minutes after MA
  - Reflects the degree of fibrinolysis
  - Increased LY30: Suggests hyperfibrinolysis
Interpretation Based on Specific Clinical Scenarios:
- Bleeding Patient:
  - Prolonged R: Factor deficiency (e.g., hemophilia)
  - Prolonged K or Decreased α: Fibrinogen deficiency or dysfunction
  - Low MA: Thrombocytopenia or platelet dysfunction
  - Increased LY30: Hyperfibrinolysis (e.g., DIC)
- Thrombotic Patient:
  - Shortened R and K: Hypercoagulability
  - Increased MA: Increased platelet reactivity
  - Decreased LY30: Impaired fibrinolysis
- Monitoring Anticoagulation:
  - Heparin: Prolonged R and K, normal MA and LY30
  - Direct Thrombin Inhibitors: Prolonged R and K, normal MA and LY30
  - Fibrinolytic Agents: Increased LY30

Factors Affecting TEG/ROTEM Results

Pre-Analytical Variables:
- Improper Collection Technique: Tissue thromboplastin contamination or hemolysis
- Clotted Sample: Invalidates the results
- Delayed Testing: Coagulation factors and platelets can degrade over time
- Improper Storage: Incorrect storage temperatures can affect results
Analytical Variables:
- Instrument Malfunction: Ensure proper calibration and maintenance of the analyzer
- Reagent Problems: Use fresh, properly stored reagents and follow the manufacturer’s instructions
- Air Bubbles: Avoid introducing air bubbles into the sample or reagents
- Temperature: Maintain a constant temperature (37°C) during the assay
Patient-Related Variables:
- Medications: Anticoagulants, antiplatelet agents, and other medications can affect TEG/ROTEM results
- Underlying Medical Conditions: Liver disease, kidney disease, and other conditions can affect coagulation and fibrinolysis
- Hypothermia: Can prolong clotting times
- Hemodilution: Can affect results in trauma patients

Troubleshooting Erroneous Results

If the TEG/ROTEM results are unexpected or inconsistent with the patient’s clinical presentation:
- Check the sample for clots or hemolysis
- Repeat the test on a fresh sample
- Verify the instrument and reagent quality control results
- Ensure that the sample and reagents were handled and stored properly
- Review the patient’s medication list and medical history
- Consult with a pathologist or coagulation expert

Reflex Testing

If the TEG/ROTEM results are abnormal, additional testing may be performed to:
- Identify specific factor deficiencies: Factor assays
- Detect platelet function defects: Platelet aggregation studies, flow cytometry
- Quantify fibrinolysis: D-dimer assay, plasminogen activator inhibitor-1 (PAI-1) assay
- Assess heparin or direct thrombin inhibitor (DTI) levels

Key Terms

Thromboelastography (TEG): A viscoelastic hemostatic assay that measures clot formation, strength, and breakdown
Rotational Thromboelastometry (ROTEM): A similar viscoelastic hemostatic assay that uses a different type of instrument
R (Reaction Time): Time to initial clot formation
K (Clot Formation Time): Time to reach a certain clot strength
α (Alpha Angle): Rate of clot formation
MA (Maximum Amplitude): Maximum clot strength
CI (Clotting Index): A global measure of hemostasis
LY30 (Lysis at 30 Minutes): Percentage decrease in clot amplitude 30 minutes after MA
Hypercoagulability: An increased tendency to clot
Hypofibrinolysis: Decreased breakdown of fibrin clots
Hyperfibrinolysis: Excessive breakdown of fibrin clots
Point of Care Testing (POCT): Performance of laboratory tests at or near the site of patient care

Platelet Aggregation

Hypercoagulability