APTT

Overview of Activated Partial Thromboplastin Time (aPTT)

Definition: A coagulation test that measures the time (in seconds) it takes for plasma to clot after the activation of the contact factors (intrinsic pathway) and the addition of calcium and phospholipid
Purpose:
- Evaluate the intrinsic and common pathways of coagulation
- Monitor heparin therapy
- Detect coagulation factor deficiencies
- Investigate unexplained bleeding or thrombotic events
- Screen for lupus anticoagulants
Principle:
- Activation of Contact Factors: A contact activator (e.g., kaolin, silica, celite) is added to plasma to activate Factor XII, initiating the intrinsic pathway
- Addition of Phospholipids: Phospholipids provide a surface for the assembly of coagulation complexes
- Addition of Calcium: Calcium ions are added to initiate the coagulation cascade
- Clot Detection: The time to clot formation is measured, either manually or using an automated coagulation analyzer

Specimen Collection:
- Collection Tube: Sodium citrate (light blue top) tube with a 3.2% or 3.8% sodium citrate concentration
- Blood-to-Anticoagulant Ratio: The correct ratio is critical for accurate coagulation testing:
  - 9:1 ratio of blood to anticoagulant
  - Underfilling the tube can lead to falsely prolonged clotting times due to excess anticoagulant
  - Overfilling the tube can lead to falsely shortened clotting times due to insufficient anticoagulant
Reagents:
- Contact Activator: A substance that activates Factor XII (e.g., kaolin, silica, celite)
- Partial Thromboplastin: A phospholipid reagent that provides a surface for the assembly of coagulation complexes
- Calcium Chloride: Provides calcium ions, which are essential for the activity of coagulation factors
Instrumentation:
- Automated Coagulation Analyzer: Detects clot formation using optical or mechanical methods:
  - Optical Method: Measures changes in light transmission as the clot forms
  - Mechanical Method: Detects the movement of a probe or wire as the clot forms

Sample Preparation:
- Centrifuge the sodium citrate tube to obtain platelet-poor plasma (PPP)
- PPP should have a platelet count < 10 x 10^9/L
Automated Method:
- Follow the instrument manufacturer’s instructions
- Add the contact activator and phospholipid reagent to the plasma
- Incubate the mixture at 37°C for a specific period (e.g., 3-5 minutes)
- Add calcium chloride to initiate coagulation
- Measure the time to clot formation (aPTT in seconds)
Quality Control:
- Run control materials at regular intervals to assess the accuracy and precision of the test
- Review control results and take corrective action if necessary

Activated Partial Thromboplastin Time (aPTT):
- Reference Range: Varies depending on the reagent and instrument used (e.g., 25-35 seconds)
- Prolonged aPTT: Indicates a deficiency or dysfunction of one or more factors in the intrinsic or common pathways (Factors XII, XI, IX, VIII, X, V, prothrombin, fibrinogen)
  - Heparin Therapy: aPTT is used to monitor unfractionated heparin therapy
    - The target aPTT range is typically 1.5-2.5 times the control value
  - Coagulation Factor Deficiencies:
    - Hemophilia A (Factor VIII deficiency)
    - Hemophilia B (Factor IX deficiency)
    - Factor XI deficiency
  - Lupus Anticoagulant (LA): May prolong the aPTT
  - Vitamin K Deficiency: May prolong the aPTT if severe
  - Disseminated Intravascular Coagulation (DIC): Consumption of coagulation factors

Pre-Analytical Variables:
- Improper Collection Technique: Tissue thromboplastin contamination or hemolysis
- Incorrect Blood-to-Anticoagulant Ratio: Underfilling or overfilling the collection tube
- Clotted Sample: Invalidates the results
- Delayed Testing: Coagulation factors can degrade over time
- Improper Storage: Incorrect storage temperatures can affect results
Analytical Variables:
- Instrument Malfunction: Ensure proper calibration and maintenance of the coagulation analyzer
- Reagent Problems: Use fresh, properly stored reagents and follow the manufacturer’s instructions
- Lipemia or Icterus: Can interfere with optical clot detection
Patient-Related Variables:
- Medications: Heparin, direct oral anticoagulants, and other medications can affect coagulation test results
- Liver Disease: Impairs the synthesis of coagulation factors
- Genetic Factors: Rare genetic variations can affect coagulation factor levels

If the aPTT is prolonged and the mixing study suggests a factor deficiency, the next step is to perform individual factor assays to identify the specific factor that is deficient
If the aPTT is prolonged and the mixing study suggests a factor inhibitor, further testing is needed to characterize the inhibitor:
- Factor VIII Inhibitor Assay: To detect and quantify Factor VIII inhibitors
- Lupus Anticoagulant Testing:
  - Dilute Russell’s Viper Venom Time (dRVVT)
  - Silica Clotting Time (SCT)
  - Hexagonal Phase Phospholipid Neutralization Assay

aPTT (Activated Partial Thromboplastin Time): Test of the intrinsic and common pathways
Contact Activator: Reagent used to initiate the intrinsic pathway
Phospholipid: A substance needed for the assembly of coagulation complexes
Factor Assay: A laboratory test to measure the activity of a specific coagulation factor
Mixing Study: A test to differentiate factor deficiencies from factor inhibitors
Lupus Anticoagulant (LA): An antibody that interferes with phospholipid-dependent coagulation assays