Standard Coagulation Laboratory Tests
Standard coagulation labs include prothrombin time (PT), activated partial thromboplastin time (APTT), international normalized ratio (INR), fibrinogen level, and platelet count as the core components of routine coagulation testing. 1
Core Coagulation Tests
Primary Tests
- Prothrombin Time (PT): Measures the extrinsic and common pathways of coagulation
- Activated Partial Thromboplastin Time (APTT): Evaluates the intrinsic and common pathways
- International Normalized Ratio (INR): Standardized calculation derived from PT
- Fibrinogen Level: Measures the amount of fibrinogen (Factor I) in blood
- Platelet Count: Quantifies the number of platelets
Additional Tests Often Included
- Thrombin Time (TT): Assesses the final step of the coagulation cascade
- D-dimer: Measures fibrin degradation products
- Factor assays: Specific tests for individual coagulation factors
Clinical Applications and Interpretation
PT/INR Testing
PT is primarily used to assess the extrinsic pathway and is mathematically converted to INR for standardization. The INR was specifically designed for monitoring vitamin K antagonist therapy (e.g., warfarin) 1. It's important to note that INR standardization is only validated for patients on vitamin K antagonists and may be misleading when used in other clinical contexts 1, 2.
APTT Testing
APTT evaluates the intrinsic pathway and is particularly useful for monitoring unfractionated heparin therapy and detecting deficiencies in factors VIII, IX, XI, and XII 3. Prolonged APTT may indicate factor deficiencies, presence of inhibitors, or anticoagulant therapy.
Specialized Testing
For more comprehensive assessment of coagulation status, especially in trauma or complex bleeding disorders:
- Viscoelastic Testing: Methods like thromboelastography (TEG) or rotational thromboelastometry (ROTEM) provide real-time assessment of clot formation and stability 1
- Anti-Xa Levels: Used to monitor low molecular weight heparin or direct Xa inhibitors 1
- Specialized Factor Assays: For specific factor deficiencies like in von Willebrand Disease 4
Guidelines for Coagulation Testing
The European guideline on trauma management recommends "early, repeated and combined measurement of prothrombin time (PT), activated partial thromboplastin time (APTT), fibrinogen and platelets" for detection of post-traumatic coagulopathy 1.
For patients on anticoagulant therapy, especially elderly trauma patients, routine coagulation assays should include APTT, TT, PT, INR, and anti-Xa levels 1.
Common Pitfalls in Coagulation Testing
Misinterpreting INR in non-VKA contexts: The INR was specifically designed for monitoring vitamin K antagonist therapy and may be misleading when applied to other clinical scenarios 1, 2
Overreliance on conventional tests: Standard coagulation screens (PT/INR and APTT) only monitor the initiation phase of blood coagulation, representing only about 4% of thrombin production 1
Pre-analytical variables: Sample collection, processing time, and storage conditions can significantly affect results
Failure to consider clinical context: Laboratory results should always be interpreted in the context of the patient's clinical presentation and medication history
For comprehensive assessment of hemostasis, especially in complex cases like liver disease or trauma, consider supplementing standard tests with viscoelastic methods that provide a more complete picture of clot formation, strength, and stability 1.