Screening coagulation tests are essential in the haematology lab to assess the blood’s ability to clot properly. These tests help in diagnosing clotting disorders and monitoring anticoagulant therapy. Here’s a detailed overview of the common screening coagulation procedures:
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Prothrombin Time (PT) and International Normalized Ratio (INR)
Determination: Evaluate the extrinsic and common pathways of coagulation.
Procedure:
- Sample Collection:
- Collect blood in a citrate anticoagulant tube (usually 3.2% sodium citrate).
- The typical ratio of blood to citrate is 9:1.
- Sample Preparation:
- Centrifuge the sample to separate plasma from cells.
- Use the plasma for testing.
- Testing:
- Add a thromboplastin reagent (containing tissue factor and phospholipids) to the plasma.
- Incubate the mixture at 37°C.
- Add calcium chloride to reintroduce calcium, which initiates the coagulation process.
- Measure the time taken for the clot to form.
- Calculation:
- PT: Reported in seconds.
- INR: A standardized measure for variations in thromboplastin reagents, calculated using the formula:
INR = (PT patient / PT normal) x ISI
- where ISI (International Sensitivity Index) is specific to the reagent used.
Normal Range:
- PT: 11-13.5 seconds
- INR: 0.8-1.2 (therapeutic ranges vary depending on the condition being treated)
-
Activated Partial Thromboplastin Time (aPTT)
Determination: Assesses the intrinsic and common pathways of coagulation.
Procedure:
- Sample Collection:
- Collect blood in a citrate anticoagulant tube.
- Sample Preparation:
- Centrifuge to separate plasma.
- Use the plasma for testing.
- Testing:
- Add an activator (e.g., kaolin, silica, or ellagic acid) to the plasma to activate the intrinsic pathway.
- Incubate at 37°C.
- Add calcium chloride to the mixture to start the coagulation process.
- Measure the time taken for the formation of a clot.
- Calculation:
- aPTT: Reported in seconds.
Normal Range:
- aPTT: 30-40 seconds
-
Thrombin Time (TT)
Determination: Measures the time for thrombin to convert fibrinogen to fibrin, assessing the final common coagulation pathway.
Procedure:
- Sample Collection:
- Collect blood in a citrate anticoagulant tube.
- Sample Preparation:
- Centrifuge to obtain plasma.
- Testing:
- Add thrombin to the plasma.
- Measure the time for fibrin clot formation.
- Calculation:
- TT: Reported in seconds.
Normal Range:
- TT: 14-16 seconds
-
Fibrinogen Level
Determination: Measures the fibrinogen concentration in the blood, which is essential for clot formation.
Procedure:
- Sample Collection:
- Collect blood in a citrate anticoagulant tube.
- Sample Preparation:
- Centrifuge to separate plasma.
- Testing:
- Perform a clot-based assay, such as the Clauss method, which measures the time for fibrinogen to convert to fibrin in the presence of thrombin.
- Alternatively, use an immunoassay to measure fibrinogen concentration directly.
- Calculation:
- Fibrinogen Level: Reported in mg/dL.
Normal Range:
- Fibrinogen: 200-400 mg/dL
-
Platelet Count
Determination: Assesses the number of platelets, which are crucial for the initial formation of the platelet plug.
Procedure:
- Sample Collection:
- Collect blood in an EDTA tube.
- Testing:
- Use automated haematology analyzers to count platelets.
- Alternatively, perform a manual count using a hemocytometer.
- Calculation:
- Platelet Count: Reported in cells per microliter (cells/µL).
Normal Range:
- Platelet Count: 150,000-450,000 cells/µL
-
Additional Tests (as needed)
Depending on clinical indications, additional tests may include:
- Activated Clotting Time (ACT): Used for monitoring anticoagulant therapy during procedures.
- D-dimer: Assesses fibrin degradation products; useful in diagnosing thrombotic disorders.
- Anti-factor assays: Evaluate specific clotting factor deficiencies (e.g., Factor VIII, Factor IX).