Activated partial thromboplastin time

1. Activated Partial Thromboplastin Time

The **Activated Partial Thromboplastin Time (aPTT)** is a blood test that characterizes the coagulation of blood. It is primarily used to investigate unexplained bleeding or clotting and to monitor patients on heparin therapy. The aPTT test evaluates the intrinsic and common pathways of the coagulation cascade.

Overview

The aPTT test measures the time it takes for a clot to form in a blood sample after reagents are added. It is called "partial" because it does not include tissue factor, which is part of the extrinsic pathway. The test is "activated" by adding substances like kaolin or silica to the blood sample, which initiates the clotting process.

Clinical Significance

The aPTT is used in several clinical scenarios:

• **Monitoring Heparin Therapy**: Heparin is an anticoagulant that affects the intrinsic pathway. The aPTT is used to adjust heparin dosage to ensure therapeutic levels are achieved without causing excessive bleeding.
• **Diagnosing Coagulation Disorders**: Prolonged aPTT can indicate deficiencies in clotting factors such as Factor VIII, IX, XI, or XII, or the presence of inhibitors like lupus anticoagulant.
• **Preoperative Screening**: In some cases, aPTT is used to screen for bleeding disorders before surgery.

Procedure

1. **Sample Collection**: Blood is drawn from a vein, usually in the arm. 2. **Centrifugation**: The blood sample is centrifuged to separate plasma from blood cells. 3. **Reagent Addition**: Reagents such as calcium chloride and a phospholipid are added to the plasma. 4. **Clotting Time Measurement**: The time taken for the plasma to clot is measured, typically in seconds.

Interpretation of Results

• **Normal Range**: The typical reference range for aPTT is 30-40 seconds, but this can vary depending on the laboratory.
• **Prolonged aPTT**: May indicate hemophilia, von Willebrand disease, or the presence of anticoagulants like heparin.
• **Shortened aPTT**: Less common, but may occur in conditions with increased clotting activity.

Limitations

The aPTT test can be affected by several factors, including:

• **Sample Handling**: Improper handling can lead to inaccurate results.
• **Liver Disease**: Can affect the production of clotting factors, altering aPTT results.
• **Medications**: Other anticoagulants or medications can interfere with the test.

Also see

• Prothrombin Time
• Coagulation Cascade
• Heparin
• Lupus Anticoagulant
• Hemophilia