Activated partial thromboplastin

1. Activated Partial Thromboplastin Time (aPTT)

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 used in the Prothrombin Time (PT) test. The "activated" part refers to the addition of an activator that speeds up the clotting time and makes the test more sensitive.

Clinical Significance

The aPTT is used to:

• **Diagnose bleeding disorders**: It helps in diagnosing conditions such as Hemophilia, von Willebrand disease, and other factor deficiencies.
• **Monitor anticoagulation therapy**: It is used to monitor patients on Heparin therapy, ensuring that the dosage is effective yet safe.
• **Evaluate unexplained bleeding or bruising**: It helps in assessing the cause of abnormal bleeding.

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**: Calcium chloride and a phospholipid reagent are added to the plasma. 4. **Clotting Time Measurement**: The time taken for a clot to form is measured in seconds.

Normal Range

The normal range for aPTT is typically between 30 to 40 seconds, but this can vary slightly depending on the laboratory and the reagents used.

Interpretation of Results

• **Prolonged aPTT**: May indicate a deficiency in one or more clotting factors, the presence of inhibitors (such as lupus anticoagulant), or the effect of anticoagulant therapy.
• **Shortened aPTT**: Less common, but may be seen in conditions with increased clotting activity.

Limitations

The aPTT test can be affected by:

• **Sample handling**: Improper handling can lead to inaccurate results.
• **Liver disease**: As the liver produces clotting factors, liver disease can affect aPTT results.
• **Medications**: Certain medications can alter aPTT results.

Also see

• Prothrombin Time
• Coagulation Cascade
• Heparin
• Bleeding Disorders