Activated Partial Thromboplastin Time

What is aPTT?
The Activated Partial Thromboplastin Time (aPTT) also known as partial thromboplastin time (PTT) is a screening test that helps evaluate a person’s ability to form blood clots appropriately.

This test is performed to detect bleeding and clotting disorders and to monitor the treatment with blood-thinning medicines. In case of unexplained bleeding, inappropriate blood clotting, or recurrent miscarriages and sometimes before a scheduled surgery aPTT test is performed. This test is usually performed with other tests like prothrombin time and thrombin time to measure the clotting time accurately.

Values greater than normal, could be due to bleeding disorder or liver disease. Values less than normal indicate that you may have a higher chance of getting blood clots and several miscarriages.

Why is aPTT done?

The Activated Partial Thromboplastin Time Test is performed:

• · To detect and diagnose bleeding or clotting disorder
• · Before performing a surgical procedure to assess the risk of bleeding
• · To monitor the treatment with blood-thinning medicines (e.g. unfractionated heparin therapy)
• · To investigate the cause of thrombotic episodes or recurrent miscarriages

What does aPTT Measure?

The Activated Partial Thromboplastin Time (aPTT) test measures the time taken by the blood to form a clot after the addition of substances (reagents) which activate the clot formation.

In case of any bleeding, the body responds to stop the blood loss as quickly as possible by forming a blood clot. This process of blood clotting or coagulation is called hemostasis and involves a series of chemical reactions in the blood (coagulation cascade). During this process, the blood proteins called coagulation factors get activated one after another in a series. The activated coagulation factors lead to the formation of fibrin mesh around the platelets and other blood cells at the site of bleeding and this complex hardens to form a “blood clot”.

Coagulation cascade proceeds by two pathways, the intrinsic pathway, and the extrinsic pathway. These pathways later merge together into a common pathway. The Activated Partial Thromboplastin Time (aPTT) test measures the functioning of Blood Coagulation Factors I, II, V, XII, VIII, IX, X, and XI, along with other factors Prekallikrein (PK), and High Molecular Weight Kininogen (HK) which form parts of the intrinsic and common coagulation pathways.

The Activated Partial Thromboplastin Time (aPTT) test measures the time taken by the blood to form a clot after the addition of substances (calcium and phospholipid emulsion) which activate clotting. The aPTT test result is compared to a control sample of normal blood.

The aPTT test is usually performed along with Prothrombin Time (PT) test to evaluate the cause of a coagulation defect, if any. PT test assesses the functioning of Blood Coagulation Factors I, II, V, VII, and X, which are parts of the extrinsic and common pathways by measuring the time taken for conversion of prothrombin to thrombin.